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Title: Experimental Researches in Electricity, Volume 1

Author: Michael Faraday

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EXPERIMENTAL RESEARCHES IN ELECTRICITY

by

MICHAEL FARADAY, D.C.L. F.R.S.

Fullerian Profesor of Chemistry in the Royal Institution.
Corresponding Member, etc. of the Royal and Imperial Academies of
Science of Paris, Petersburgh, Florence, Copenhagen, Berlin,
Gottingen, Modena, Stockholm, Palermo, etc. etc.

In Two Volumes

VOL. I.

Second Edition

Reprinted from the PHILOSOPHICAL TRANSACTIONS of 1831-1838.

London:
Richard and John Edward Taylor,
Printers and Publishers to the University of London,
Red Lion Court, Fleet Street

1849







PREFACE


I have been induced by various circumstances to collect in One Volume the
Fourteen Series of Experimental Researches in Electricity, which have
appeared in the Philosophical Transactions during the last seven years: the
chief reason has been the desire to supply at a moderate price the whole of
these papers, with an Index, to those who may desire to have them.

The readers of the volume will, I hope, do me the justice to remember that
it was not written as a _whole_, but in parts; the earlier portions rarely
having any known relation at the time to those which might follow. If I had
rewritten the work, I perhaps might have considerably varied the form, but
should not have altered much of the real matter: it would not, however,
then have been considered a faithful reprint or statement of the course and
results of the whole investigation, which only I desired to supply.

I may be allowed to express my great satisfaction at finding, that the
different parts, written at intervals during seven years, harmonize so well
as they do. There would have been nothing particular in this, if the parts
had related only to matters well-ascertained before any of them were
written:--but as each professes to contain something of original discovery,
or of correction of received views, it does surprise even my partiality,
that they should have the degree of consistency and apparent general
accuracy which they seem to me to present.

I have made some alterations in the text, but they have been altogether of
a typographical or grammatical character; and even where greatest, have
been intended to explain the sense, not to alter it. I have often added
Notes at the bottom of the page, as to paragraphs 59, 360, 439, 521, 552,
555, 598, 657, 883, for the correction of errors, and also the purpose of
illustration: but these are all distinguished from the Original Notes of
the Researches by the date of _Dec. 1838_.

The date of a scientific paper containing any pretensions to discovery is
frequently a matter of serious importance, and it is a great misfortune
that there are many most valuable communications, essential to the history
and progress of science, with respect to which this point cannot now be
ascertained. This arises from the circumstance of the papers having no
dates attached to them individually, and of the journals in which they
appear having such as are inaccurate, i.e. dates of a period earlier than
that of publication. I may refer to the note at the end of the First
Series, as an illustration of the kind of confusion thus produced. These
circumstances have induced me to affix a date at the top of every other
page, and I have thought myself justified in using that placed by the
Secretary of the Royal Society on each paper as it was received. An author
has no right, perhaps, to claim an earlier one, unless it has received
confirmation by some public act or officer.

Before concluding these lines I would beg leave to make a reference or two;
first, to my own Papers on Electro-magnetic Rotations in the Quarterly
Journal of Science, 1822. xii. 74. 186. 283. 416, and also to my Letter on
Magneto-electric Induction in the Annales de Chimie, li. p. 404. These
might, as to the matter, very properly have appeared in this volume, but
they would have interfered with it as a simple reprint of the "Experimental
Researches" of the Philosophical Transactions.

Then I wish to refer, in relation to the Fourth Series on a new law of
Electric Conduction, to Franklin's experiments on the non-conduction of
ice, which have been very properly separated and set forth by Professor
Bache (Journal of the Franklin Institute, 1836. xvii. 183.). These, which I
did not at all remember as to the extent of the effect, though they in no
way anticipate the expression of the law I state as to the general effect
of liquefaction on electrolytes, still should never be forgotten when
speaking of that law as applicable to the case of water.

There are two papers which I am anxious to refer to, as corrections or
criticisms of parts of the Experimental Researches. The first of these is
one by Jacobi (Philosophical Magazine, 1838. xiii. 401.), relative to the
possible production of a spark on completing the junction of the two metals
of a single pair of plates (915.). It is an excellent paper, and though I
have not repeated the experiments, the description of them convinces me
that I must have been in error. The second is by that excellent
philosopher, Marianini (Memoria della Societa Italiana di Modena, xxi.
205), and is a critical and experimental examination of Series viii, and of
the question whether metallic contact is or is not _productive_ of a part
of the electricity of the voltaic pile. I see no reason as yet to alter the
opinion I have given; but the paper is so very valuable, comes to the
question so directly, and the point itself is of such great importance,
that I intend at the first opportunity renewing the inquiry, and, if I can,
rendering the proofs either on the one side or the other undeniable to all.

Other parts of these researches have received the honour of critical
attention from various philosophers, to all of whom I am obliged, and some
of whose corrections I have acknowledged in the foot notes. There are, no
doubt, occasions on which I have not felt the force of the remarks, but
time and the progress of science will best settle such cases; and, although
I cannot honestly say that I _wish_ to be found in error, yet I do
fervently hope that the progress of science in the hands of its many
zealous present cultivators will be such, as by giving us new and other
developments, and laws more and more general in their applications, will
even make me think that what is written and illustrated in these
experimental researches, belongs to the by-gone parts of science.

MICHAEL FARADAY.

Royal Institution,
March, 1839.



CONTENTS.

                                                                       Par.
Series I.    §. 1.  Induction of electric currents                        6
             §. 2.  Evolution of electricity from magnetism              27
             §. 3.  New electrical state or condition of matter          60
             §. 4.  Explication of Arago's magnetic phenomena            81
Series II.   §. 5.  Terrestrial magneto-electric induction              140
             §. 6.  Force and direction of magneto-electric
                       induction generally                              193
Series III.  §. 7.  Identity of electricities from different
                       sources                                          265
             ----    ---- i Voltaic electricity                         268
             ----    ---- ii Ordinary electricity                       284
             ----    ---- iii Magneto-electricity                       343
             ----    ---- iv Thermo-electricity                         349
             ----    ---- v Animal electricity                          351
             §. 8.  Relation by measure of common and voltaic
                       electricity                                      361
             ----   Note respecting Ampère's inductive results
                       after                                            379
Series IV.   §. 9.  New law of electric conduction                      380
             §. 10. On conducting power generally                       418
Series V.    §. 11. Electro-chemical decomposition                      450
             ----   ¶ 1. New conditions of electro-chemical
                            decomposition                               453
             ----   ¶ 2. Influence of water in such decomposition       472
             ----   ¶ 3. Theory of electro-chemical decomposition       477
Series VI.   §. 12. Power of platina, &c. to induce combination         564
Series VII.  §. 11.* Electro-chemical decomposition continued
                       (nomenclature)                                   661
             ----   ¶ 4. Some general conditions of
                            Electro-chemical decomposition              669
             ----   ¶ 5. Volta-electrometer                             704
             ----   ¶ 6. Primary and secondary results                  742
             ----   ¶ 7. Definite nature and extent of
                            electro-chemical forces                     783
             ----   ---- Electro-chemical equivalents                   822
             §. 13. Absolute quantity of Electricity in the
                       molecules of matter                              852
Series VIII. §. 14. Electricity of the voltaic pile                     875
             ----   ¶ 1. Simple voltaic circles                         875
             ----   ¶ 2. Electrolytic intensity                         966
             ----   ¶ 3. Associated voltaic circles; or battery         989
             ----   ¶ 4. Resistance of an electrolyte to
                            decomposition                              1007
             ----   ¶ 5. General remarks on the active battery         1034
Series IX.   §. 15. Induction of a current on itself                   1048
             ----   Inductive action of currents generally             1101
Series X.    §. 16. Improved voltaic battery                           1119
             §. 17. Practical results with the voltaic battery         1136
Series XI.   §. 18. On static induction                                1161
             ----   ¶ 1. Induction an action of contiguous
                            particles                                  1161
             ----   ¶ 2. Absolute charge of matter                     1169
             ----   ¶ 3. Electrometer and inductive apparatus          1179
             ----   ¶ 4. Induction in curved lines                     1215
             ----   ---- Conduction by glass, lac, sulphur, &c.        1283
             ----   ¶ 5. Specific inductive capacity                   1252
             ----   ¶ 6. General results as to the nature of
                            induction                                  1295
             ----   ---- Differential inductometer                     1307
Series XII.  ----   ¶ 7. Conduction or conductive discharge            1320
             ----   ¶ 8. Electrolytic discharge                        1343
             ----   ¶ 9. Disruptive discharge                          1359
             ----   ----  ---- Insulation                              1362
             ----   ----  ---- as spark                                1406
             ----   ----  ---- as brush                                1425
             ----   ----  ---- positive and negative                   1465
Series XIII. ----   ----  ---- as glow                                 1526
             ----   ----  ---- dark                                    1544
             ----   ¶ 10. Convection; or carrying discharge            1562
             ----   ¶ 11. Relation of a vacuum to electrical
                             phenomena                                 1613
             §. 19. Nature of the electric current                     1617
             ----   ---- its transverse forces                         1653
Series XIV.  §. 20. Nature of the electric force or forces             1667
             §. 21. Relation of the electric and magnetic
                        forces                                         1709
             §. 22. Note on electrical excitation                      1737
Index
Notes




EXPERIMENTAL RESEARCHES
IN
ELECTRICITY.




FIRST SERIES.

§ 1. _On the Induction of Electric Currents._ § 2. _On the Evolution of
Electricity from Magnetism._ § 3. _On a new Electrical Condition of
Matter._ § 4. _On_ Arago's _Magnetic Phenomena._

[Read November 24, 1831.]


1. The power which electricity of tension possesses of causing an opposite
electrical state in its vicinity has been expressed by the general term
Induction; which, as it has been received into scientific language, may
also, with propriety, be used in the same general sense to express the
power which electrical currents may possess of inducing any particular
state upon matter in their immediate neighbourhood, otherwise indifferent.
It is with this meaning that I purpose using it in the present paper.

2. Certain effects of the induction of electrical currents have already
been recognised and described: as those of magnetization; Ampère's
experiments of bringing a copper disc near to a flat spiral; his repetition
with electro-magnets of Arago's extraordinary experiments, and perhaps a
few others. Still it appeared unlikely that these could be all the effects
which induction by currents could produce; especially as, upon dispensing
with iron, almost the whole of them disappear, whilst yet an infinity of
bodies, exhibiting definite phenomena of induction with electricity of
tension, still remain to be acted upon by the induction of electricity in
motion.

3. Further: Whether Ampère's beautiful theory were adopted, or any other,
or whatever reservation were mentally made, still it appeared very
extraordinary, that as every electric current was accompanied by a
corresponding intensity of magnetic action at right angles to the current,
good conductors of electricity, when placed within the sphere of this
action, should not have any current induced through them, or some sensible
effect produced equivalent in force to such a current.

4. These considerations, with their consequence, the hope of obtaining
electricity from ordinary magnetism, have stimulated me at various times to
investigate experimentally the inductive effect of electric currents. I
lately arrived at positive results; and not only had my hopes fulfilled,
but obtained a key which appeared to me to open out a full explanation of
Arago's magnetic phenomena, and also to discover a new state, which may
probably have great influence in some of the most important effects of
electric currents.

5. These results I purpose describing, not as they were obtained, but in
such a manner as to give the most concise view of the whole.


§ 1. _Induction of Electric Currents._


6. About twenty-six feet of copper wire one twentieth of an inch in
diameter were wound round a cylinder of wood as a helix, the different
spires of which were prevented from touching by a thin interposed twine.
This helix was covered with calico, and then a second wire applied in the
same manner. In this way twelve helices were superposed, each containing an
average length of wire of twenty-seven feet, and all in the same direction.
The first, third, fifth, seventh, ninth, and eleventh of these helices were
connected at their extremities end to end, so as to form one helix; the
others were connected in a similar manner; and thus two principal helices
were produced, closely interposed, having the same direction, not touching
anywhere, and each containing one hundred and fifty-five feet in length of
wire.

7. One of these helices was connected with a galvanometer, the other with a
voltaic battery of ten pairs of plates four inches square, with double
coppers and well charged; yet not the slightest sensible reflection of the
galvanometer-needle could be observed.

8. A similar compound helix, consisting of six lengths of copper and six of
soft iron wire, was constructed. The resulting iron helix contained two
hundred and fourteen feet of wire, the resulting copper helix two hundred
and eight feet; but whether the current from the trough was passed through
the copper or the iron helix, no effect upon the other could be perceived
at the galvanometer.

9. In these and many similar experiments no difference in action of any
kind appeared between iron and other metals.

10. Two hundred and three feet of copper wire in one length were coiled
round a large block of wood; other two hundred and three feet of similar
wire were interposed as a spiral between the turns of the first coil, and
metallic contact everywhere prevented by twine. One of these helices was
connected with a galvanometer, and the other with a battery of one hundred
pairs of plates four inches square, with double coppers, and well charged.
When the contact was made, there was a sudden and very slight effect at the
galvanometer, and there was also a similar slight effect when the contact
with the battery was broken. But whilst the voltaic current was continuing
to pass through the one helix, no galvanometrical appearances nor any
effect like induction upon the other helix could be perceived, although the
active power of the battery was proved to be great, by its heating the
whole of its own helix, and by the brilliancy of the discharge when made
through charcoal.

11. Repetition of the experiments with a battery of one hundred and twenty
pairs of plates produced no other effects; but it was ascertained, both at
this and the former time, that the slight deflection of the needle
occurring at the moment of completing the connexion, was always in one
direction, and that the equally slight deflection produced when the contact
was broken, was in the other direction; and also, that these effects
occurred when the first helices were used (6. 8.).

12. The results which I had by this time obtained with magnets led me to
believe that the battery current through one wire, did, in reality, induce
a similar current through the other wire, but that it continued for an
instant only, and partook more of the nature of the electrical wave passed
through from the shock of a common Leyden jar than of the current from a
voltaic battery, and therefore might magnetise a steel needle, although it
scarcely affected the galvanometer.

13. This expectation was confirmed; for on substituting a small hollow
helix, formed round a glass tube, for the galvanometer, introducing a steel
needle, making contact as before between the battery and the inducing wire
(7. 10.), and then removing the needle before the battery contact was
broken, it was found magnetised.

14. When the battery contact was first made, then an unmagnetised needle
introduced into the small indicating helix (13.), and lastly the battery
contact broken, the needle was found magnetised to an equal degree
apparently as before; but the poles were of the contrary kind.

15. The same effects took place on using the large compound helices first
described (6. 8.).

16. When the unmagnetised needle was put into the indicating helix, before
contact of the inducing wire with the battery, and remained there until the
contact was broken, it exhibited little or no magnetism; the first effect
having been nearly neutralised by the second (13. 14.). The force of the
induced current upon making contact was found always to exceed that of the
induced current at breaking of contact; and if therefore the contact was
made and broken many times in succession, whilst the needle remained in the
indicating helix, it at last came out not unmagnetised, but a needle
magnetised as if the induced current upon making contact had acted alone on
it. This effect may be due to the accumulation (as it is called) at the
poles of the unconnected pile, rendering the current upon first making
contact more powerful than what it is afterwards, at the moment of breaking
contact.

17. If the circuit between the helix or wire under induction and the
galvanometer or indicating spiral was not rendered complete _before_ the
connexion between the battery and the inducing wire was completed or
broken, then no effects were perceived at the galvanometer. Thus, if the
battery communications were first made, and then the wire under induction
connected with the indicating helix, no magnetising power was there
exhibited. But still retaining the latter communications, when those with
the battery were broken, a magnet was formed in the helix, but of the
second kind (14.), i.e. with poles indicating a current in the same
direction to that belonging to the battery current, or to that always
induced by that current at its cessation.

18. In the preceding experiments the wires were placed near to each other,
and the contact of the inducing one with the buttery made when the
inductive effect was required; but as the particular action might be
supposed to be exerted only at the moments of making and breaking contact,
the induction was produced in another way. Several feet of copper wire were
stretched in wide zigzag forms, representing the letter W, on one surface
of a broad board; a second wire was stretched in precisely similar forms on
a second board, so that when brought near the first, the wires should
everywhere touch, except that a sheet of thick paper was interposed. One of
these wires was connected with the galvanometer, and the other with a
voltaic battery. The first wire was then moved towards the second, and as
it approached, the needle was deflected. Being then removed, the needle was
deflected in the opposite direction. By first making the wires approach and
then recede, simultaneously with the vibrations of the needle, the latter
soon became very extensive; but when the wires ceased to move from or
towards each other, the galvanometer-needle soon came to its usual
position.

19. As the wires approximated, the induced current was in the _contrary_
direction to the inducing current. As the wires receded, the induced
current was in the _same_ direction as the inducing current. When the wires
remained stationary, there was no induced current (54.).

20. When a small voltaic arrangement was introduced into the circuit
between the galvanometer (10.) and its helix or wire, so as to cause a
permanent deflection of 30° or 40°, and then the battery of one hundred
pairs of plates connected with the inducing wire, there was an
instantaneous action as before (11.); but the galvanometer-needle
immediately resumed and retained its place unaltered, notwithstanding the
continued contact of the inducing wire with the trough: such was the case
in whichever way the contacts were made (33.).

21. Hence it would appear that collateral currents, either in the same or
in opposite directions, exert no permanent inducing power on each other,
affecting their quantity or tension.

22. I could obtain no evidence by the tongue, by spark, or by heating fine
wire or charcoal, of the electricity passing through the wire under
induction; neither could I obtain any chemical effects, though the contacts
with metallic and other solutions were made and broken alternately with
those of the battery, so that the second effect of induction should not
oppose or neutralise the first (13. 16.).

23. This deficiency of effect is not because the induced current of
electricity cannot pass fluids, but probably because of its brief duration
and feeble intensity; for on introducing two large copper plates into the
circuit on the induced side (20.), the plates being immersed in brine, but
prevented from touching each other by an interposed cloth, the effect at
the indicating galvanometer, or helix, occurred as before. The induced
electricity could also pass through a voltaic trough (20.). When, however,
the quantity of interposed fluid was reduced to a drop, the galvanometer
gave no indication.

24. Attempts to obtain similar effects by the use of wires conveying
ordinary electricity were doubtful in the results. A compound helix similar
to that already described, containing eight elementary helices (6.), was
used. Four of the helices had their similar ends bound together by wire,
and the two general terminations thus produced connected with the small
magnetising helix containing an unmagnetised needle (13.). The other four
helices were similarly arranged, but their ends connected with a Leyden
jar. On passing the discharge, the needle was found to be a magnet; but it
appeared probable that a part of the electricity of the jar had passed off
to the small helix, and so magnetised the needle. There was indeed no
reason to expect that the electricity of a jar possessing as it does great
tension, would not diffuse itself through all the metallic matter
interposed between the coatings.

25. Still it does not follow that the discharge of ordinary electricity
through a wire does not produce analogous phenomena to those arising from
voltaic electricity; but as it appears impossible to separate the effects
produced at the moment when the discharge begins to pass, from the equal
and contrary effects produced when it ceases to pass (16.), inasmuch as
with ordinary electricity these periods are simultaneous, so there can be
scarcely any hope that in this form of the experiment they can be
perceived.

26. Hence it is evident that currents of voltaic electricity present
phenomena of induction somewhat analogous to those produced by electricity
of tension, although, as will be seen hereafter, many differences exist
between them. The result is the production of other currents, (but which
are only momentary,) parallel, or tending to parallelism, with the inducing
current. By reference to the poles of the needle formed in the indicating
helix (13. 14.) and to the deflections of the galvanometer-needle (11.), it
was found in all cases that the induced current, produced by the first
action of the inducing current, was in the contrary direction to the
latter, but that the current produced by the cessation of the inducing
current was in the same direction (19.). For the purpose of avoiding
periphrasis, I propose to call this action of the current from the voltaic
battery, _volta-electric induction_. The properties of the second wire,
after induction has developed the first current, and whilst the electricity
from the battery continues to flow through its inducing neighbour (10.
18.), constitute a peculiar electric condition, the consideration of which
will be resumed hereafter (60.). All these results have been obtained with
a voltaic apparatus consisting of a single pair of plates.


§ 2. _Evolution of Electricity from Magnetism._


27. A welded ring was made of soft round bar-iron, the metal being
seven-eighths of an inch in thickness, and the ring six inches in external
diameter. Three helices were put round one part of this ring, each
containing about twenty-four feet of copper wire one twentieth of an inch
thick; they were insulated from the iron and each other, and superposed in
the manner before described (6.), occupying about nine inches in length
upon the ring. They could be used separately or conjointly; the group may
be distinguished by the letter A (Pl. I. fig. 1.). On the other part of the
ring about sixty feet of similar copper wire in two pieces were applied in
the same manner, forming a helix B, which had the same common direction
with the helices of A, but being separated from it at each extremity by
about half an inch of the uncovered iron.

28. The helix B was connected by copper wires with a galvanometer three
feet from the ring. The helices of A were connected end to end so as to
form one common helix, the extremities of which were connected with a
battery of ten pairs of plates four inches square. The galvanometer was
immediately affected, and to a degree far beyond what has been described
when with a battery of tenfold power helices _without iron_ were used
(10.); but though the contact was continued, the effect was not permanent,
for the needle soon came to rest in its natural position, as if quite
indifferent to the attached electro-magnetic arrangement. Upon breaking the
contact with the batterry, the needle was again powerfully deflected, but
in the contrary direction to that induced in the first instance.

29. Upon arranging the apparatus so that B should be out of use, the
galvanometer be connected with one of the three wires of A (27.), and the
other two made into a helix through which the current from the trough (28.)
was passed, similar but rather more powerful effects were produced.

30. When the battery contact was made in one direction, the
galvanometer-needle was deflected on the one side; if made in the other
direction, the deflection was on the other side. The deflection on breaking
the battery contact was always the reverse of that produced by completing
it. The deflection on making a battery contact always indicated an induced
current in the opposite direction to that from the battery; but on breaking
the contact the deflection indicated an induced current in the same
direction as that of the battery. No making or breaking of the contact at B
side, or in any part of the galvanometer circuit, produced any effect at
the galvanometer. No continuance of the battery current caused any
deflection of the galvanometer-needle. As the above results are common to
all these experiments, and to similar ones with ordinary magnets to be
hereafter detailed, they need not be again particularly described.

31. Upon using the power of one hundred pairs of plates (10.) with this
ring, the impulse at the galvanometer, when contact was completed or
broken, was so great as to make the needle spin round rapidly four or five
times, before the air and terrestrial magnetism could reduce its motion to
mere oscillation.

32. By using charcoal at the ends of the B helix, a minute _spark_ could be
perceived when the contact of the battery with A was completed. This spark
could not be due to any diversion of a part of the current of the battery
through the iron to the helix B; for when the battery contact was
continued, the galvanometer still resumed its perfectly indifferent state
(28.). The spark was rarely seen on breaking contact. A small platina wire
could not be ignited by this induced current; but there seems every reason
to believe that the effect would be obtained by using a stronger original
current or a more powerful arrangement of helices.

33. A feeble voltaic current was sent through the helix B and the
galvanometer, so as to deflect the needle of the latter 30° or 40°, and
then the battery of one hundred pairs of plates connected with A; but after
the first effect was over, the galvanometer-needle resumed exactly the
position due to the feeble current transmitted by its own wire. This took
place in whichever way the battery contacts were made, and shows that here
again (20.) no permanent influence of the currents upon each other, as to
their quantity and tension, exists.

34. Another arrangement was then employed connecting the former experiments
on volta-electric induction (6-26.) with the present. A combination of
helices like that already described (6.) was constructed upon a hollow
cylinder of pasteboard: there were eight lengths of copper wire, containing
altogether 220 feet; four of these helices were connected end to end, and
then with the galvanometer (7.); the other intervening four were also
connected end to end, and the battery of one hundred pairs discharged
through them. In this form the effect on the galvanometer was hardly
sensible (11.), though magnets could be made by the induced current (13.).
But when a soft iron cylinder seven eighths of an inch thick, and twelve
inches long, was introduced into the pasteboard tube, surrounded by the
helices, then the induced current affected the galvanometer powerfully and
with all the phenomena just described (30.). It possessed also the power of
making magnets with more energy, apparently, than when no iron cylinder was
present.

35. When the iron cylinder was replaced by an equal cylinder of copper, no
effect beyond that of the helices alone was produced. The iron cylinder
arrangement was not so powerful as the ring arrangement already described
(27.).

36. Similar effects were then produced by _ordinary magnets_: thus the
hollow helix just described (34.) had all its elementary helices connected
with the galvanometer by two copper wires, each five feet in length; the
soft iron cylinder was introduced into its axis; a couple of bar magnets,
each twenty-four inches long, were arranged with their opposite poles at
one end in contact, so as to resemble a horse-shoe magnet, and then contact
made between the other poles and the ends of the iron cylinder, so as to
convert it for the time into a magnet (fig. 2.): by breaking the magnetic
contacts, or reversing them, the magnetism of the iron cylinder could be
destroyed or reversed at pleasure.

37. Upon making magnetic contact, the needle was deflected; continuing the
contact, the needle became indifferent, and resumed its first position; on
breaking the contact, it was again deflected, but in the opposite direction
to the first effect, and then it again became indifferent. When the
magnetic contacts were reversed the deflections were reversed.

38. When the magnetic contact was made, the deflection was such as to
indicate an induced current of electricity in the opposite direction to
that fitted to form a magnet, having the same polarity as that really
produced by contact with the bar magnets. Thus when the marked and unmarked
poles were placed as in fig. 3, the current in the helix was in the
direction represented, P being supposed to be the end of the wire going to
the positive pole of the battery, or that end towards which the zinc plates
face, and N the negative wire. Such a current would have converted the
cylinder into a magnet of the opposite kind to that formed by contact with
the poles A and B; and such a current moves in the opposite direction to
the currents which in M. Ampère's beautiful theory are considered as
constituting a magnet in the position figured[A].

  [A] The relative position of an electric current and a magnet is by
  most persons found very difficult to remember, and three or four helps
  to the memory have been devised by M. Ampère and others. I venture to
  suggest the following as a very simple and effectual assistance in
  these and similar latitudes. Let the experimenter think he is looking
  down upon a dipping needle, or upon the pole of the north, and then
  let him think upon the direction of the motion of the hands of a
  watch, or of a screw moving direct; currents in that direction round a
  needle would make it into such a magnet as the dipping needle, or
  would themselves constitute an electro-magnet of similar qualities; or
  if brought near a magnet would tend to make it take that direction; or
  would themselves be moved into that position by a magnet so placed; or
  in M. Ampère's theory are considered as moving in that direction in
  the magnet. These two points of the position of the dipping-needle and
  the motion of the watch hands being remembered, any other relation of
  the current and magnet can be at once deduced from it.

39. But as it might be supposed that in all the preceding experiments of
this section, it was by some peculiar effect taking place during the
formation of the magnet, and not by its mere virtual approximation, that
the momentary induced current was excited, the following experiment was
made. All the similar ends of the compound hollow helix (34.) were bound
together by copper wire, forming two general terminations, and these were
connected with the galvanometer. The soft iron cylinder (34.) was removed,
and a cylindrical magnet, three quarters of an inch in diameter and eight
inches and a half in length, used instead. One end of this magnet was
introduced into the axis of the helix (fig. 4.), and then, the
galvanometer-needle being stationary, the magnet was suddenly thrust in;
immediately the needle was deflected in the same direction as if the magnet
had been formed by either of the two preceding processes (34. 36.). Being
left in, the needle resumed its first position, and then the magnet being
withdrawn the needle was deflected in the opposite direction. These effects
were not great; but by introducing and withdrawing the magnet, so that the
impulse each time should be added to those previously communicated to the
needle, the latter could be made to vibrate through an arc of 180° or more.

40. In this experiment the magnet must not be passed entirely through the
helix, for then a second action occurs. When the magnet is introduced, the
needle at the galvanometer is deflected in a certain direction; but being
in, whether it be pushed quite through or withdrawn, the needle is
deflected in a direction the reverse of that previously produced. When the
magnet is passed in and through at one continuous motion, the needle moves
one way, is then suddenly stopped, and finally moves the other way.

41. If such a hollow helix as that described (34.) be laid east and west
(or in any other constant position), and a magnet be retained east and
west, its marked pole always being one way; then whichever end of the helix
the magnet goes in at, and consequently whichever pole of the magnet enters
first, still the needle is deflected the same way: on the other hand,
whichever direction is followed in withdrawing the magnet, the deflection
is constant, but contrary to that due to its entrance.

42. These effects are simple consequences of the _law_ hereafter to be
described (114).

43. When the eight elementary helices were made one long helix, the effect
was not so great as in the arrangement described. When only one of the
eight helices was used, the effect was also much diminished. All care was
taken to guard against tiny direct action of the inducing magnet upon the
galvanometer, and it was found that by moving the magnet in the same
direction, and to the same degree on the outside of the helix, no effect on
the needle was produced.

44. The Royal Society are in possession of a large compound magnet formerly
belonging to Dr. Gowin Knight, which, by permission of the President and
Council, I was allowed to use in the prosecution of these experiments: it
is at present in the charge of Mr. Christie, at his house at Woolwich,
where, by Mr. Christie's kindness, I was at liberty to work; and I have to
acknowledge my obligations to him for his assistance in all the experiments
and observations made with it. This magnet is composed of about 450 bar
magnets, each fifteen inches long, one inch wide, and half an inch thick,
arranged in a box so as to present at one of its extremities two external
poles (fig. 5.). These poles projected horizontally six inches from the
box, were each twelve inches high and three inches wide. They were nine
inches apart; and when a soft iron cylinder, three quarters of an inch in
diameter and twelve inches long, was put across from one to the other, it
required a force of nearly one hundred pounds to break the contact. The
pole to the left in the figure is the marked pole[A].

  [A] To avoid any confusion as to the poles of the magnet, I shall
  designate the pole pointing to the north as the marked pole; I may
  occasionally speak of the north and south ends of the needle, but do
  not mean thereby north and south poles. That is by many considered the
  true north pole of a needle which points to the south; but in this
  country it in often called the south pole.

45. The indicating galvanometer, in all experiments made with this magnet,
was about eight feet from it, not directly in front of the poles, but about
16° or 17° on one side. It was found that on making or breaking the
connexion of the poles by soft iron, the instrument was slightly affected;
but all error of observation arising from this cause was easily and
carefully avoided.

46. The electrical effects exhibited by this magnet were very striking.
When a soft iron cylinder thirteen inches long was put through the compound
hollow helix, with its ends arranged as two general terminations (39.),
these connected with the galvanometer, and the iron cylinder brought in
contact with the two poles of the magnet (fig. 5.), so powerful a rush of
electricity took place that the needle whirled round many times in
succession[A].

  [A] A soft iron bar in the form of a lifter to a horse-shoe magnet,
  when supplied with a coil of this kind round the middle of it,
  becomes, by juxta-position with a magnet, a ready source of a brief
  but determinate current of electricity.

47. Notwithstanding this great power, if the contact was continued, the
needle resumed its natural position, being entirely uninfluenced by the
position of the helix (30.). But on breaking the magnetic contact, the
needle was whirled round in the opposite direction with a force equal to
the former.

48. A piece of copper plate wrapped _once_ round the iron cylinder like a
socket, but with interposed paper to prevent contact, had its edges
connected with the wires of the galvanometer. When the iron was brought in
contact with the poles the galvanometer was strongly affected.

49. Dismissing the helices and sockets, the galvanometer wire was passed
over, and consequently only half round the iron cylinder (fig. 6.); but
even then a strong effect upon the needle was exhibited, when the magnetic
contact was made or broken.

50. As the helix with its iron cylinder was brought towards the magnetic
poles, but _without making contact_, still powerful effects were produced.
When the helix, without the iron cylinder, and consequently containing no
metal but copper, was approached to, or placed between the poles (44.), the
needle was thrown 80°, 90°, or more, from its natural position. The
inductive force was of course greater, the nearer the helix, either with or
without its iron cylinder, was brought to the poles; but otherwise the same
effects were produced, whether the helix, &c. was or was not brought into
contact with the magnet; i.e. no permanent effect on the galvanometer was
produced; and the effects of approximation and removal were the reverse of
each other (30.).

51. When a bolt of copper corresponding to the iron cylinder was
introduced, no greater effect was produced by the helix than without it.
But when a thick iron wire was substituted, the magneto-electric induction
was rendered sensibly greater.

52. The direction of the electric current produced in all these experiments
with the helix, was the same as that already described (38.) as obtained
with the weaker bar magnets.

53. A spiral containing fourteen feet of copper wire, being connected with
the galvanometer, and approximated directly towards the marked pole in the
line of its axis, affected the instrument strongly; the current induced in
it was in the reverse direction to the current theoretically considered by
M. Ampère as existing in the magnet (38.), or as the current in an
electro-magnet of similar polarity. As the spiral was withdrawn, the
induced current was reversed.

54. A similar spiral had the current of eighty pairs of 4-inch plates sent
through it so as to form an electro-magnet, and then the other spiral
connected with the galvanometer (58.) approximated to it; the needle
vibrated, indicating a current in the galvanometer spiral the reverse of
that in the battery spiral (18. 26.). On withdrawing the latter spiral, the
needle passed in the opposite direction.

55. Single wires, approximated in certain directions towards the magnetic
pole, had currents induced in them. On their removal, the currents were
inverted. In such experiments the wires should not be removed in directions
different to those in which they were approximated; for then occasionally
complicated and irregular effects are produced, the causes of which will be
very evident in the fourth part of this paper.

56. All attempts to obtain chemical effects by the induced current of
electricity failed, though the precautions before described (22.), and all
others that could be thought of, were employed. Neither was any sensation
on the tongue, or any convulsive effect upon the limbs of a frog, produced.
Nor could charcoal or fine wire be ignited (133.). But upon repeating the
experiments more at leisure at the Royal Institution, with an armed
loadstone belonging to Professor Daniell and capable of lifting about
thirty pounds, a frog was very _powerfully convulsed_ each time magnetic
contact was made. At first the convulsions could not be obtained on
breaking magnetic contact; but conceiving the deficiency of effect was
because of the comparative slowness of separation, the latter act was
effected by a blow, and then the frog was convulsed strongly. The more
instantaneous the union or disunion is effected, the more powerful the
convulsion. I thought also I could perceive the _sensation_ upon the tongue
and the _flash_ before the eyes; but I could obtain no evidence of chemical
decomposition.

57. The various experiments of this section prove, I think, most completely
the production of electricity from ordinary magnetism. That its intensity
should be very feeble and quantity small, cannot be considered wonderful,
when it is remembered that like thermo-electricity it is evolved entirely
within the substance of metals retaining all their conducting power. But an
agent which is conducted along metallic wires in the manner described;
which whilst so passing possesses the peculiar magnetic actions and force
of a current of electricity; which can agitate and convulse the limbs of a
frog; and which, finally, can produce a spark[A] by its discharge through
charcoal (32.), can only be electricity. As all the effects can be produced
by ferruginous electro-magnets (34.), there is no doubt that arrangements
like the magnets of Professors Moll, Henry, Ten Eyke, and others, in which
as many as two thousand pounds have been lifted, may be used for these
experiments; in which case not only a brighter spark may be obtained, but
wires also ignited, and, as the current can pass liquids (23.), chemical
action be produced. These effects are still more likely to be obtained when
the magneto-electric arrangements to be explained in the fourth section are
excited by the powers of such apparatus.

  [A] For a mode of obtaining the spark from the common magnet which I
  have found effectual, see the Philosophical Magazine for June 1832, p.
  5. In the same Journal for November 1834, vol. v. p. 349, will be
  found a method of obtaining the magneto-electric spark, still simpler
  in its principle, the use of soft iron being dispensed with
  altogether.--_Dec. 1838._

58. The similarity of action, almost amounting to identity, between common
magnets and either electro-magnets or volta-electric currents, is
strikingly in accordance with and confirmatory of M. Ampère's theory, and
furnishes powerful reasons for believing that the action is the same in
both cases; but, as a distinction in language is still necessary, I propose
to call the agency thus exerted by ordinary magnets, _magneto-electric_ or
_magnelectric_ induction (26).

59. The only difference which powerfully strikes the attention as existing
between volta-electric and magneto-electric induction, is the suddenness of
the former, and the sensible time required by the latter; but even in this
early state of investigation there are circumstances which seem to
indicate, that upon further inquiry this difference will, as a
philosophical distinction, disappear (68).[A]

  [A] For important additional phenomena and developments of the
  induction of electrical currents, see now the ninth series,
  1048-1118.--_Dec. 1838._


§ 3. _New Electrical State or Condition of Matter._[A]

  [A] This section having been read at the Royal Society and reported
  upon, and having also, in consequence of a letter from myself to M.
  Hachette, been noticed at the French Institute, I feel bound to let it
  stand as part of the paper; but later investigations (intimated 73.
  76. 77.) of the laws governing those phenomena, induce me to think
  that the latter can be fully explained without admitting the
  electro-tonic state. My views on this point will appear in the second
  series of these researches.--M.F.


60. Whilst the wire is subject to either volta-electric or magneto-electric
induction, it appears to be in a peculiar state; for it resists the
formation of an electrical current in it, whereas, if in its common
condition, such a current would be produced; and when left uninfluenced it
has the power of originating a current, a power which the wire does not
possess under common circumstances. This electrical condition of matter has
not hitherto been recognised, but it probably exerts a very important
influence in many if not most of the phenomena produced by currents of
electricity. For reasons which will immediately appear (71.), I have, after
advising with several learned friends, ventured to designate it as the
_electro-ionic_ state.

61. This peculiar condition shows no known electrical effects whilst it
continues; nor have I yet been able to discover any peculiar powers
exerted, or properties possessed, by matter whilst retained in this state.

62. It shows no reaction by attractive or repulsive powers. The various
experiments which have been made with powerful magnets upon such metals, as
copper, silver, and generally those substances not magnetic, prove this
point; for the substances experimented upon, if electrical conductors, must
have acquired this state; and yet no evidence of attractive or repulsive
powers has been observed. I have placed copper and silver discs, very
delicately suspended on torsion balances in vacuo near to the poles of very
powerful magnets, yet have not been able to observe the least attractive or
repulsive force.

63. I have also arranged a fine slip of gold-leaf very near to a bar of
copper, the two being in metallic contact by mercury at their extremities.
These have been placed in vacuo, so that metal rods connected with the
extremities of the arrangement should pass through the sides of the vessel
into the air. I have then moved powerful magnetic poles, about this
arrangement, in various directions, the metallic circuit on the outside
being sometimes completed by wires, and sometimes broken. But I never could
obtain any sensible motion of the gold-leaf, either directed to the magnet
or towards the collateral bar of copper, which must have been, as far as
induction was concerned, in a similar state to itself.

64. In some cases it has been supposed that, under such circumstances,
attractive and repulsive forces have been exhibited, i.e. that such bodies
have become slightly magnetic. But the phenomena now described, in
conjunction with the confidence we may reasonably repose in M. Ampère's
theory of magnetism, tend to throw doubt on such cases; for if magnetism
depend upon the attraction of electrical currents, and if the powerful
currents at first excited, both by volta-electric and magneto-electric
induction, instantly and naturally cease (12. 28. 47.), causing at the same
time an entire cessation of magnetic effects at the galvanometer needle,
then there can be little or no expectation that any substances not
partaking of the peculiar relation in which iron, nickel, and one or two
other bodies, stand, should exhibit magneto-attractive powers. It seems far
more probable, that the extremely feeble permanent effects observed have
been due to traces of iron, or perhaps some other unrecognised cause not
magnetic.

65. This peculiar condition exerts no retarding or accelerating power upon
electrical currents passing through metal thus circumstanced (20. 33.).
Neither could any such power upon the inducing current itself be detected;
for when masses of metal, wires, helices, &c. were arranged in all possible
ways by the side of a wire or helix, carrying a current measured by the
galvanometer (20.), not the slightest permanent change in the indication of
the instrument could be perceived. Metal in the supposed peculiar state,
therefore, conducts electricity in all directions with its ordinary
facility, or, in other words, its conducting power is not sensibly altered
by it.

66. All metals take on the peculiar state. This is proved in the preceding
experiments with copper and iron (9.), and with gold, silver, tin, lead,
zinc, antimony, bismuth, mercury, &c. by experiments to be described in the
fourth part (132.), admitting of easy application. With regard to iron, the
experiments prove the thorough and remarkable independence of these
phenomena of induction, and the ordinary magnetical appearances of that
metal.

67. This state is altogether the effect of the induction exerted, and
ceases as soon as the inductive force is removed. It is the same state,
whether produced by the collateral passage of voltaic currents (26.), or
the formation of a magnet (34. 36.), or the mere approximation of a magnet
(39. 50.); and is a strong proof in addition to those advanced by M.
Ampère, of the identity of the agents concerned in these several
operations. It probably occurs, momentarily, during the passage of the
common electric spark (24.), and may perhaps be obtained hereafter in bad
conductors by weak electrical currents or other means (74. 76).

68. The state appears to be instantly assumed (12.), requiring hardly a
sensible portion of time for that purpose. The _difference_ of time between
volta-electric and magneto-electric induction, rendered evident by the
galvanometer (59.), may probably be thus explained. When a voltaic current
is sent through one of two parallel wires, as those of the hollow helix
(34.), a current is produced in the other wire, as brief in its continuance
as the time required for a single action of this kind, and which, by
experiment, is found to be inappreciably small. The action will seem still
more instantaneous, because, as there is an accumulation of power in the
poles of the battery before contact, the first rush of electricity in the
wire of communication is greater than that sustained after the contact is
completed; the wire of induction becomes at the moment electro-tonic to an
equivalent degree, which the moment after sinks to the state in which the
continuous current can sustain it, but in sinking, causes an opposite
induced current to that at first produced. The consequence is, that the
first induced wave of electricity more resembles that from the discharge of
an electric jar, than it otherwise would do.

69. But when the iron cylinder is put into the same helix (31.), previous
to the connexion being made with the battery, then the current from the
latter may be considered as active in inducing innumerable currents of a
similar kind to itself in the iron, rendering it a magnet. This is known by
experiment to occupy time; for a magnet so formed, even of soft iron, does
not rise to its fullest intensity in an instant, and it may be because the
currents within the iron are successive in their formation or arrangement.
But as the magnet can induce, as well as the battery current, the combined
action of the two continues to evolve induced electricity, until their
joint effect is at a maximum, and thus the existence of the deflecting
force is prolonged sufficiently to overcome the inertia of the galvanometer
needle.

70. In all those cases where the helices or wires are advanced towards or
taken from the magnet (50. 55.), the direct or inverted current of induced
electricity continues for the time occupied in the advance or recession;
for the electro-tonic state is rising to a higher or falling to a lower
degree during that time, and the change is accompanied by its corresponding
evolution of electricity; but these form no objections to the opinion that
the electro-tonic state is instantly assumed.

71. This peculiar state appears to be a state of tension, and may be
considered as _equivalent_ to a current of electricity, at least equal to
that produced either when the condition is induced or destroyed. The
current evolved, however, first or last, is not to be considered a measure
of the degree of tension to which the electro-tonic state has risen; for as
the metal retains its conducting powers unimpaired (65.), and as the
electricity evolved is but for a moment, (the peculiar state being
instantly assumed and lost (68.),) the electricity which may be led away by
long wire conductors, offering obstruction in their substance proportionate
to their small lateral and extensive linear dimensions, can be but a very
small portion of that really evolved within the mass at the moment it
assumes this condition. Insulated helices and portions of metal instantly
assumed the state; and no traces of electricity could be discovered in
them, however quickly the contact with the electrometer was made, after
they were put under induction, either by the current from the battery or
the magnet. A single drop of water or a small piece of moistened paper (23.
56.) was obstacle sufficient to stop the current through the conductors,
the electricity evolved returning to a state of equilibrium through the
metal itself, and consequently in an unobserved manner.

72. The tension of this state may therefore be comparatively very great.
But whether great or small, it is hardly conceivable that it should exist
without exerting a reaction upon the original inducing current, and
producing equilibrium of some kind. It might be anticipated that this would
give rise to a retardation of the original current; but I have not been
able to ascertain that this is the case. Neither have I in any other way as
yet been able to distinguish effects attributable to such a reaction.

73. All the results favour the notion that the electro-tonic state relates
to the particles, and not to the mass, of the wire or substance under
induction, being in that respect different to the induction exerted by
electricity of tension. If so, the state may be assumed in liquids when no
electrical current is sensible, and even in non-conductors; the current
itself, when it occurs, being as it were a contingency due to the existence
of conducting power, and the momentary propulsive force exerted by the
particles during their arrangement. Even when conducting power is equal,
the currents of electricity, which as yet are the only indicators of this
state, may be unequal, because of differences as to numbers, size,
electrical condition, &c. &c. in the particles themselves. It will only be
after the laws which govern this new state are ascertained, that we shall
be able to predict what is the true condition of, and what are the
electrical results obtainable from, any particular substance.

74. The current of electricity which induces the electro-tonic state in a
neighbouring wire, probably induces that state also in its own wire; for
when by a current in one wire a collateral wire is made electro-tonic, the
latter state is not rendered any way incompatible or interfering with a
current of electricity passing through it (62.). If, therefore, the current
were sent through the second wire instead of the first, it does not seem
probable that its inducing action upon the second would be less, but on the
contrary more, because the distance between the agent and the matter acted
upon would be very greatly diminished. A copper bolt had its extremities
connected with a galvanometer, and then the poles of a battery of one
hundred pairs of plates connected with the bolt, so as to send the current
through it; the voltaic circuit was then suddenly broken, and the
galvanometer observed for any indications of a return current through the
copper bolt due to the discharge of its supposed electro-tonic state. No
effect of the kind was obtained, nor indeed, for two reasons, ought it to
be expected; for first, as the cessation of induction and the discharge of
the electro-tonic condition are simultaneous, and not successive, the
return current would only be equivalent to the neutralization of the last
portion of the inducing current, and would not therefore show any
alteration of direction; or assuming that time did intervene, and that the
latter current was really distinct from the former, its short, sudden
character (12. 26.) would prevent it from being thus recognised.

75. No difficulty arises, I think, in considering the wire thus rendered
electro-tonic by its own current more than by any external current,
especially when the apparent non-interference of that state with currents
is considered (62. 71.). The simultaneous existence of the conducting and
electro-tonic states finds an analogy in the manner in which electrical
currents can be passed through magnets, where it is found that both the
currents passed, and those of the magnets, preserve all their properties
distinct from each other, and exert their mutual actions.

76. The reason given with regard to metals extends also to fluids and all
other conductors, and leads to the conclusion that when electric currents
are passed through them they also assume the electro-tonic state. Should
that prove to be the case, its influence in voltaic decomposition, and the
transference of the elements to the poles, can hardly be doubted. In the
electro-tonic state the homogeneous particles of matter appear to have
assumed a regular but forced electrical arrangement in the direction of the
current, which if the matter be undecomposable, produces, when relieved, a
return current; but in decomposable matter this forced state may be
sufficient to make an elementary particle leave its companion, with which
it is in a constrained condition, and associate with the neighbouring
similar particle, in relation to which it is in a more natural condition,
the forced electrical arrangement being itself discharged or relieved, at
the same time, as effectually as if it had been freed from induction. But
as the original voltaic current is continued, the electro-tonic state may
be instantly renewed, producing the forced arrangement of the compound
particles, to be as instantly discharged by a transference of the
elementary particles of the opposite kind in opposite directions, but
parallel to the current. Even the differences between common and voltaic
electricity, when applied to effect chemical decomposition, which Dr.
Wollaston has pointed out[A], seem explicable by the circumstances
connected with the induction of electricity from these two sources (25.).
But as I have reserved this branch of the inquiry, that I might follow out
the investigations contained in the present paper, I refrain (though much
tempted) from offering further speculations.

  [A] Philosophical Transactions, 1801, p. 247.

77. Marianini has discovered and described a peculiar affection of the
surfaces of metallic discs, when, being in contact with humid conductors, a
current of electricity is passed through them; they are then capable of
producing a reverse current of electricity, and Marianini has well applied
the effect in explanation of the phenomena of Ritter's piles[A]. M.A. de la
Rive has described a peculiar property acquired by metallic conductors,
when being immersed in a liquid as poles, they have completed, for some
time, the voltaic circuit, in consequence of which, when separated from the
battery and plunged into the same fluid, they by themselves produce an
electric current[B]. M.A. Van Beek has detailed cases in which the
electrical relation of one metal in contact with another has been preserved
after separation, and accompanied by its corresponding chemical effects[C].
These states and results appear to differ from the electro-tonic state and
its phenomena; but the true relation of the former to the latter can only
be decided when our knowledge of all these phenomena has been enlarged.

  [A] Annales de Chimie, xxxviii. 5.

  [B] Ibid. xxviii. 190.

  [C] Ibid. xxxviii. 49.

78. I had occasion in the commencement of this paper (2.) to refer to an
experiment by Ampère, as one of those dependent upon the electrical
induction of currents made prior to the present investigation, and have
arrived at conclusions which seem to imply doubts of the accuracy of the
experiment (62. &c.); it is therefore due to M. Ampère that I should attend
to it more distinctly. When a disc of copper (says M. Ampère) was suspended
by a silk thread and surrounded by a helix or spiral, and when the charge
of a powerful voltaic battery was sent through the spiral, a strong magnet
at the same time being presented to the copper disc, the latter turned at
the moment to take a position of equilibrium, exactly as the spiral itself
would have turned had it been free to move. I have not been able to obtain
this effect, nor indeed any motion; but the cause of my failure in the
_latter_ point may be due to the momentary existence of the current not
allowing time for the inertia of the plate to be overcome (11. 12.). M.
Ampère has perhaps succeeded in obtaining motion from the superior delicacy
and power of his electro-magnetical apparatus, or he may have obtained only
the motion due to cessation of action. But all my results tend to invert
the sense of the proposition stated by M. Ampère, "that a current of
electricity tends to put the electricity of conductors near which it passes
in motion in the same direction," for they indicate an opposite direction
for the produced current (26. 53.); and they show that the effect is
momentary, and that it is also produced by magnetic induction, and that
certain other extraordinary effects follow thereupon.

79. The momentary existence of the phenomena of induction now described is
sufficient to furnish abundant reasons for the uncertainty or failure of
the experiments, hitherto made to obtain electricity from magnets, or to
effect chemical decomposition or arrangement by their means[A].

  [A] The Lycée, No. 36, for January 1st, has a long and rather
  premature article, in which it endeavours to show anticipations by
  French philosophers of my researches. It however mistakes the
  erroneous results of MM. Fresnel and Ampère for true ones, and then
  imagines my true results are like those erroneous ones. I notice it
  here, however, for the purpose of doing honour to Fresnel in a much
  higher degree than would have been merited by a feeble anticipation of
  the present investigations. That great philosopher, at the same time
  with myself and fifty other persons, made experiments which the
  present paper proves could give no expected result. He was deceived
  for the moment, and published his imaginary success; but on more
  carefully repeating his trials, he could find no proof of their
  accuracy; and, in the high and pure philosophic desire to remove error
  as well as discover truth, he recanted his first statement. The
  example of Berzelius regarding the first Thorina is another instance
  of this fine feeling; and as occasions are not rare, it would be to
  the dignity of science if such examples were more frequently
  followed.--February 10th, 1832.

80. It also appears capable of explaining fully the remarkable phenomena
observed by M. Arago between metals and magnets when neither are moving
(120.), as well as most of the results obtained by Sir John Herschel,
Messrs. Babbage, Harris, and others, in repeating his experiments;
accounting at the same time perfectly for what at first appeared
inexplicable; namely, the non-action of the same metals and magnets when at
rest. These results, which also afford the readiest means of obtaining
electricity from magnetism, I shall now proceed to describe.


§ 4. _Explication of Arago's Magnetic Phenomena._


81. If a plate of copper be revolved close to a magnetic needle, or magnet,
suspended in such a way that the latter may rotate in a plane parallel to
that of the former, the magnet tends to follow the motion of the plate; or
if the magnet be revolved, the plate tends to follow its motion; and the
effect is so powerful, that magnets or plates of many pounds weight may be
thus carried round. If the magnet and plate be at rest relative to each
other, not the slightest effect, attractive or repulsive, or of any kind,
can be observed between them (62.). This is the phenomenon discovered by M.
Arago; and he states that the effect takes place not only with all metals,
but with solids, liquids, and even gases, i.e. with all substances (130.).

82. Mr. Babbage and Sir John Herschel, on conjointly repeating the
experiments in this country[A], could obtain the effects only with the
metals, and with carbon in a peculiar state (from gas retorts), i.e. only
with excellent conductors of electricity. They refer the effect to
magnetism induced in the plate by the magnet; the pole of the latter
causing an opposite pole in the nearest part of the plate, and round this a
more diffuse polarity of its own kind (120.). The essential circumstance in
producing the rotation of the suspended magnet is, that the substance
revolving below it shall acquire and lose its magnetism in sensible time,
and not instantly (124.). This theory refers the effect to an attractive
force, and is not agreed to by the discoverer, M. Arago, nor by M. Ampère,
who quote against it the absence of all attraction when the magnet and
metal are at rest (62. 126.), although the induced magnetism should still
remain; and who, from experiments made with a long dipping needle, conceive
the action to be always repulsive (125.).

  [A] Philosophical Transactions, 1825, p. 467.

83. Upon obtaining electricity from magnets by the means already described
(36 46.), I hoped to make the experiment of M. Arago a new source of
electricity; and did not despair, by reference to terrestrial
magneto-electric induction, of being able to construct a new electrical
machine. Thus stimulated, numerous experiments were made with the magnet of
the Royal Society at Mr. Christie's house, in all of which I had the
advantage of his assistance. As many of these were in the course of the
superseded by more perfect arrangements, I shall consider myself at liberty
investigation to rearrange them in a manner calculated to convey most
readily what appears to me to be a correct view of the nature of the
phenomena.

84. The magnet has been already described (44.). To concentrate the poles,
and bring them nearer to each other, two iron or steel bars, each about six
or seven inches long, one inch wide, and half an inch thick, were put
across the poles as in fig. 7, and being supported by twine from slipping,
could be placed as near to or far from each other as was required.
Occasionally two bars of soft iron were employed, so bent that when
applied, one to each pole, the two smaller resulting poles were vertically
over each other, either being uppermost at pleasure.

85. A disc of copper, twelve inches in diameter, and about one fifth of an
inch in thickness, fixed upon a brass axis, was mounted in frames so as to
allow of revolution either vertically or horizontally, its edge being at
the same time introduced more or less between the magnetic poles (fig. 7.).
The edge of the plate was well amalgamated for the purpose of obtaining a
good but moveable contact, and a part round the axis was also prepared in a
similar manner.

86. Conductors or electric collectors of copper and lead were constructed
so as to come in contact with the edge of the copper disc (85.), or with
other forms of plates hereafter to be described (101.). These conductors
were about four inches long, one third of an inch wide, and one fifth of an
inch thick; one end of each was slightly grooved, to allow of more exact
adaptation to the somewhat convex edge of the plates, and then amalgamated.
Copper wires, one sixteenth of an inch in thickness, attached, in the
ordinary manner, by convolutions to the other ends of these conductors,
passed away to the galvanometer.

87. The galvanometer was roughly made, yet sufficiently delicate in its
indications. The wire was of copper covered with silk, and made sixteen or
eighteen convolutions. Two sewing-needles were magnetized and fixed on to a
stem of dried grass parallel to each other, but in opposite directions, and
about half an inch apart; this system was suspended by a fibre of unspun
silk, so that the lower needle should be between the convolutions of the
multiplier, and the upper above them. The latter was by much the most
powerful magnet, and gave terrestrial direction to the whole; fig. 8.
represents the direction of the wire and of the needles when the instrument
was placed in the magnetic meridian: the ends of the wires are marked A and
B for convenient reference hereafter. The letters S and N designate the
south and north ends of the needle when affected merely by terrestrial
magnetism; the end N is therefore the marked pole (44.). The whole
instrument was protected by a glass jar, and stood, as to position and
distance relative to the large magnet, under the same circumstances as
before (45.).

88. All these arrangements being made, the copper disc was adjusted as in
fig. 7, the small magnetic poles being about half an inch apart, and the
edge of the plate inserted about half their width between them. One of the
galvanometer wires was passed twice or thrice loosely round the brass axis
of the plate, and the other attached to a conductor (86.), which itself was
retained by the hand in contact with the amalgamated edge of the disc at
the part immediately between the magnetic poles. Under these circumstances
all was quiescent, and the galvanometer exhibited no effect. But the
instant the plate moved, the galvanometer was influenced, and by revolving
the plate quickly the needle could be deflected 90° or more.

89. It was difficult under the circumstances to make the contact between
the conductor and the edge of the revolving disc uniformly good and
extensive; it was also difficult in the first experiments to obtain a
regular velocity of rotation: both these causes tended to retain the needle
in a continual state of vibration; but no difficulty existed in
ascertaining to which side it was deflected, or generally, about what line
it vibrated. Afterwards, when the experiments were made more carefully, a
permanent deflection of the needle of nearly 45° could be sustained.

90. Here therefore was demonstrated the production of a permanent current
of electricity by ordinary magnets (57.).

91. When the motion of the disc was reversed, every other circumstance
remaining the same, the galvanometer needle was deflected with equal power
as before; but the deflection was on the opposite side, and the current of
electricity evolved, therefore, the reverse of the former.

92. When the conductor was placed on the edge of the disc a little to the
right or left, as in the dotted positions fig. 9, the current of
electricity was still evolved, and in the same direction as at first (88.
91.). This occurred to a considerable distance, i.e. 50° or 60° on each
side of the place of the magnetic poles. The current gathered by the
conductor and conveyed to the galvanometer was of the same kind on both
sides of the place of greatest intensity, but gradually diminished in force
from that place. It appeared to be equally powerful at equal distances from
the place of the magnetic poles, not being affected in that respect by the
direction of the rotation. When the rotation of the disc was reversed, the
direction of the current of electricity was reversed also; but the other
circumstances were not affected.

93. On raising the plate, so that the magnetic poles were entirely hidden
from each other by its intervention, (a. fig. 10,) the same effects were
produced in the same order, and with equal intensity as before. On raising
it still higher, so as to bring the place of the poles to c, still the
effects were produced, and apparently with as much power as at first.

94. When the conductor was held against the edge as if fixed to it, and
with it moved between the poles, even though but for a few degrees, the
galvanometer needle moved and indicated a current of electricity, the same
as that which would have been produced if the wheel had revolved in the
same direction, the conductor remaining stationary.

95. When the galvanometer connexion with the axis was broken, and its wires
made fast to two conductors, both applied to the edge of the copper disc,
then currents of electricity were produced, presenting more complicated
appearances, but in perfect harmony with the above results. Thus, if
applied as in fig. 11, a current of electricity through the galvanometer
was produced; but if their place was a little shifted, as in fig. 12, a
current in the contrary direction resulted; the fact being, that in the
first instance the galvanometer indicated the difference between a strong
current through A and a weak one through B, and in the second, of a weak
current through A and a strong one through B (92.), and therefore produced
opposite deflections.

96. So also when the two conductors were equidistant from the magnetic
poles, as in fig. 13, no current at the galvanometer was perceived,
whichever way the disc was rotated, beyond what was momentarily produced by
irregularity of contact; because equal currents in the same direction
tended to pass into both. But when the two conductors were connected with
one wire, and the axis with the other wire, (fig. 14,) then the
galvanometer showed a current according with the direction of rotation
(91.); both conductors now acting consentaneously, and as a single
conductor did before (88.).

97. All these effects could be obtained when only one of the poles of the
magnet was brought near to the plate; they were of the same kind as to
direction, &c., but by no means so powerful.

98. All care was taken to render these results independent of the earth's
magnetism, or of the mutual magnetism of the magnet and galvanometer
needles. The contacts were made in the magnetic equator of the plate, and
at other parts; the plate was placed horizontally, and the poles
vertically; and other precautions were taken. But the absence of any
interference of the kind referred to, was readily shown by the want of all
effect when the disc was removed from the poles, or the poles from the
disc; every other circumstance remaining the same.

99. The _relation of the current_ of electricity produced, to the magnetic
pole, to the direction of rotation of the plate, &c. &c., may be expressed
by saying, that when the unmarked pole (44. 84.) is beneath the edge of the
plate, and the latter revolves horizontally, screw-fashion, the electricity
which can be collected at the edge of the plate nearest to the pole is
positive. As the pole of the earth may mentally be considered the unmarked
pole, this relation of the rotation, the pole, and the electricity evolved,
is not difficult to remember. Or if, in fig. 15, the circle represent the
copper disc revolving in the direction of the arrows, and _a_ the outline
of the unmarked pole placed beneath the plate, then the electricity
collected at _b_ and the neighbouring parts is positive, whilst that
collected at the centre _c_ and other parts is negative (88.). The currents
in the plate are therefore from the centre by the magnetic poles towards
the circumference.

100. If the marked pole be placed above, all other things remaining the
same, the electricity at _b_, fig. 15, is still positive. If the marked
pole be placed below, or the unmarked pole above, the electricity is
reversed. If the direction of revolution in any case is reversed, the
electricity is also reversed.

101. It is now evident that the rotating plate is merely another form of
the simpler experiment of passing a piece of metal between the magnetic
poles in a rectilinear direction, and that in such cases currents of
electricity are produced at right angles to the direction of the motion,
and crossing it at the place of the magnetic pole or poles. This was
sufficiently shown by the following simple experiment: A piece of copper
plate one fifth of an inch thick, one inch and a half wide, and twelve
inches long, being amalgamated at the edges, was placed between the
magnetic poles, whilst the two conductors from the galvanometer were held
in contact with its edges; it was then drawn through between the poles of
the conductors in the direction of the arrow, fig. 16; immediately the
galvanometer needle was deflected, its north or marked end passed eastward,
indicating that the wire A received negative and the wire B positive
electricity; and as the marked pole was above, the result is in perfect
accordance with the effect obtained by the rotatory plate (99.).

102. On reversing the motion of the plate, the needle at the galvanometer
was deflected in the opposite direction, showing an opposite current.

103. To render evident the character of the electrical current existing in
various parts of the moving copper plate, differing in their relation to
the inducing poles, one collector (86.) only was applied at the part to be
examined near to the pole, the other being connected with the end of the
plate as the most neutral place: the results are given at fig. 17-20, the
marked pole being above the plate. In fig. 17, B received positive
electricity; but the plate moving in the same direction, it received on the
opposite side, fig. 18, negative electricity: reversing the motion of the
latter, as in fig. 20, B received positive electricity; or reversing the
motion of the first arrangement, that of fig. 17 to fig. 19, B received
negative electricity.

104. When the plates were previously removed sideways from between the
magnets, as in fig. 21, so as to be quite out of the polar axis, still the
same effects were produced, though not so strongly.

105. When the magnetic poles were in contact, and the copper plate was
drawn between the conductors near to the place, there was but very little
effect produced. When the poles were opened by the width of a card, the
effect was somewhat more, but still very small.

106. When an amalgamated copper wire, one eighth of an inch thick, was
drawn through between the conductors and poles (101.), it produced a very
considerable effect, though not so much as the plates.

107. If the conductors were held permanently against any particular parts
of the copper plates, and carried between the magnetic poles with them,
effects the same as those described were produced, in accordance with the
results obtained with the revolving disc (94.).

108. On the conductors being held against the ends of the plates, and the
latter then passed between the magnetic poles, in a direction transverse to
their length, the same effects were produced (fig. 22.). The parts of the
plates towards the end may be considered either as mere conductors, or as
portions of metal in which the electrical current is excited, according to
their distance and the strength of the magnet; but the results were in
perfect harmony with those before obtained. The effect was as strong as
when the conductors were held against the sides of the plate (101.).

109. When a mere wire, connected with the galvanometer so as to form a
complete circuit, was passed through between the poles, the galvanometer
was affected; and upon moving the wire to and fro, so as to make the
alternate impulses produced correspond with the vibrations of the needle,
the latter could be increased to 20° or 30° on each side the magnetic
meridian.

110. Upon connecting the ends of a plate of metal with the galvanometer
wires, and then carrying it between the poles from end to end (as in fig.
23.), in either direction, no effect whatever was produced upon the
galvanometer. But the moment the motion became transverse, the needle was
deflected.

111. These effects were also obtained from _electro-magnetic poles_,
resulting from the use of copper helices or spirals, either alone or with
iron cores (34. 54.). The directions of the motions were precisely the
same; but the action was much greater when the iron cores were used, than
without.

112. When a flat spiral was passed through edgewise between the poles, a
curious action at the galvanometer resulted; the needle first went strongly
one way, but then suddenly stopped, as if it struck against some solid
obstacle, and immediately returned. If the spiral were passed through from
above downwards, or from below upwards, still the motion of the needle was
in the same direction, then suddenly stopped, and then was reversed. But on
turning the spiral half-way round, i.e. edge for edge, then the directions
of the motions were reversed, but still were suddenly interrupted and
inverted as before. This double action depends upon the halves of the
spiral (divided by a line passing through its centre perpendicular to the
direction of its motion) acting in opposite directions; and the reason why
the needle went to the same side, whether the spiral passed by the poles in
the one or the other direction, was the circumstance, that upon changing
the motion, the direction of the wires in the approaching half of the
spiral was changed also. The effects, curious as they appear when
witnessed, are immediately referable to the action of single wires (40.
109.).

113. Although the experiments with the revolving plate, wires, and plates
of metal, were first successfully made with the large magnet belonging to
the Royal Society, yet they were all ultimately repeated with a couple of
bar magnets two feet long, one inch and a half wide, and half an inch
thick; and, by rendering the galvanometer (87.) a little more delicate,
with the most striking results. Ferro-electro-magnets, as those of Moll,
Henry, &c. (57.), are very powerful. It is very essential, when making
experiments on different substances, that thermo-electric effects (produced
by contact of the fingers, &c.) be avoided, or at least appreciated and
accounted for; they are easily distinguished by their permanency, and their
independence of the magnets, or of the direction of the motion.

114. The relation which holds between the magnetic pole, the moving wire or
metal, and the direction of the current evolved, i.e. _the law_ which
governs the evolution of electricity by magneto-electric induction, is very
simple, although rather difficult to express. If in fig. 24, PN represent a
horizontal wire passing by a marked magnetic pole, so that the direction of
its motion shall coincide with the curved line proceeding from below
upwards; or if its motion parallel to itself be in a line tangential to the
curved line, but in the general direction of the arrows; or if it pass the
pole in other directions, but so as to cut the magnetic curves[A] in the
same general direction, or on the same side as they would be cut by the
wire if moving along the dotted curved line;--then the current of
electricity in the wire is from P to N. If it be carried in the reverse
directions, the electric current will be from N to P. Or if the wire be in
the vertical position, figured P' N', and it be carried in similar
directions, coinciding with the dotted horizontal curve so far, as to cut
the magnetic curves on the same side with it, the current will be from P'
to N'. If the wire be considered a tangent to the curved surface of the
cylindrical magnet, and it be carried round that surface into any other
position, or if the magnet itself be revolved on its axis, so as to bring
any part opposite to the tangential wire,--still, if afterwards the wire be
moved in the directions indicated, the current of electricity will be from
P to N; or if it be moved in the opposite direction, from N to P; so that
as regards the motions of the wire past the pole, they may be reduced to
two, directly opposite to each other, one of which produces a current from
P to N, and the other from N to P.

  [A] By magnetic curves, I mean the lines of magnetic forces, however
  modified by the juxtaposition of poles, which would be depicted by
  iron filings; or those to which a very small magnetic needle would
  form a tangent.

115. The same holds true of the unmarked pole of the magnet, except that if
it be substituted for the one in the figure, then, as the wires are moved
in the direction of the arrows, the current of electricity would be from N
to P, and when they move in the reverse direction, from P to N.

116. Hence the current of electricity which is excited in metal when moving
in the neighbourhood of a magnet, depends for its direction altogether upon
the relation of the metal to the resultant of magnetic action, or to the
magnetic curves, and may be expressed in a popular way thus; Let AB (fig.
25.) represent a cylinder magnet, A being the marked pole, and B the
unmarked pole; let PN be a silver knife-blade, resting across the magnet
with its edge upward, and with its marked or notched side towards the pole
A; then in whatever direction or position this knife be moved edge
foremost, either about the marked or the unmarked pole, the current of
electricity produced will be from P to N, provided the intersected curves
proceeding from A abut upon the notched surface of the knife, and those
from B upon the unnotched side. Or if the knife be moved with its back
foremost, the current will be from N to P in every possible position and
direction, provided the intersected curves abut on the same surfaces as
before. A little model is easily constructed, by using a cylinder of wood
for a magnet, a flat piece for the blade, and a piece of thread connecting
one end of the cylinder with the other, and passing through a hole in the
blade, for the magnetic curves: this readily gives the result of any
possible direction.

117. When the wire under induction is passing by an electromagnetic pole,
as for instance one end of a copper helix traversed by the electric current
(34.), the direction of the current in the approaching wire is the same
with that of the current in the parts or sides of the spirals nearest to
it, and in the receding wire the reverse of that in the parts nearest to
it.

118. All these results show that the power of inducing electric currents is
circumferentially exerted by a magnetic resultant or axis of power, just as
circumferential magnetism is dependent upon and is exhibited by an electric
current.

119. The experiments described combine to prove that when a piece of metal
(and the same may be true of all conducting matter (213.) ) is passed
either before a single pole, or between the opposite poles of a magnet, or
near electro-magnetic poles, whether ferruginous or not, electrical
currents are produced across the metal transverse to the direction of
motion; and which therefore, in Arago's experiments, will approximate
towards the direction of radii. If a single wire be moved like the spoke of
a wheel near a magnetic pole, a current of electricity is determined
through it from one end towards the other. If a wheel be imagined,
constructed of a great number of these radii, and this revolved near the
pole, in the manner of the copper disc (85.), each radius will have a
current produced in it as it passes by the pole. If the radii be supposed
to be in contact laterally, a copper disc results, in which the directions
of the currents will be generally the same, being modified only by the
coaction which can take place between the particles, now that they are in
metallic contact.

120. Now that the existence of these currents is known, Arago's phenomena
may be accounted for without considering them as due to the formation in
the copper, of a pole of the opposite kind to that approximated, surrounded
by a diffuse polarity of the same kind (82.); neither is it essential that
the plate should acquire and lose its state in a finite time; nor on the
other hand does it seem necessary that any repulsive force should be
admitted as the cause of the rotation (82.).

121. The effect is precisely of the same kind as the electromagnetic
rotations which I had the good fortune to discover some years ago[A].
According to the experiments then made which have since been abundantly
confirmed, if a wire (PN fig. 26.) be connected with the positive and
negative ends of a voltaic buttery, so that the positive electricity shall
pass from P to N, and a marked magnetic pole N be placed near the wire
between it and the spectator, the pole will move in a direction tangential
to the wire, i.e. towards the right, and the wire will move tangentially
towards the left, according to the directions of the arrows. This is
exactly what takes place in the rotation of a plate beneath a magnetic
pole; for let N (fig. 27.) be a marked pole above the circular plate, the
latter being rotated in the direction of the arrow: immediately currents of
positive electricity set from the central parts in the general direction of
the radii by the pole to the parts of the circumference _a_ on the other
side of that pole (99. 119.), and are therefore exactly in the same
relation to it as the current in the wire (PN, fig. 26.), and therefore the
pole in the same manner moves to the right hand.

  [A] Quarterly Journal of Science, vol. xii. pp. 74. 186. 416. 283.

122. If the rotation of the disc be reversed, the electric currents are
reversed (91.), and the pole therefore moves to the left hand. If the
contrary pole be employed, the effects are the same, i.e. in the same
direction, because currents of electricity, the reverse of those described,
are produced, and by reversing both poles and currents, the visible effects
remain unchanged. In whatever position the axis of the magnet be placed,
provided the same pole be applied to the same side of the plate, the
electric current produced is in the same direction, in consistency with the
law already stated (114, &c.); and thus every circumstance regarding the
direction of the motion may be explained.

123. These currents are _discharged or return_ in the parts of the plate on
each side of and more distant from the place of the pole, where, of course,
the magnetic induction is weaker; and when the collectors are applied, and
a current of electricity is carried away to the galvanometer (88.), the
deflection there is merely a repetition, by the same current or part of it,
of the effect of rotation in the magnet over the plate itself.

124. It is under the point of view just put forth that I have ventured to
say it is not necessary that the plate should acquire and lose its state in
a finite time (120.); for if it were possible for the current to be fully
developed the instant _before_ it arrived at its state of nearest
approximation to the vertical pole of the magnet, instead of opposite to or
a little beyond it, still the relative motion of the pole and plate would
be the same, the resulting force being in fact tangential instead of
direct.

125. But it is possible (though not necessary for the rotation) that _time_
may be required for the development of the maximum current in the plate, in
which case the resultant of all the forces would be in advance of the
magnet when the plate is rotated, or in the rear of the magnet when the
latter is rotated, and many of the effects with pure electro-magnetic poles
tend to prove this is the case. Then, the tangential force may be resolved
into two others, one parallel to the plane of rotation, and the other
perpendicular to it; the former would be the force exerted in making the
plate revolve with the magnet, or the magnet with the plate; the latter
would be a repulsive force, and is probably that, the effects of which M.
Arago has also discovered (82.).

126. The extraordinary circumstance accompanying this action, which has
seemed so inexplicable, namely, the cessation of all phenomena when the
magnet and metal are brought to rest, now receives a full explanation
(82.); for then the electrical currents which cause the motion cease
altogether.

127. All the effects of solution of metallic continuity, and the consequent
diminution of power described by Messrs. Babbage and Herschel[A], now
receive their natural explanation, as well also as the resumption of power
when the cuts were filled up by metallic substances, which, though
conductors of electricity, were themselves very deficient in the power of
influencing magnets. And new modes of cutting the plate may be devised,
which shall almost entirely destroy its power. Thus, if a copper plate
(81.) be cut through at about a fifth or sixth of its diameter from the
edge, so as to separate a ring from it, and this ring be again fastened on,
but with a thickness of paper intervening (fig. 29.), and if Arago's
experiment be made with this compound plate so adjusted that the section
shall continually travel opposite the pole, it is evident that the magnetic
currents will be greatly interfered with, and the plate probably lose much
of its effect[B].

  [A] Philosophical Transactions, 1825, p. 481.

  [B] This experiment has actually been made by Mr. Christie, with the
  results here described, and is recorded in the Philosophical
  Transactions for 1827, p. 82.

An elementary result of this kind was obtained by using two pieces of thick
copper, shaped as in fig. 28. When the two neighbouring edges were
amalgamated and put together, and the arrangement passed between the poles
of the magnet, in the direction parallel to these edges, a current was
urged through the wires attached to the outer angles, and the galvanometer
became strongly affected; but when a single film of paper was interposed,
and the experiment repeated, no sensible effect could be produced.

128. A section of this kind could not interfere much with the induction of
magnetism, supposed to be of the nature ordinarily received by iron.

129. The effect of rotation or deflection of the needle, which M. Arago
obtained by ordinary magnets, M. Ampère succeeded in procuring by
electro-magnets. This is perfectly in harmony with the results relative to
volta-electric and magneto-electric induction described in this paper. And
by using flat spirals of copper wire, through which electric currents were
sent, in place of ordinary magnetic poles (Ill.), sometimes applying a
single one to one side of the rotating plate, and sometimes two to opposite
sides, I obtained the induced currents of electricity from the plate
itself, and could lead them away to, and ascertain their existence by, the
galvanometer.

130. The cause which has now been assigned for the rotation in Arago's
experiment, namely, the production of electrical currents, seems abundantly
sufficient in all cases where the metals, or perhaps even other conductors,
are concerned; but with regard to such bodies as glass, resins, and, above
all, gases, it seems impossible that currents of electricity, capable of
producing these effects, should be generated in them. Yet Arago found that
the effects in question were produced by these and by all bodies tried
(81.). Messrs. Babbage and Herschel, it is true, did not observe them with
any substance not metallic, except carbon, in a highly conducting state
(82.). Mr. Harris has ascertained their occurrence with wood, marble,
freestone and annealed glass, but obtained no effect with sulphuric acid
and saturated solution of sulphate of iron, although these are better
conductors of electricity than the former substances.

131. Future investigations will no doubt explain these difficulties, and
decide the point whether the retarding or dragging action spoken of is
always simultaneous with electric currents.[A] The existence of the action
in metals, only whilst the currents exist, i.e. whilst motion is given (82.
88.), and the explication of the repulsive action observed by M. Arago (82.
125.), are powerful reasons for referring it to this cause; but it may be
combined with others which occasionally act alone.

  [A] Experiments which I have since made convince me that this
  particular action is always due to the electrical currents formed; and
  they supply a test by which it may be distinguished from the action of
  ordinary magnetism, or any other cause, including those which are
  mechanical or irregular, producing similar effects (254.)

132. Copper, iron, tin, zinc, lead, mercury, and all the metals tried,
produced electrical currents when passed between the magnetic poles: the
mercury was put into a glass tube for the purpose. The dense carbon
deposited in coal gas retorts, also produced the current, but ordinary
charcoal did not. Neither could I obtain any sensible effects with brine,
sulphuric acid, saline solutions, &c., whether rotated in basins, or
inclosed in tubes and passed between the poles.

133. I have never been able to produce any sensation upon the tongue by the
wires connected with the conductors applied to the edges of the revolving
plate (88.) or slips of metal (101.). Nor have I been able to heat a fine
platina wire, or produce a spark, or convulse the limbs of a frog. I have
failed also to produce any chemical effects by electricity thus evolved
(22. 56).

134. As the electric current in the revolving copper plate occupies but a
small space, proceeding by the poles and being discharged right and left at
very small distances comparatively (123.); and as it exists in a thick mass
of metal possessing almost the highest conducting power of any, and
consequently offering extraordinary facility for its production and
discharge; and as, notwithstanding this, considerable currents may be drawn
off which can pass through narrow wires, forty, fifty, sixty, or even one
hundred feet long; it is evident that the current existing in the plate
itself must be a very powerful one, when the rotation is rapid and the
magnet strong. This is also abundantly proved by the obedience and
readiness with which a magnet ten or twelve pounds in weight follows the
motion of the plate and will strongly twist up the cord by which it is
suspended.

135. Two rough trials were made with the intention of constructing
_magneto-electric machines_. In one, a ring one inch and a half broad and
twelve inches external diameter, cut from a thick copper plate, was mounted
so as to revolve between the poles of the magnet and represent a plate
similar to those formerly used (101.), but of interminable length; the
inner and outer edges were amalgamated, and the conductors applied one to
each edge, at the place of the magnetic poles. The current of electricity
evolved did not appear by the galvanometer to be stronger, if so strong, as
that from the circular plate (88.).

136. In the other, small thick discs of copper or other metal, half an inch
in diameter, were revolved rapidly near to the poles, but with the axis of
rotation out of the polar axis; the electricity evolved was collected by
conductors applied as before to the edges (86.). Currents were procured,
but of strength much inferior to that produced by the circular plate.

137. The latter experiment is analogous to those made by Mr. Barlow with a
rotating iron shell, subject to the influence of the earth[A]. The effects
obtained by him have been referred by Messrs. Babbage and Herschel to the
same cause as that considered as influential in Arago's experiment[B]; but
it would be interesting to know how far the electric current which might be
produced in the experiment would account for the deflexion of the needle.
The mere inversion of a copper wire six or seven times near the poles of
the magnet, and isochronously with the vibrations of the galvanometer
needle connected with it, was sufficient to make the needle vibrate through
an arc of 60° or 70°. The rotation of a copper shell would perhaps decide
the point, and might even throw light upon the more permanent, though
somewhat analogous effects obtained by Mr. Christie.

  [A] Philosophical Transactions, 1825. p. 317.

  [B] Ibid. 1825. p. 485.

138. The remark which has already been made respecting iron (66.), and the
independence of the ordinary magnetical phenomena of that substance and the
phenomena now described of magneto-electric induction in that and other
metals, was fully confirmed by many results of the kind detailed in this
section. When an iron plate similar to the copper one formerly described
(101.) was passed between the magnetic poles, it gave a current of
electricity like the copper plate, but decidedly of less power; and in the
experiments upon the induction of electric currents (9.), no difference in
the kind of action between iron and other metals could be perceived. The
power therefore of an iron plate to drag a magnet after it, or to intercept
magnetic action, should be carefully distinguished from the similar power
of such metals as silver, copper, &c. &c., inasmuch as in the iron by far
the greater part of the effect is due to what may be called ordinary
magnetic action. There can be no doubt that the cause assigned by Messrs.
Babbage and Herschel in explication of Arago's phenomena is the true one,
when iron is the metal used.

139. The very feeble powers which were found by those philosophers to
belong to bismuth and antimony, when moving, of affecting the suspended
magnet, and which has been confirmed by Mr. Harris, seem at first
disproportionate to their conducting powers; whether it be so or not must
be decided by future experiment (73.)[A]. These metals are highly
crystalline, and probably conduct electricity with different degrees of
facility in different directions; and it is not unlikely that where a mass
is made up of a number of crystals heterogeneously associated, an effect
approaching to that of actual division may occur (127.); or the currents of
electricity may become more suddenly deflected at the confines of similar
crystalline arrangements, and so be more readily and completely discharged
within the mass.

  [A] I have since been able to explain these differences, and prove,
  with several metals, that the effect is in the order of the conducting
  power; for I have been able to obtain, by magneto-electric induction,
  currents of electricity which are proportionate in strength to the
  conducting power of the bodies experimented with (211.).

§. _Royal Institution, November 1831._

_Note._--In consequence of the long period which has intervened between the
reading and printing of the foregoing paper, accounts of the experiments
have been dispersed, and, through a letter of my own to M. Hachette, have
reached France and Italy. That letter was translated (with some errors),
and read to the Academy of Sciences at Paris, 26th December, 1831. A copy
of it in _Le Temps_ of the 28th December quickly reached Signor Nobili,
who, with Signor Antinori, immediately experimented upon the subject, and
obtained many of the results mentioned in my letter; others they could not
obtain or understand, because of the brevity of my account. These results
by Signori Nobili and Antinori have been embodied in a paper dated 31st
January 1832, and printed and published in the number of the _Antologia_
dated November 1831 (according at least to the copy of the paper kindly
sent me by Signor Nobili). It is evident the work could not have been then
printed; and though Signor Nobili, in his paper, has inserted my letter as
the text of his experiments, yet the circumstance of back date has caused
many here, who have heard of Nobili's experiments by report only, to
imagine his results were anterior to, instead of being dependent upon,
mine.

I may be allowed under these circumstances to remark, that I experimented
on this subject several years ago, and have published results. (See
Quarterly Journal of Science for July 1825, p. 338.) The following also is
an extract from my note-book, dated November 28, 1825: "Experiments on
induction by connecting wire of voltaic battery:--a battery of four
troughs, ten pairs of plates, each arranged side by side--the poles
connected by a wire about four feet long, parallel to which was another
similar wire separated from it only by two thicknesses of paper, the ends
of the latter were attached to a galvanometer:--exhibited no action, &c.
&c. &c.--Could not in any way render any induction evident from the
connecting wire." The cause of failure at that time is now evident
(79.).--M.F. April, 1832.




SECOND SERIES.

THE BAKERIAN LECTURE.


§ 5. _Terrestrial Magneto-electric Induction._ § 6. _Force and Direction of
Magneto-electric Induction generally._

Read January 12, 1832.


§ 5. _Terrestrial Magneto-electric Induction._

140. When the general facts described in the former paper were discovered,
and the _law_ of magneto-electric induction relative to direction was
ascertained (114.), it was not difficult to perceive that the earth would
produce the same effect as a magnet, and to an extent that would, perhaps,
render it available in the construction of new electrical machines. The
following are some of the results obtained in pursuance of this view.

141. The hollow helix already described (6.) was connected with a
galvanometer by wires eight feet long; and the soft iron cylinder (34.)
after being heated red-hot and slowly cooled, to remove all traces of
magnetism, was put into the helix so as to project equally at both ends,
and fixed there. The combined helix and bar were held in the magnetic
direction or line of dip, and (the galvanometer needle being motionless)
were then inverted, so that the lower end should become the upper, but the
whole still correspond to the magnetic direction; the needle was
immediately deflected. As the latter returned to its first position, the
helix and bar were again inverted; and by doing this two or three times,
making the inversions and vibrations to coincide, the needle swung through
an arc of 150° or 160°.

142. When one end of the helix, which may be called A, was uppermost at
first (B end consequently being below), then it mattered not in which
direction it proceeded during the inversion, whether to the right hand or
left hand, or through any other course; still the galvanometer needle
passed in the same direction. Again, when B end was uppermost, the
inversion of the helix and bar in any direction always caused the needle to
be deflected one way; that way being the opposite to the course of the
deflection in the former case.

143. When the helix with its iron core in any given position was inverted,
the effect was as if a magnet with its marked pole downwards had been
introduced from above into the inverted helix. Thus, if the end B were
upwards, such a magnet introduced from above would make the marked end of
the galvanometer needle pass west. Or the end B being downwards, and the
soft iron in its place, inversion of the whole produced the same effect.

144. When the soft iron bar was taken out of the helix and inverted in
various directions within four feet of the galvanometer, not the slightest
effect upon it was produced.

145. These phenomena are the necessary consequence of the inductive
magnetic power of the earth, rendering the soft iron cylinder a magnet with
its marked pole downwards. The experiment is analogous to that in which two
bar magnets were used to magnetize the same cylinder in the same helix
(36.), and the inversion of position in the present experiment is
equivalent to a change of the poles in that arrangement. But the result is
not less an instance of the evolution of electricity by means of the
magnetism of the globe.

146. The helix alone was then held permanently in the magnetic direction,
and the soft iron cylinder afterwards introduced; the galvanometer needle
was instantly deflected; by withdrawing the cylinder as the needle
returned, and continuing the two actions simultaneously, the vibrations
soon extended through an arc of 180°. The effect was precisely the same as
that obtained by using a cylinder magnet with its marked pole downwards;
and the direction of motion, &c. was perfectly in accordance with the
results of former experiments obtained with such a magnet (39.). A magnet
in that position being used, gave the same deflections, but stronger. When
the helix was put at right angles to the magnetic direction or dip, then
the introduction or removal of the soft iron cylinder produced no effect at
the needle. Any inclination to the dip gave results of the same kind as
those already described, but increasing in strength as the helix
approximated to the direction of the dip.

147. A cylinder magnet, although it has great power of affecting the
galvanometer when moving into or out of the helix, has no power of
continuing the deflection (39.); and therefore, though left in, still the
magnetic needle comes to its usual place of rest. But upon repeating (with
the magnet) the experiment of inversion in the direction of the dip (141),
the needle was affected as powerfully as before; the disturbance of the
magnetism in the steel magnet, by the earth's inductive force upon it,
being thus shown to be nearly, if not quite, equal in amount and rapidity
to that occurring in soft iron. It is probable that in this way
magneto-electrical arrangements may become very useful in indicating the
disturbance of magnetic forces, where other means will not apply; for it is
not the whole magnetic power which produces the visible effect, but only
the difference due to the disturbing causes.

148. These favourable results led me to hope that the direct
magneto-electric induction of the earth might be rendered sensible; and I
ultimately succeeded in obtaining the effect in several ways. When the
helix just referred to (141. 6.) was placed in the magnetic dip, but
without any cylinder of iron or steel, and was then inverted, a feeble
action at the needle was observed. Inverting the helix ten or twelve times,
and at such periods that the deflecting forces exerted by the currents of
electricity produced in it should be added to the momentum of the needle
(39.), the latter was soon made to vibrate through an arc of 80° or 90°.
Here, therefore, currents of electricity were produced by the direct
inductive power of the earth's magnetism, without the use of any
ferruginous matter, and upon a metal not capable of exhibiting any of the
ordinary magnetic phenomena. The experiment in everything represents the
effects produced by bringing the same helix to one or both poles of any
powerful magnet (50.).

149. Guided by the law already expressed (114.), I expected that all the
electric phenomena of the revolving metal plate could now be produced
without any other magnet than the earth. The plate so often referred to
(85.) was therefore fixed so as to rotate in a horizontal plane. The
magnetic curves of the earth (114. _note_), i.e. the dip, passes through
this plane at angles of about 70°, which it was expected would be an
approximation to perpendicularity, quite enough to allow of
magneto-electric induction sufficiently powerful to produce a current of
electricity.

150. Upon rotation of the plate, the currents ought, according to the law
(114. 121.), to tend to pass in the direction of the radii, through _all_
parts of the plate, either from the centre to the circumference, or from
the circumference to the centre, as the direction of the rotation of the
plate was one way or the other. One of the wires of the galvanometer was
therefore brought in contact with the axis of the plate, and the other
attached to a leaden collector or conductor (86.), which itself was placed
against the amalgamated edge of the disc. On rotating the plate there was a
distinct effect at the galvanometer needle; on reversing the rotation, the
needle went in the opposite direction; and by making the action of the
plate coincide with the vibrations of the needle, the arc through which the
latter passed soon extended to half a circle.

151. Whatever part of the edge of the plate was touched by the conductor,
the electricity was the same, provided the direction of rotation continued
unaltered.

152. When the plate revolved _screw-fashion_, or as the hands of a watch,
the current of electricity (150.) was from the centre to the circumference;
when the direction of rotation was _unscrew_, the current was from the
circumference to the centre. These directions are the same with those
obtained when the unmarked pole of a magnet was placed beneath the
revolving plate (99.).

153. When the plate was in the magnetic meridian, or in any other plane
_coinciding_ with the magnetic dip, then its rotation produced no effect
upon the galvanometer. When inclined to the dip but a few degrees,
electricity began to appear upon rotation. Thus when standing upright in a
plane perpendicular to the magnetic meridian, and when consequently its own
plane was inclined only about 20° to the dip, revolution of the plate
evolved electricity. As the inclination was increased, the electricity
became more powerful until the angle formed by the plane of the plate with
the dip was 90°, when the electricity for a given velocity of the plate was
a maximum.

154. It is a striking thing to observe the revolving copper plate become
thus a _new electrical machine_; and curious results arise on comparing it
with the common machine. In the one, the plate is of the best
non-conducting substance that can be applied; in the other, it is the most
perfect conductor: in the one, insulation is essential; in the other, it is
fatal. In comparison of the quantities of electricity produced, the metal
machine does not at all fall below the glass one; for it can produce a
constant current capable of deflecting the galvanometer needle, whereas the
latter cannot. It is quite true that the force of the current thus evolved
has not as yet been increased so as to render it available in any of our
ordinary applications of this power; but there appears every reasonable
expectation that this may hereafter be effected; and probably by several
arrangements. Weak as the current may seem to be, it is as strong as, if
not stronger than, any thermo-electric current; for it can pass fluids
(23.), agitate the animal system, and in the case of an electro-magnet has
produced sparks (32.).

155. A disc of copper, one fifth of an inch thick and only one inch and a
half in diameter, was amalgamated at the edge; a square piece of sheet lead
(copper would have been better) of equal thickness had a circular hole cut
in it, into which the disc loosely fitted; a little mercury completed the
metallic communication of the disc and its surrounding ring; the latter was
attached to one of the galvanometer wires, and the other wire dipped into a
little metallic cup containing mercury, fixed upon the top of the copper
axis of the small disc. Upon rotating the disc in a horizontal plane, the
galvanometer needle could be affected, although the earth was the only
magnet employed, and the radius of the disc but three quarters of an inch;
in which space only the current was excited.

156. On putting the pole of a magnet under the revolving disc, the
galvanometer needle could be permanently deflected.

157. On using copper wires one sixth of an inch in thickness instead of the
smaller wires (86.) hitherto constantly employed, far more powerful effects
were obtained. Perhaps if the galvanometer had consisted of fewer turns of
thick wire instead of many convolutions of thinner, more striking effects
would have been produced.

158. One form of apparatus which I purpose having arranged, is to have
several discs superposed; the discs are to be metallically connected,
alternately at the edges and at the centres, by means of mercury; and are
then to be revolved alternately in opposite directions, i.e. the first,
third, fifth, &c. to the right hand, and the second, fourth, sixth, &c. to
the left hand; the whole being placed so that the discs are perpendicular
to the dip, or intersect most directly the magnetic curves of powerful
magnets. The electricity will be from the centre to the circumference in
one set of discs, and from the circumference to the centre in those on each
side of them; thus the action of the whole will conjoin to produce one
combined and more powerful current.

159. I have rather, however, been desirous of discovering new facts and new
relations dependent on magneto-electric induction, than of exalting the
force of those already obtained; being assured that the latter would find
their full development hereafter.

       *       *       *       *       *

160. I referred in my former paper to the probable influence of terrestrial
magneto-electric induction (137.) in producing, either altogether or in
part, the phenomena observed by Messrs. Christie and Barlow[A], whilst
revolving ferruginous bodies; and especially those observed by the latter
when rapidly rotating an iron shell, which were by that philosopher
referred to a change in the ordinary disposition of the magnetism of the
ball. I suggested also that the rotation of a copper globe would probably
insulate the effects due to electric currents from those due to mere
derangement of magnetism, and throw light upon the true nature of the
phenomena.

  [A] Christie, Phil. Trans. 1825, pp. 58, 347, &c. Barlow, Phil. Trans.
  1825, p. 317.

161. Upon considering the law already referred to (114.), it appeared
impossible that a metallic globe could revolve under natural circumstances,
without having electric currents produced within it, circulating round the
revolving globe in a plane at right angles to the plane of revolution,
provided its axis of rotation did not coincide with the dip; and it
appeared that the current would be most powerful when the axis of
revolution was perpendicular to the dip of the needle: for then all those
parts of the ball below a plane passing through its centre and
perpendicular to the dip, would in moving cut the magnetic curves in one
direction, whilst all those parts above that plane would intersect them in
the other direction: currents therefore would exist in these moving parts,
proceeding from one pole of rotation to the other; but the currents above
would be in the reverse direction to those below, and in conjunction with
them would produce a continued circulation of electricity.

162. As the electric currents are nowhere interrupted in the ball, powerful
effects were expected, and I endeavoured to obtain them with simple
apparatus. The ball I used was of brass; it had belonged to an old
electrical machine, was hollow, thin (too thin), and four inches in
diameter; a brass wire was screwed into it, and the ball either turned in
the hand by the wire, or sometimes, to render it more steady, supported by
its wire in a notched piece of wood, and motion again given by the hand.
The ball gave no signs of magnetism when at rest.

163. A compound magnetic needle was used to detect the currents. It was
arranged thus: a sewing-needle had the head and point broken off, and was
then magnetised; being broken in halves, the two magnets thus produced were
fixed on a stem of dried grass, so as to be perpendicular to it, and about
four inches asunder; they were both in one plane, but their similar poles
in contrary directions. The grass was attached to a piece of unspun silk
about six inches long, the latter to a stick passing through a cork in the
mouth of a cylindrical jar; and thus a compound arrangement was obtained,
perfectly sheltered from the motion of the air, but little influenced by
the magnetism of the earth, and yet highly sensible to magnetic and
electric forces, when the latter were brought into the vicinity of the one
or the other needle.

164. Upon adjusting the needles to the plane of the magnetic meridian;
arranging the ball on the outside of the glass jar to the west of the
needles, and at such a height that its centre should correspond
horizontally with the upper needle, whilst its axis was in the plane of the
magnetic meridian, but perpendicular to the dip; and then rotating the
ball, the needle was immediately affected. Upon inverting the direction of
rotation, the needle was again affected, but in the opposite direction.
When the ball revolved from east over to west, the marked pole went
eastward; when the ball revolved in the opposite direction, the marked pole
went westward or towards the ball. Upon placing the ball to the east of the
needles, still the needle was deflected in the same way; i.e. when the ball
revolved from east over to west, the marked pole wont eastward (or towards
the ball); when the rotation was in the opposite direction, the marked pole
went westward.

165. By twisting the silk of the needles, the latter were brought into a
position perpendicular to the plane of the magnetic meridian; the ball was
again revolved, with its axis parallel to the needles; the upper was
affected as before, and the deflection was such as to show that both here
and in the former case the needle was influenced solely by currents of
electricity existing in the brass globe.

166. If the upper part of the revolving ball be considered as a wire moving
from east to west, over the unmarked pole of the earth, the current of
electricity in it should be from north to south (99. 114. 150.); if the
under part be considered as a similar wire, moving from west to east over
the same pole, the electric current should be from south to north; and the
circulation of electricity should therefore be from north above to south,
and below back to north, in a metal ball revolving from east above to west
in these latitudes. Now these currents are exactly those required to give
the directions of the needle in the experiments just described; so that the
coincidence of the theory from which the experiments were deduced with the
experiments themselves, is perfect.

167. Upon inclining the axis of rotation considerably, the revolving ball
was still found to affect the magnetic needle; and it was not until the
angle which it formed with the magnetic dip was rendered small, that its
effects, even upon this apparatus, were lost (153.). When revolving with
its axis parallel to the dip, it is evident that the globe becomes
analogous to the copper plate; electricity of one kind might be collected
at its equator, and of the other kind at its poles.

168. A current in the ball, such as that described above (161.), although
it ought to deflect a needle the same way whether it be to the right or the
left of the ball and of the axis of rotation, ought to deflect it the
contrary way when above or below the ball; for then the needle is, or ought
to be, acted upon in a contrary direction by the current. This expectation
was fulfilled by revolving the ball beneath the magnetic needle, the latter
being still inclosed in its jar. When the ball was revolved from east over
to west, the marked pole of the needle, instead of passing eastward, went
westward; and when revolved from west over to east, the marked pole went
eastward.

169. The deflections of the magnetic needle thus obtained with a brass ball
are exactly in the same direction as those observed by Mr. Barlow in the
revolution of the iron shell; and from the manner in which iron exhibits
the phenomena of magneto-electric induction like any other metal, and
distinct from its peculiar magnetic phenomena (132.), it is impossible but
that electric currents must have been excited, and become active in those
experiments. What proportion of the whole effect obtained is due to this
cause, must be decided by a more elaborate investigation of all the
phenomena.

170. These results, in conjunction with the general law before stated
(114.), suggested an experiment of extreme simplicity, which yet, on trial,
was found to answer perfectly. The exclusion of all extraneous
circumstances and complexity of arrangement, and the distinct character of
the indications afforded, render this single experiment an epitome of
nearly all the facts of magneto-electric induction.

171. A piece of common copper wire, about eight feet long and one twentieth
of an inch in thickness, had one of its ends fastened to one of the
terminations of the galvanometer wire, and the other end to the other
termination; thus it formed an endless continuation of the galvanometer
wire: it was then roughly adjusted into the shape of a rectangle, or rather
of a loop, the upper part of which could be carried to and fro over the
galvanometer, whilst the lower part, and the galvanometer attached to it,
remained steady (Plate II. fig. 30.). Upon moving this loop over the
galvanometer from right to left, the magnetic needle was immediately
deflected; upon passing the loop back again, the needle passed in the
contrary direction to what it did before; upon repeating these motions of
the loop in accordance with the vibrations of the needle (39.), the latter
soon swung through 90° or more.

172. The relation of the current of electricity produced in the wire, to
its motion, may be understood by supposing the convolutions at the
galvanometer away, and the wire arranged as a rectangle, with its lower
edge horizontal and in the plane of the magnetic meridian, and a magnetic
needle suspended above and over the middle part of this edge, and directed
by the earth (fig. 30.). On passing the upper part of the rectangle from
west to east into the position represented by the dotted line, the marked
pole of the magnetic needle went west; the electric current was therefore
from north to south in the part of the wire passing under the needle, and
from south to north in the moving or upper part of the parallelogram. On
passing the upper part of the rectangle from east to west over the
galvanometer, the marked pole of the needle went east, and the current of
electricity was therefore the reverse of the former.

173. When the rectangle was arranged in a plane east and west, and the
magnetic needle made parallel to it, either by the torsion of its
suspension thread or the action of a magnet, still the general effects were
the same. On moving the upper part of the rectangle from north to south,
the marked pole of the needle went north; when the wire was moved in the
opposite direction, the marked pole went south. The same effect took place
when the motion of the wire was in any other azimuth of the line of dip;
the direction of the current always being conformable to the law formerly
expressed (114.), and also to the directions obtained with the rotating
ball (101.).

174. In these experiments it is not necessary to move the galvanometer or
needle from its first position. It is quite sufficient if the wire of the
rectangle is distorted where it leaves the instrument, and bent so as to
allow the moving upper part to travel in the desired direction.

175. The moveable part of the wire was then arranged _below_ the
galvanometer, but so as to be carried across the dip. It affected the
instrument as before, and in the same direction; i.e. when carried from
west to east under the instrument, the marked end of the needle went west,
as before. This should, of course, be the case; for when the wire is
cutting the magnetic dip in a certain direction, an electric current also
in a certain direction should be induced in it.

176. If in fig. 31 _dp_ be parallel to the dip, and BA be considered as the
upper part of the rectangle (171.), with an arrow _c_ attached to it, both
these being retained in a plane perpendicular to the dip,--then, however BA
with its attached arrow is moved upon _dp_ as an axis, if it afterwards
proceed in the direction of the arrow, a current of electricity will move
along it from B towards A.

177. When the moving part of the wire was carried up or down parallel to
the dip, no effect was produced on the galvanometer. When the direction of
motion was a little inclined to the dip, electricity manifested itself; and
was at a maximum when the motion was perpendicular to the magnetic
direction.

178. When the wire was bent into other forms and moved, equally strong
effects were obtained, especially when instead of a rectangle a double
catenarian curve was formed of it on one side of the galvanometer, and the
two single curves or halves were swung in opposite directions at the same
time; their action then combined to affect the galvanometer: but all the
results were reducible to those above described.

179. The longer the extent of the moving wire, and the greater the space
through which it moves, the greater is the effect upon the galvanometer.

180. The facility with which electric currents are produced in metals when
moving under the influence of magnets, suggests that henceforth precautions
should always be taken, in experiments upon metals and magnets, to guard
against such effects. Considering the universality of the magnetic
influence of the earth, it is a consequence which appears very
extraordinary to the mind, that scarcely any piece of metal can be moved in
contact with others, either at rest, or in motion with different velocities
or in varying directions, without an electric current existing within them.
It is probable that amongst arrangements of steam-engines and metal
machinery, some curious accidental magneto-electric combinations may be
found, producing effects which have never been observed, or, if noticed,
have never as yet been understood.

       *       *       *       *       *

181. Upon considering the effects of terrestrial magneto-electric induction
which have now been described, it is almost impossible to resist the
impression that similar effects, but infinitely greater in force, may be
produced by the action of the globe, as a magnet, upon its own mass, in
consequence of its diurnal rotation. It would seem that if a bar of metal
be laid in these latitudes on the surface of the earth parallel to the
magnetic meridian, a current of electricity tends to pass through it from
south to north, in consequence of the travelling of the bar from west to
east (172.), by the rotation of the earth; that if another bar in the same
direction be connected with the first by wires, it cannot discharge the
current of the first, because it has an equal tendency to have a current in
the same direction induced within itself: but that if the latter be carried
from east to west, which is equivalent to a diminution of the motion
communicated to it from the earth (172.), then the electric current from
south to north is rendered evident in the first bar, in consequence of its
discharge, at the same time, by means of the second.

182. Upon the supposition that the rotation of the earth tended, by
magneto-electric induction, to cause currents in its own mass, these would,
according to the law (114.) and the experiments, be, upon the surface at
least, from the parts in the neighbourhood of or towards the plane of the
equator, in opposite directions to the poles; and if collectors could be
applied at the equator and at the poles of the globe, as has been done with
the revolving copper plate (150.), and also with magnets (220.), then
negative electricity would be collected at the equator, and positive
electricity at both poles (222.). But without the conductors, or something
equivalent to them, it is evident these currents could not exist, as they
could not be discharged.

183. I did not think it impossible that some natural difference might occur
between bodies, relative to the intensity of the current produced or
tending to be produced in them by magneto-electric induction, which might
be shown by opposing them to each other; especially as Messrs. Arago,
Babbage, Herschel, and Harris, have all found great differences, not only
between the metals and other substances, but between the metals themselves,
in their power of receiving motion from or giving it to a magnet in trials
by revolution (130.). I therefore took two wires, each one hundred and
twenty feet long, one of iron and the other of copper. These were connected
with each other at their ends, and then extended in the direction of the
magnetic meridian, so as to form two nearly parallel lines, nowhere in
contact except at the extremities. The copper wire was then divided in the
middle, and examined by a delicate galvanometer, but no evidence of an
electrical current was obtained.

184. By favour of His Royal Highness the President of the Society, I
obtained the permission of His Majesty to make experiments at the lake in
the gardens of Kensington-palace, for the purpose of comparing, in a
similar manner, water and metal. The basin of this lake is artificial; the
water is supplied by the Chelsea Company; no springs run into it, and it
presented what I required, namely, a uniform mass of still pure water, with
banks ranging nearly from east to west, and from north to south.

185. Two perfectly clean bright copper plates, each exposing four square
feet of surface, were soldered to the extremities of a copper wire; the
plates were immersed in the water, north and south of each other, the wire
which connected them being arranged upon the grass of the bank. The plates
were about four hundred and eighty feet from each other, in a right line;
the wire was probably six hundred feet long. This wire was then divided in
the middle, and connected by two cups of mercury with a delicate
galvanometer.

186. At first, indications of electric currents were obtained; but when
these were tested by inverting the direction of contact, and in other ways,
they were found to be due to other causes than the one sought for. A little
difference in temperature; a minute portion of the nitrate of mercury used
to amalgamate the wires, entering into the water employed to reduce the two
cups of mercury to the same temperature; was sufficient to produce currents
of electricity, which affected the galvanometer, notwithstanding they had
to pass through nearly five hundred feet of water. When these and other
interfering causes were guarded against, no effect was obtained; and it
appeared that even such dissimilar substances as water and copper, when
cutting the magnetic curves of the earth with equal velocity, perfectly
neutralized each other's action.

187. Mr. Fox of Falmouth has obtained some highly important results
respecting the electricity of metalliferous veins in the mines of Cornwall,
which have been published in the Philosophical Transactions[A]. I have
examined the paper with a view to ascertain whether any of the effects were
probably referable to magneto-electric induction; but, though unable to
form a very strong opinion, believe they are not. When parallel veins
running east and west were compared, the general tendency of the
electricity _in the wires_ was from north to south; when the comparison was
made between parts towards the surface and at some depth, the current of
electricity in the wires was from above downwards. If there should be any
natural difference in the force of the electric currents produced by
magneto-electric induction in different substances, or substances in
different positions moving with the earth, and which might be rendered
evident by increasing the masses acted upon, then the wires and veins
experimented with by Mr. Fox might perhaps have acted as dischargers to the
electricity of the mass of strata included between them, and the directions
of the currents would agree with those observed as above.

  [A] 1830. p. 399.

188. Although the electricity obtained by magneto-electric induction in a
few feet of wire is of but small intensity, and has not yet been observed
except in metals, and carbon in a particular state, still it has power to
pass through brine (23.); and, as increased length in the substance acted
upon produces increase of intensity, I hoped to obtain effects from
extensive moving masses of water, though quiescent water gave none. I made
experiments therefore (by favour) at Waterloo Bridge, extending a copper
wire nine hundred and sixty feet in length upon the parapet of the bridge,
and dropping from its extremities other wires with extensive plates of
metal attached to them to complete contact with the water. Thus the wire
and the water made one conducting circuit; and as the water ebbed or flowed
with the tide, I hoped to obtain currents analogous to those of the brass
ball (161.).

189. I constantly obtained deflections at the galvanometer, but they were
very irregular, and were, in succession, referred to other causes than that
sought for. The different condition of the water as to purity on the two
sides of the river; the difference in temperature; slight differences in
the plates, in the solder used, in the more or less perfect contact made by
twisting or otherwise; all produced effects in turn: and though I
experimented on the water passing through the middle arches only; used
platina plates instead of copper; and took every other precaution, I could
not after three days obtain any satisfactory results.

190. Theoretically, it seems a necessary consequence, that where water is
flowing, there electric currents should be formed; thus, if a line be
imagined passing from Dover to Calais through the sea, and returning
through the land beneath the water to Dover, it traces out a circuit of
conducting matter, one part of which, when the water moves up or down the
channel, is cutting the magnetic curves of the earth, whilst the other is
relatively at rest. This is a repetition of the wire experiment (171.), but
with worse conductors. Still there is every reason to believe that electric
currents do run in the general direction of the circuit described, either
one way or the other, according as the passage of the waters is up or down
the channel. Where the lateral extent of the moving water is enormously
increased, it does not seem improbable that the effect should become
sensible; and the gulf stream may thus, perhaps, from electric currents
moving across it, by magneto-electric induction from the earth, exert a
sensible influence upon the forms of the lines of magnetic variation[A].

  [A] Theoretically, even a ship or a boat when passing on the surface
  of the water, in northern or southern latitudes, should have currents
  of electricity running through it directly across the line of her
  motion; or if the water is flowing past the ship at anchor, similar
  currents should occur.

191. Though positive results have not yet been obtained by the action of
the earth upon water and aqueous fluids, yet, as the experiments are very
limited in their extent, and as such fluids do yield the current by
artificial magnets (23.), (for transference of the current is proof that it
may be produced (213.),) the supposition made, that the earth produces
these induced currents within itself (181.) in consequence of its diurnal
rotation, is still highly probable (222, 223.); and when it is considered
that the moving masses extend for thousands of miles across the magnetic
curves, cutting them in various directions within its mass, as well as at
the surface, it is possible the electricity may rise to considerable
intensity.

192. I hardly dare venture, even in the most hypothetical form, to ask
whether the Aurora Borealis and Australia may not be the discharge of
electricity, thus urged towards the poles of the earth, from whence it is
endeavouring to return by natural and appointed means above the earth to
the equatorial regions. The non-occurrence of it in very high latitudes is
not at all against the supposition; and it is remarkable that Mr. Fox, who
observed the deflections of the magnetic needle at Falmouth, by the Aurora
Borealis, gives that direction of it which perfectly agrees with the
present view. He states that all the variations at night were towards the
east[A], and this is what would happen if electric currents were setting
from south to north in the earth under the needle, or from north to south
in space above it.

  [A] Philosophical Transactions, 1831, p. 202.


§ 6. _General remarks and illustrations of the Force and Direction of
Magneto-electric Induction._


193. In the repetition and variation of Arago's experiment by Messrs.
Babbage, Herschel, and Harris, these philosophers directed their attention
to the differences of force observed amongst the metals and other
substances in their action on the magnet. These differences were very
great[A], and led me to hope that by mechanical combinations of various
metals important results might be obtained (183.). The following
experiments were therefore made, with a view to obtain, if possible, any
such difference of the action of two metals,

  [B] Philosophical Transactions, 1825, p. 472; 1831, p.78.

194. A piece of soft iron bonnet-wire covered with cotton was laid bare and
cleaned at one extremity, and there fastened by metallic contact with the
clean end of a copper wire. Both wires were then twisted together like the
strands of a rope, for eighteen or twenty inches; and the remaining parts
being made to diverge, their extremities were connected with the wires of
the galvanometer. The iron wire was about two feet long, the continuation
to the galvanometer being copper.

195. The twisted copper and iron (touching each other nowhere but at the
extremity) were then passed between the poles of a powerful magnet arranged
horse-shoe fashion (fig. 32.); but not the slightest effect was observed at
the galvanometer, although the arrangement seemed fitted to show any
electrical difference between the two metals relative to the action of the
magnet,

196. A soft iron cylinder was then covered with paper at the middle part,
and the twisted portion of the above compound wire coiled as a spiral
around it, the connexion with the galvanometer still being made at the ends
A and B. The iron cylinder was then brought in contact with the poles of a
powerful magnet capable of raising thirty pounds; yet no signs of
electricity appeared at the galvanometer. Every precaution was applied in
making and breaking contact to accumulate effect, but no indications of a
current could be obtained.

197. Copper and tin, copper and zinc, tin and zinc, tin and iron, and zinc
and iron, were tried against each other in a similar manner (194), but not
the slightest sign of electric currents could be procured.

198. Two flat spirals, one of copper and the other of iron, containing each
eighteen inches of wire, were connected with each other and with the
galvanometer, and then put face to face so as to be in contrary directions.
When brought up to the magnetic pole (53.). No electrical indications at
the galvanometer were observed. When one was turned round so that both were
in the same direction, the effect at the galvanometer was very powerful.

199. The compound helix of copper and iron wire formerly described (8.) was
arranged as a double helix, one of the helices being all iron and
containing two hundred and fourteen feet, the other all copper and
continuing two hundred and eight feet. The two similar ends AA of the
copper and iron helix were connected together, and the other ends BB of
each helix connected with the galvanometer; so that when a magnet was
introduced into the centre of the arrangement, the induced currents in the
iron and copper would tend to proceed in contrary directions. Yet when a
magnet was inserted, or a soft iron bar within made a magnet by contact
with poles, no effect at the needle was produced.

200. A glass tube about fourteen inches long was filled with strong
sulphuric acid. Twelve inches of the end of a clean copper wire were bent
up into a bundle and inserted into the tube, so as to make good superficial
contact with the acid, and the rest of the wire passed along the outside of
the tube and away to the galvanometer. A wire similarly bent up at the
extremity was immersed in the other end of the sulphuric acid, and also
connected with the galvanometer, so that the acid and copper wire were in
the same parallel relation to each other in this experiment as iron and
copper were in the first (194). When this arrangement was passed in a
similar manner between the poles of the magnet, not the slightest effect at
the galvanometer could be perceived.

201. From these experiments it would appear, that when metals of different
kinds connected in one circuit are equally subject in every circumstance to
magneto-electric induction, they exhibit exactly equal powers with respect
to the currents which either are formed, or tend to form, in them. The same
even appears to be the case with regard to fluids, and probably all other
substances.

202. Still it seemed impossible that these results could indicate the
relative inductive power of the magnet upon the different metals; for that
the effect should be in some relation to the conducting power seemed a
necessary consequence (139.), and the influence of rotating plates upon
magnets had been found to bear a general relation to the conducting power
of the substance used.

203. In the experiments of rotation (81.), the electric current is excited
and discharged in the same substance, be it a good or bad conductor; but in
the experiments just described the current excited in iron could not be
transmitted but through the copper, and that excited in copper had to pass
through iron: i.e. supposing currents of dissimilar strength to be formed
in the metals proportionate to their conducting power, the stronger current
had to pass through the worst conductor, and the weaker current through the
best.

204. Experiments were therefore made in which different metals insulated
from each other were passed between the poles of the magnet, their opposite
ends being connected with the same end of the galvanometer wire, so that
the currents formed and led away to the galvanometer should oppose each
other; and when considerable lengths of different wires were used, feeble
deflections were obtained.

205. To obtain perfectly satisfactory results a new galvanometer was
constructed, consisting of two independent coils, each containing eighteen
feet of silked copper wire. These coils were exactly alike in shape and
number of turns, and were fixed side by side with a small interval between
them, in which a double needle could be hung by a fibre of silk exactly as
in the former instrument (87.). The coils may be distinguished by the
letters KL, and when electrical currents were sent through them in the same
direction, acted upon the needle with the sum of their powers; when in
opposite directions, with the difference of their powers.

206. The compound helix (199. 8.) was now connected, the ends A and B of
the iron with A and B ends of galvanometer coil K, and the ends A and B of
the copper with B and A ends of galvanometer coil L, so that the currents
excited in the two helices should pass in opposite directions through the
coils K and L. On introducing a small cylinder magnet within the helices,
the galvanometer needle was powerfully deflected. On disuniting the iron
helix, the magnet caused with the copper helix alone still stronger
deflection in the same direction. On reuniting the iron helix, and
unconnecting the copper helix, the magnet caused a moderate deflection in
the contrary direction. Thus it was evident that the electric current
induced by a magnet in a copper wire was far more powerful than the current
induced by the same magnet in an equal iron wire.

207. To prevent any error that might arise from the greater influence, from
vicinity or other circumstances, of one coil on the needle beyond that of
the other, the iron and copper terminations were changed relative to the
galvanometer coils KL, so that the one which before carried the current
from the copper now conveyed that from the iron, and vice versa. But the
same striking superiority of the copper was manifested as before. This
precaution was taken in the rest of the experiments with other metals to be
described.

208. I then had wires of iron, zinc, copper, tin, and lead, drawn to the
same diameter (very nearly one twentieth of an inch), and I compared
exactly equal lengths, namely sixteen feet, of each in pairs in the
following manner: The ends of the copper wire were connected with the ends
A and B of galvanometer coil K, and the ends of the zinc wire with the
terminations A and B of the galvanometer coil L. The middle part of each
wire was then coiled six times round a cylinder of soft iron covered with
paper, long enough to connect the poles of Daniell's horse-shoe magnet
(56.) (fig. 33.), so that similar helices of copper and zinc, each of six
turns, surrounded the bar at two places equidistant from each other and
from the poles of the magnet; but these helices were purposely arranged so
as to be in contrary directions, and therefore send contrary currents
through the galvanometer coils K and L,

209. On making and breaking contact between the soft iron bar and the poles
of the magnet, the galvanometer was strongly affected; on detaching the
zinc it was still more strongly affected in the same direction. On taking
all the precautions before alluded to (207.), with others, it was
abundantly proved that the current induced by the magnet in copper was far
more powerful than in zinc.

210. The copper was then compared in a similar manner with tin, lead, and
iron, and surpassed them all, even more than it did zinc. The zinc was then
compared experimentally with the tin, lead, and iron, and found to produce
a more powerful current than any of them. Iron in the same manner proved
superior to tin and lead. Tin came next, and lead the last.

211. Thus the order of these metals is copper, zinc, iron, tin, and lead.
It is exactly their order with respect to conducting power for electricity,
and, with the exception of iron, is the order presented by the
magneto-rotation experiments of Messrs. Babbage, Herschel, Harris, &c. The
iron has additional power in the latter kind of experiments, because of its
ordinary magnetic relations, and its place relative to magneto-electric
action of the kind now under investigation cannot be ascertained by such
trials. In the manner above described it may be correctly ascertained[A].

  [A] Mr. Christie, who being appointed reporter upon this paper, had it
  in his hands before it was complete, felt the difficulty (202.); and
  to satisfy his mind, made experiments upon iron and copper with the
  large magnet(44.), and came to the same conclusions as I have arrived
  at. The two sets of experiments were perfectly independent of each
  other, neither of us being aware of the other's proceedings.

212. It must still be observed that in these experiments the whole effect
between different metals is not obtained; for of the thirty-four feet of
wire included in each circuit, eighteen feet are copper in both, being the
wire of the galvanometer coils; and as the whole circuit is concerned in
the resulting force of the current, tin's circumstance must tend to
diminish the difference which would appear between the metals if the
circuits were of the same substances throughout. In the present case the
difference obtained is probably not more than a half of that which would be
given if the whole of each circuit were of one metal.

213. These results tend to prove that the currents produced by
magneto-electric induction in bodies is proportional to their conducting
power. That they are _exactly_ proportional to and altogether dependent
upon the conducting power, is, I think, proved by the perfect neutrality
displayed when two metals or other substances, as acid, water, &c. &c.
(201. 186.), are opposed to each other in their action. The feeble current
which tends to be produced in the worse conductor, has its transmission
favoured in the better conductor, and the stronger current which tends to
form in the latter has its intensity diminished by the obstruction of the
former; and the forces of generation and obstruction are so perfectly
neutralize each other exactly. Now as the obstruction is inversely as the
balanced as to conducting power, the tendency to generate a current must
be directly as that power to produce this perfect equilibrium.

214. The cause of the equality of action under the various circumstances
described, where great extent of wire (183.) or wire and water (181.) were
connected together, which yet produced such different effects upon the
magnet, is now evident and simple.

215. The effects of a rotating substance upon a needle or magnet ought,
where ordinary magnetism has no influence, to be directly as the conducting
power of the substance; and I venture now to predict that such will be
found to be the case; and that in all those instances where non-conductors
have been supposed to exhibit this peculiar influence, the motion has been
due to some interfering cause of an ordinary kind; as mechanical
communication of motion through the parts of the apparatus, or otherwise
(as in the case Mr. Harris has pointed out[A]); or else to ordinary
magnetic attractions. To distinguish the effects of the latter from those
of the induced electric currents, I have been able to devise a most perfect
test, which shall be almost immediately described (243.).

  [A] Philosophical Transactions, 1831. p. 68.

216. There is every reason to believe that the magnet or magnetic needle
will become an excellent measurer of the conducting power of substances
rotated near it; for I have found by careful experiment, that when a
constant current of electricity was sent successively through a series of
wires of copper, platina, zinc, silver, lead, and tin, drawn to the same
diameter; the deflection of the needle was exactly equal by them all. It
must be remembered that when bodies are rotated in a horizontal plane, the
magnetism of the earth is active upon them. As the effect is general to the
whole of the plate, it may not interfere in these cases; but in some
experiments and calculations may be of important consequence.

217. Another point which I endeavoured to ascertain, was, whether it was
essential or not that the moving part of the wire should, in cutting the
magnetic curves, pass into positions of greater or lesser magnetic force;
or whether, always intersecting curves of equal magnetic intensity, the
mere motion was sufficient for the production of the current. That the
latter is true, has been proved already in several of the experiments on
terrestrial magneto-electric induction. Thus the electricity evolved from
the copper plate (149.), the currents produced in the rotating globe (161,
&c.), and those passing through the moving wire (171.), are all produced
under circumstances in which the magnetic force could not but be the same
during the whole experiments.

218. To prove the point with an ordinary magnet, a copper disc was cemented
upon the end of a cylinder magnet, with paper intervening; the magnet and
disc were rotated together, and collectors (attached to the galvanometer)
brought in contact with the circumference and the central part of the
copper plate. The galvanometer needle moved as in former cases, and the
_direction_ of motion was the _same_ as that which would have resulted, if
the copper only had revolved, and the magnet been fixed. Neither was there
any apparent difference in the quantity of deflection. Hence, rotating the
magnet causes no difference in the results; for a rotatory and a stationary
magnet produce the same effect upon the moving copper.

219. A copper cylinder, closed at one extremity, was then put over the
magnet, one half of which it inclosed like a cap; it was firmly fixed, and
prevented from touching the magnet anywhere by interposed paper. The
arrangement was then floated in a narrow jar of mercury, so that the lower
edge of the copper cylinder touched the fluid metal; one wire of the
galvanometer dipped into this mercury, and the other into a little cavity
in the centre of the end of the copper cap. Upon rotating the magnet and
its attached cylinder, abundance of electricity passed through the
galvanometer, and in the same direction as if the cylinder had rotated
only, the magnet being still. The results therefore were the same as those
with the disc (218.).

220. That the metal of the magnet itself might be substituted for the
moving cylinder, disc, or wire, seemed an inevitable consequence, and yet
one which would exhibit the effects of magneto-electric induction in a
striking form. A cylinder magnet had therefore a little hole made in the
centre of each end to receive a drop of mercury, and was then floated pole
upwards in the same metal contained in a narrow jar. One wire from the
galvanometer dipped into the mercury of the jar, and the other into the
drop contained in the hole at the upper extremity of the axis. The magnet
was then revolved by a piece of string passed round it, and the
galvanometer-needle immediately indicated a powerful current of
electricity. On reversing the order of rotation, the electrical current was
reversed. The direction of the electricity was the same as if the copper
cylinder (219.) or a copper wire had revolved round the fixed magnet in the
same direction as that which the magnet itself had followed. Thus a
_singular independence_ of the magnetism and the bar in which it resides is
rendered evident.

221. In the above experiment the mercury reached about halfway up the
magnet; but when its quantity was increased until within one eighth of an
inch of the top, or diminished until equally near the bottom, still the
same effects and the _same direction_ of electrical current was obtained.
But in those extreme proportions the effects did not appear so strong as
when the surface of the mercury was about the middle, or between that and
an inch from each end. The magnet was eight inches and a half long, and
three quarters of an inch in diameter.

222. Upon inversion of the magnet, and causing rotation in the same
direction, i.e. always screw or always unscrew, then a contrary current of
electricity was produced. But when the motion of the magnet was continued
in a direction constant in relation to its _own axis_, then electricity of
the same kind was collected at both poles, and the opposite electricity at
the equator, or in its neighbourhood, or in the parts corresponding to it.
If the magnet be held parallel to the axis of the earth, with its unmarked
pole directed to the pole star, and then rotated so that the parts at its
southern side pass from west to east in conformity to the motion of the
earth; then positive electricity may be collected at the extremities of the
magnet, and negative electricity at or about the middle of its mass.

223. When the galvanometer was very sensible, the mere spinning of the
magnet in the air, whilst one of the galvanometer wires touched the
extremity, and the other the equatorial parts, was sufficient to evolve a
current of electricity and deflect the needle.

224. Experiments were then made with a similar magnet, for the purpose of
ascertaining whether any return of the electric current could occur at the
central or axial parts, they having the same angular velocity of rotation
as the other parts (259.) the belief being that it could not.

225. A cylinder magnet, seven inches in length, and three quarters of an
inch in diameter, had a hole pierced in the direction of its axis from one
extremity, a quarter of an inch in diameter, and three inches deep. A
copper cylinder, surrounded by paper and amalgamated at both extremities,
was introduced so as to be in metallic contact at the bottom of the hole,
by a little mercury, with the middle of the magnet; insulated at the sides
by the paper; and projecting about a quarter of an inch above the end of
the steel. A quill was put over the copper rod, which reached to the paper,
and formed a cup to receive mercury for the completion of the circuit. A
high paper edge was also raised round that end of the magnet and mercury
put within it, which however had no metallic connexion with that in the
quill, except through the magnet itself and the copper rod (fig. 34.). The
wires A and B from the galvanometer were dipped into these two portions of
mercury; any current through them could, therefore, only pass down the
magnet towards its equatorial parts, and then up the copper rod; or vice
versa.

226. When thus arranged and rotated screw fashion, the marked end of the
galvanometer needle went west, indicating that there was a current through
the instrument from A to B and consequently from B through the magnet and
copper rod to A (fig. 34.).

227. The magnet was then put into a jar of mercury (fig. 35.) as before
(219.); the wire A left in contact with the copper axis, but the wire B
dipped in the mercury of the jar, and therefore in metallic communication
with the equatorial parts of the magnet instead of its polar extremity. On
revolving the magnet screw fashion, the galvanometer needle was deflected
in the same direction as before, but far more powerfully. Yet it is evident
that the parts of the magnet from the equator to the pole were out of the
electric circuit.

228. Then the wire A was connected with the mercury on the extremity of the
magnet, the wire B still remaining in contact with that in the jar (fig.
36.), so that the copper axis was altogether out of the circuit. The magnet
was again revolved screw fashion, and again caused the same deflection of
the needle, the current being as strong as it was in the last trial (227.),
and much stronger than at first (226.).

229. Hence it is evident that there is no discharge of the current at the
centre of the magnet, for the current, now freely evolved, is up through
the magnet; but in the first experiment (226.) it was down. In fact, at
that time, it was only the part of the moving metal equal to a little disc
extending from the end of the wire B in the mercury to the wire A that was
efficient, i.e. moving with a different angular velocity to the rest of the
circuit (258.); and for that portion the direction of the current is
consistent with the other results.

230. In the two after experiments, the _lateral_ parts of the magnet or of
the copper rod are those which move relative to the other parts of the
circuit, i.e. the galvanometer wires; and being more extensive,
intersecting more curves, or moving with more velocity, produce the greater
effect. For the discal part, the direction of the induced electric current
is the same in all, namely, from the circumference towards the centre.

       *       *       *       *       *

231. The law under which the induced electric current excited in bodies
moving relatively to magnets, is made dependent on the intersection of the
magnetic curves by the metal (114.) being thus rendered more precise and
definite (217. 220. 224.), seem now even to apply to the cause in the first
section of the former paper (26.); and by rendering a perfect reason for
the effects produced, take away any for supposing that peculiar condition,
which I ventured to call the electro-tonic state (60.).

232. When an electrical current is passed through a wire, that wire is
surrounded at every part by magnetic curves, diminishing in intensity
according to their distance from the wire, and which in idea may be likened
to rings situated in planes perpendicular to the wire or rather to the
electric current within it. These curves, although different in form, are
perfectly analogous to those existing between two contrary magnetic poles
opposed to each other; and when a second wire, parallel to that which
carries the current, is made to approach the latter (18.), it passes
through magnetic curves exactly of the same kind as those it would
intersect when carried between opposite magnetic poles (109.) in one
direction; and as it recedes from the inducing wire, it cuts the curves
around it in the same manner that it would do those between the same poles
if moved in the other direction.

233. If the wire NP (fig. 40.) have an electric current passed through it
in the direction from P to N, then the dotted ring may represent a magnetic
curve round it, and it is in such a direction that if small magnetic
needles lie placed as tangents to it, they will become arranged as in the
figure, _n_ and _s_ indicating north and south ends (14. _note_.).

234. But if the current of electricity were made to cease for a while, and
magnetic poles were used instead to give direction to the needles, and make
them take the same position as when under the influence of the current,
then they must be arranged as at fig. 41; the marked and unmarked poles
_ab_ above the wire, being in opposite directions to those _a'b'_ below. In
such a position therefore the magnetic curves between the poles _ab_ and
_a'b'_ have the same general direction with the corresponding parts of the
ring magnetic curve surrounding the wire NP carrying an electric current.

235. If the second wire _pn_ (fig. 40.) be now brought towards the
principal wire, carrying a current, it will cut an infinity of magnetic
curves, similar in direction to that figured, and consequently similar in
direction to those between the poles _ab_ of the magnets (fig. 41.), and it
will intersect these current curves in the same manner as it would the
magnet curves, if it passed from above between the poles downwards. Now,
such an intersection would, with the magnets, induce an electric current in
the wire from _p_ to _n_ (114.); and therefore as the curves are alike in
arrangement, the same effect ought to result from the intersection of the
magnetic curves dependent on the current in the wire NP; and such is the
case, for on approximation the induced current is in the opposite direction
to the principal current (19.).

236. If the wire _p'n'_ be carried up from below, it will pass in the
opposite direction between the magnetic poles; but then also the magnetic
poles themselves are reversed (fig. 41.), and the induced current is
therefore (114.) still in the same direction as before. It is also, for
equally sufficient and evident reasons, in the same direction, if produced
by the influence of the curves dependent upon the wire.

237. When the second wire is retained at rest in the vicinity the principal
wire, no current is induced through it, for it is intersecting no magnetic
curves. When it is removed from the principal wire, it intersects the
curves in the opposite direction to what it did before (235.); and a
current in the opposite direction is induced, which therefore corresponds
with the direction of the principal current (19.). The same effect would
take place if by inverting the direction of motion of the wire in passing
between either set of poles (fig. 41.), it were made to intersect the
curves there existing in the opposite direction to what it did before.

238. In the first experiments (10. 13.), the inducing wire and that under
induction were arranged at a fixed distance from each other, and then an
electric current sent through the former. In such cases the magnetic curves
themselves must be considered as moving (if I may use the expression)
across the wire under induction, from the moment at which they begin to be
developed until the magnetic force of the current is at its utmost;
expanding as it were from the wire outwards, and consequently being in the
same relation to the fixed wire under induction as if _it_ had moved in the
opposite direction across them, or towards the wire carrying the current.
Hence the first current induced in such cases was in the contrary direction
to the principal current (17. 235.). On breaking the battery contact, the
magnetic curves (which are mere expressions for arranged magnetic forces)
may be conceived as contracting upon and returning towards the failing
electrical current, and therefore move in the opposite direction across the
wire, and cause an opposite induced current to the first.

239. When, in experiments with ordinary magnets, the latter, in place of
being moved past the wires, were actually made near them (27. 36.), then a
similar progressive development of the magnetic curves may be considered as
having taken place, producing the effects which would have occurred by
motion of the wires in one direction; the destruction of the magnetic power
corresponds to the motion of the wire in the opposite direction.

240. If, instead of intersecting the magnetic curves of a straight wire
carrying a current, by approximating or removing a second wire (235.), a
revolving plate be used, being placed for that purpose near the wire, and,
as it were, amongst the magnetic curves, then it ought to have continuous
electric currents induced within it; and if a line joining the wire with
the centre of the plate were perpendicular to both, then the induced
current ought to be, according to the law (114.), directly across the
plate, from one side to the other, and at right angles to the direction of
the inducing current.

241. A single metallic wire one twentieth of an inch in diameter had an
electric current passed through it, and a small copper disc one inch and a
half in diameter revolved near to and under, but not in actual contact with
it (fig. 39). Collectors were then applied at the opposite edges of the
disc, and wires from them connected with the galvanometer. As the disc
revolved in one direction, the needle was deflected on one side: and when
the direction of revolution was reversed, the needle was inclined on the
other side, in accordance with the results anticipated.

242. Thus the reasons which induce me to suppose a particular state in the
wire (60.) have disappeared; and though it still seems to me unlikely that
a wire at rest in the neighbourhood of another carrying a powerful electric
current is entirely indifferent to it, yet I am not aware of any distinct
_facts_ which authorize the conclusion that it is in a particular state.

       *       *       *       *       *

243. In considering the nature of the cause assigned in these papers to
account for the mutual influence of magnets and moving metals (120.), and
comparing it with that heretofore admitted, namely, the induction of a
feeble magnetism like that produced in iron, it occurred to me that a most
decisive experimental test of the two views could be applied (215.).

244. No other known power has like direction with that exerted between an
electric current and a magnetic pole; it is tangential, while all other
forces, acting at a distance, are direct. Hence, if a magnetic pole on one
side of a revolving plate follow its course by reason of its obedience to
the tangential force exerted upon it by the very current of electricity
which it has itself caused, a similar pole on the opposite side of the
plate should immediately set it free from this force; for the currents
which tend to be formed by the action of the two poles are in opposite
directions; or rather no current tends to be formed, or no magnetic curves
are intersected (114.); and therefore the magnet should remain at rest. On
the contrary, if the action of a north magnetic pole were to produce a
southness in the nearest part of the copper plate, and a diffuse northness
elsewhere (82.), as is really the case with iron; then the use of another
north pole on the opposite side of the same part of the plate should double
the effect instead of destroying it, and double the tendency of the first
magnet to move with the plate.

245. A thick copper plate (85.) was therefore fixed on a vertical axis, a
bar magnet was suspended by a plaited silk cord, so that its marked pole
hung over the edge of the plate, and a sheet of paper being interposed, the
plate was revolved; immediately the magnetic pole obeyed its motion and
passed off in the same direction. A second magnet of equal size and
strength was then attached to the first, so that its marked pole should
hang _beneath_ the edge of the copper plate in a corresponding position to
that above, and at an equal distance (fig. 37.). Then a paper sheath or
screen being interposed as before, and the plate revolved, the poles were
found entirely indifferent to its motion, although either of them alone
would have followed the course of rotation.

246. On turning one magnet round, so that _opposite_ poles were on each
side of the plate, then the mutual action of the poles and the moving metal
was a maximum.

247. On suspending one magnet so that its axis was level with the plate,
and either pole opposite its edge, the revolution of the plate caused no
motion of the magnet. The electrical currents dependent upon induction
would now tend to be produced in a vertical direction across the thickness
of the plate, but could not be so discharged, or at least only to so slight
a degree as to leave all effects insensible; but ordinary magnetic
induction, or that on an iron plate, would be equally if not more
powerfully developed in such a position (251.).

248. Then, with regard to the production of electricity in these
cases:--whenever motion was communicated by the plate to the magnets,
currents existed; when it was not communicated, they ceased. A marked pole
of a large bar magnet was put under the edge of the plate; collectors (86.)
applied at the axis and edge of the plate as on former occasions (fig.
38.), and these connected with the galvanometer; when the plate was
revolved, abundance of electricity passed to the instrument. The unmarked
pole of a similar magnet was then put over the place of the former pole, so
that contrary poles were above and below; on revolving the plate, the
electricity was more powerful than before. The latter magnet was then
turned end for end, so that marked poles were both above and below the
plate, and then, upon revolving it, scarcely any electricity was procured.
By adjusting the distance of the poles so as to correspond with their
relative force, they at last were brought so perfectly to neutralize each
other's inductive action upon the plate, that no electricity could be
obtained with the most rapid motion.

249. I now proceeded to compare the effect of similar and dissimilar poles
upon iron and copper, adopting for the purpose Mr. Sturgeon's very useful
form of Arago's experiment. This consists in a circular plate of metal
supported in a vertical plane by a horizontal axis, and weighted a little
at one edge or rendered excentric so as to vibrate like a pendulum. The
poles of the magnets are applied near the side and edges of these plates,
and then the number of vibrations, required to reduce the vibrating arc a
certain constant quantity, noted. In the first description of this
instrument[A] it is said that opposite poles produced the greatest
retarding effect, and similar poles none; and yet within a page of the
place the effect is considered as of the same kind with that produced in
iron.

  [A] Edin. Phil. Journal, 1825, p. 124.

250. I had two such plates mounted, one of copper, one of iron. The copper
plate alone gave sixty vibrations, in the average of several experiments,
before the arc of vibration was reduced from one constant mark to another.
On placing opposite magnetic poles near to, and on each side of, the same
place, the vibrations were reduced to fifteen. On putting similar poles on
each side of it, they rose to fifty; and on placing two pieces of wood of
equal size with the poles equally near, they became fifty-two. So that,
when similar poles were used, the magnetic effect was little or none, (the
obstruction being due to the confinement of the air, rather,) whilst with
opposite poles it was the greatest possible. When a pole was presented to
the edge of the plate, no retardation occurred.

251. The iron plate alone made thirty-two vibrations, whilst the arc of
vibration diminished a certain quantity. On presenting a magnetic pole to
the edge of the plate (247.), the vibrations were diminished to eleven; and
when the pole was about half an inch from the edge, to five.

252. When the marked pole was put at the side of the iron plate at a
certain distance, the number of vibrations was only five. When the marked
pole of the second bar was put on the opposite side of the plate at the
same distance (250.), the vibrations were reduced to two. But when the
second pole was an unmarked one, yet occupying exactly the same position,
the vibrations rose to twenty-two. By removing the stronger of these two
opposite poles a little way from the plate, the vibrations increased to
thirty-one, or nearly the original number. But on removing it _altogether_,
they fell to between five and six.

253. Nothing can be more clear, therefore, than that with iron, and bodies
admitting of ordinary magnetic induction, _opposite_ poles on opposite
sides of the edge of the plate neutralize each other's effect, whilst
_similar_ poles exalt the action; a single pole end on is also sufficient.
But with copper, and substances not sensible to ordinary magnetic
impressions, _similar_ poles on opposite sides of the plate neutralize each
other; _opposite_ poles exalt the action; and a single pole at the edge or
end on does nothing.

254. Nothing can more completely show the thorough independence of the
effects obtained with the metals by Arago, and those due to ordinary
magnetic forces; and henceforth, therefore, the application of two poles to
various moving substances will, if they appear at all magnetically
affected, afford a proof of the nature of that affection. If opposite poles
produce a greater effect than one pole, the result will be due to electric
currents. If similar poles produce more effect than one, then the power is
_not_ electrical; it is not like that active in the metals and carbon when
they are moving, and in most cases will probably be found to be not even
magnetical, but the result of irregular causes not anticipated and
consequently not guarded against.

255. The result of these investigations tends to show that there are really
but very few bodies that are magnetic in the manner of iron. I have often
sought for indications of this power in the common metals and other
substances; and once in illustration of Arago's objection (82.), and in
hopes of ascertaining the existence of currents in metals by the momentary
approach of a magnet, suspended a disc of copper by a single fibre of silk
in an excellent vacuum, and approximated powerful magnets on the outside of
the jar, making them approach and recede in unison with a pendulum that
vibrated as the disc would do: but no motion could be obtained; not merely,
no indication of ordinary magnetic powers, but none or _any electric
current_ occasioned in the metal by the approximation and recession of the
magnet. I therefore venture to arrange substances in three classes as
regards their relation to magnets; first, those which are affected when at
rest, like iron, nickel, &c., being such as possess ordinary magnetic
properties; then, those which are affected when in motion, being conductors
of electricity in which are produced electric currents by the inductive
force of the magnet; and, lastly, those which are perfectly indifferent to
the magnet, whether at rest or in motion.

256. Although it will require further research, and probably close
investigation, both experimental and mathematical, before the exact mode of
action between a magnet and metal moving relatively to each other is
ascertained; yet many of the results appear sufficiently clear and simple
to allow of expression in a somewhat general manner.--If a terminated wire
move so as to cut a magnetic curve, a power is called into action which
tends to urge an electric current through it; but this current cannot be
brought into existence unless provision be made at the ends of the wire for
its discharge and renewal.

257. If a second wire move in the same direction as the first, the same
power is exerted upon it, and it is therefore unable to alter the condition
of the first: for there appear to be no natural differences among
substances when connected in a series, by which, when moving under the same
circumstances relative to the magnet, one tends to produce a more powerful
electric current in the whole circuit than another (201. 214.).

258. But if the second wire move with a different velocity, or in some
other direction, then variations in the force exerted take place; and if
connected at their extremities, an electric current passes through them.

259. Taking, then, a mass of metal or an endless wire, and referring to the
pole of the magnet as a centre of action, (which though perhaps not
strictly correct may be allowed for facility of expression, at present,) if
all parts move in the same direction, and with the same angular velocity,
and through magnetic curves of constant intensity, then no electric
currents are produced. This point is easily observed with masses subject to
the earth's magnetism, and may be proved with regard to small magnets; by
rotating them, and leaving the metallic arrangements stationary, no current
is produced.

260. If one part of the wire or metal cut the magnetic curves, whilst the
other is stationary, then currents are produced. All the results obtained
with the galvanometer are more or less of this nature, the galvanometer
extremity being the fixed part. Even those with the wire, galvanometer, and
earth (170.), may be considered so without any error in the result.

261. If the motion of the metal be in the same direction, but the angular
velocity of its parts relative to the pole of the magnet different, then
currents are produced. This is the case in Arago's experiment, and also in
the wire subject to the earth's induction (172.), when it was moved from
west to east.

262. If the magnet moves not directly to or from the arrangement, but
laterally, then the case is similar to the last.

263. If different parts move in opposite directions across the magnetic
curves, then the effect is a maximum for equal velocities.

264. All these in fact are variations of one simple condition, namely, that
all parts of the mass shall not move in the same direction across the
curves, and with the same angular velocity. But they are forms of
expression which, being retained in the mind, I have found useful when
comparing the consistency of particular phenomena with general results.

_Royal Institution,
December 21, 1831._




THIRD SERIES.


§ 7. _Identity of Electricities derived from different sources._ § 8.
_Relation by measure of common and voltaic Electricity._

[Read January 10th and 17th, 1833.]


§ 7. _Identity of Electricities derived from different sources._


265. The progress of the electrical researches which I have had the honour
to present to the Royal Society, brought me to a point at which it was
essential for the further prosecution of my inquiries that no doubt should
remain of the identity or distinction of electricities excited by different
means. It is perfectly true that Cavendish[A], Wollaston[B], Colladon[C],
and others, have in succession removed some of the greatest objections to
the acknowledgement of the identity of common, animal and voltaic
electricity, and I believe that most philosophers consider these
electricities as really the same. But on the other hand it is also true,
that the accuracy of Wollaston's experiments has been denied[D]; and also
that one of them, which really is no proper proof of chemical decomposition
by common electricity (309. 327.), has been that selected by several
experimenters as the test of chemical action (336. 346.). It is a fact,
too, that many philosophers are still drawing distinctions between the
electricities from different sources; or at least doubting whether their
identity is proved. Sir Humphry Davy, for instance, in his paper on the
Torpedo[E], thought it probable that animal electricity would be found of a
peculiar kind; and referring to it, to common electricity, voltaic
electricity and magnetism, has said, "Distinctions might be established in
pursuing the various modifications or properties of electricity in those
different forms, &c." Indeed I need only refer to the last volume of the
Philosophical Transactions to show that the question is by no means
considered as settled[F].

  [A] Phil. Trans. 1779, p. 196.

  [B] Ibid. 1801, p. 434.

  [C] Annnles de Chimie, 1826, p. 62, &c.

  [D] Phil. Trans. 1832, p. 282, note.

  [E] Phil. Trans. 1892, p. 17.

  "Common electricity is excited upon non-conductors, and is readily
  carried off by conductors and imperfect conductors. Voltaic
  electricity is excited upon combinations of perfect and imperfect
  conductors, and is only transmitted by perfect conductors or imperfect
  conductors of the best kind. Magnetism, if it be a form of
  electricity, belongs only to perfect conductors; and, in its
  modifications, to a peculiar class of them[1]. Animal electricity
  resides only in the imperfect conductors forming the organs of living
  animals, &c."

    [1] Dr. Ritchie has shown this is not the case. Phil. Trans. 1832, p.
    294.

  [F] Phil. Trans. 1832, p. 259. Dr. Davy, in making experiments on the
  torpedo, obtains effects the same as those produced by common and
  voltaic electricity, and says that in its magnetic and chemical power
  it does not seem to be essentially peculiar,--p. 274; but he then
  says, p. 275, there are other points of difference; and after
  referring to them, adds, "How are these differences to be explained?
  Do they admit of explanation similar to that advanced by Mr. Cavendish
  in his theory of the torpedo; or may we suppose, according to the
  analogy of the solar ray, that the electrical power, whether excited
  by the common machine, or by the voltaic battery, or by the torpedo,
  is not a simple power, but a combination of powers, which may occur
  variously associated, and produce all the varieties of electricity
  with which we are acquainted?"

At p. 279 of the same volume of Transactions is Dr. Ritchie's paper,
from which the following are extracts: "Common electricity is diffused
over the surface of the metal;--voltaic electricity exists within the
metal. Free electricity is conducted over the surface of the thinnest
gold leaf as effectually as over a mass of metal having the same
surface;--voltaic electricity requires thickness of metal for its
conduction," p. 280: and again, "The supposed analogy between common and
voltaic electricity, which was so eagerly traced after the invention of
the pile, completely fails in this case, which was thought to afford the
most striking resemblance." p. 291.

266. Notwithstanding, therefore, the general impression of the identity of
electricities, it is evident that the proofs have not been sufficiently
clear and distinct to obtain the assent of all those who were competent to
consider the subject; and the question seemed to me very much in the
condition of that which Sir H. Davy solved so beautifully,--namely, whether
voltaic electricity in all cases merely eliminated, or did not in some
actually produce, the acid and alkali found after its action upon water.
The same necessity that urged him to decide the doubtful point, which
interfered with the extension of his views, and destroyed the strictness of
his reasoning, has obliged me to ascertain the identity or difference of
common and voltaic electricity. I have satisfied myself that they are
identical, and I hope the experiments which I have to offer and the proofs
flowing from them, will be found worthy the attention of the Royal Society.

267. The various phenomena exhibited by electricity may, for the purposes
of comparison, be arranged under two heads; namely, those connected with
electricity of tension, and those belonging to electricity in motion. This
distinction is taken at present not as philosophical, but merely as
convenient. The effect of electricity of tension, at rest, is either
attraction or repulsion at sensible distances. The effects of electricity
in motion or electrical currents may be considered as 1st, Evolution of
heat; 2nd, Magnetism; 3rd, Chemical decomposition; 4th, Physiological
phenomena; 5th, Spark. It will be my object to compare electricities from
different sources, and especially common and voltaic electricities, by
their power of producing these effects.

I. _Voltaic Electricity._

268. _Tension._--When a voltaic battery of 100 pairs of plates has its
extremities examined by the ordinary electrometer, it is well known that
they are found positive and negative, the gold leaves at the same extremity
repelling each other, the gold leaves at different extremities attracting
each other, even when half an inch or more of air intervenes.

269. That ordinary electricity is discharged by points with facility
through air; that it is readily transmitted through highly rarefied air;
and also through heated air, as for instance a flame; is due to its high
tension. I sought, therefore, for similar effects in the discharge of
voltaic electricity, using as a test of the passage of the electricity
either the galvanometer or chemical action produced by the arrangement
hereafter to be described (312. 316.).

270. The voltaic battery I had at my disposal consisted of 140 pairs of
plates four inches square, with double coppers. It was insulated
throughout, and diverged a gold leaf electrometer about one third of an
inch. On endeavouring to discharge this battery by delicate points very
nicely arranged and approximated, either in the air or in an exhausted
receiver, I could obtain no indications of a current, either by magnetic or
chemical action. In this, however, was found no point of discordance
between voltaic and common electricity; for when a Leyden battery (291.)
was charged so as to deflect the gold leaf electrometer to the same degree,
the points were found equally unable to discharge it with such effect as to
produce either magnetic or chemical action. This was not because common
electricity could not produce both these effects (307. 310.); but because
when of such low intensity the quantity required to make the effects
visible (being enormously great (371. 375.),) could not be transmitted in
any reasonable time. In conjunction with the other proofs of identity
hereafter to be given, these effects of points also prove identity instead
of difference between voltaic and common electricity.

271. As heated air discharges common electricity with far greater facility
than points, I hoped that voltaic electricity might in this way also be
discharged. An apparatus was therefore constructed (Plate III. fig. 46.),
in which AB is an insulated glass rod upon which two copper wires, C, D,
are fixed firmly; to these wires are soldered two pieces of fine platina
wire, the ends of which are brought very close to each other at _e_, but
without touching; the copper wire C was connected with the positive pole of
a voltaic battery, and the wire D with a decomposing apparatus (312. 316.),
from which the communication was completed to the negative pole of the
battery. In these experiments only two troughs, or twenty pairs of plates,
were used.

272. Whilst in the state described, no decomposition took place at the
point _a_, but when the side of a spirit-lamp flame was applied to the two
platina extremities at _e_, so as to make them bright red-hot,
decomposition occurred; iodine soon appeared at the point _a_, and the
transference of electricity through the heated air was established. On
raising the temperature of the points _e_ by a blowpipe, the discharge was
rendered still more free, and decomposition took place instantly. On
removing the source of heat, the current immediately ceased. On putting the
ends of the wires very close by the side of and parallel to each other, but
not touching, the effects were perhaps more readily obtained than before.
On using a larger voltaic battery (270.), they were also more freely
obtained.

273. On removing the decomposing apparatus and interposing a galvanometer
instead, heating the points _e_ as the needle would swing one way, and
removing the heat during the time of its return (302.), feeble deflections
were soon obtained: thus also proving the current through heated air; but
the instrument used was not so sensible under the circumstances as chemical
action.

274. These effects, not hitherto known or expected under this form, are
only cases of the discharge which takes place through air between the
charcoal terminations of the poles of a powerful battery, when they are
gradually separated after contact. Then the passage is through heated air
exactly as with common electricity, and Sir H. Davy has recorded that with
the original battery of the Royal Institution this discharge passed through
a space of at least four inches[A]. In the exhausted receiver the
electricity would _strike_ through nearly half an inch of space, and the
combined effects of rarefaction and heat were such upon the inclosed air us
to enable it to conduct the electricity through a space of six or seven
inches.

  [A] Elements of Chemical Philosophy, p. 153

275. The instantaneous charge of a Leyden battery by the poles of a voltaic
apparatus is another proof of the tension, and also the quantity, of
electricity evolved by the latter. Sir H. Davy says[A], "When the two
conductors from the ends of the combination were connected with a Leyden
battery, one with the internal, the other with the external coating, the
battery instantly became charged; and on removing the wires and making the
proper connexions, either a shock or a _spark_ could be perceived: and the
least possible time of contact was sufficient to renew the charge to its
full intensity."

  [A] Elements of Chemical Philosophy, p. 154.

276. _In motion:_ i. _Evolution of Heat._--The evolution of heat in wires
and fluids by the voltaic current is matter of general notoriety.

277. ii. _Magnetism._--No fact is better known to philosophers than the
power of the voltaic current to deflect the magnetic needle, and to make
magnets according to _certain laws_; and no effect can be more distinctive
of an electrical current.

278. iii. _Chemical decomposition._--The chemical powers of the voltaic
current, and their subjection to _certain laws_, are also perfectly well
known.

279. iv. _Physiological effects._--The power of the voltaic current, when
strong, to shock and convulse the whole animal system, and when weak to
affect the tongue and the eyes, is very characteristic.

280. v. _Spark_.--The brilliant star of light produced by the discharge of
a voltaic battery is known to all as the most beautiful light that man can
produce by art.

       *       *       *       *       *

281. That these effects may be almost infinitely varied, some being exalted
whilst others are diminished, is universally acknowledged; and yet without
any doubt of the identity of character of the voltaic currents thus made to
differ in their effect. The beautiful explication of these variations
afforded by Cavendish's theory of quantity and intensity requires no
support at present, as it is not supposed to be doubted.

282. In consequence of the comparisons that will hereafter arise between
wires carrying voltaic and ordinary electricities, and also because of
certain views of the condition of a wire or any other conducting substance
connecting the poles of a voltaic apparatus, it will be necessary to give
some definite expression of what is called the voltaic current, in
contradistinction to any supposed peculiar state of arrangement, not
progressive, which the wire or the electricity within it may be supposed to
assume. If two voltaic troughs PN, P'N', fig. 42, be symmetrically arranged
and insulated, and the ends NP' connected by a wire, over which a magnetic
needle is suspended, the wire will exert no effect over the needle; but
immediately that the ends PN' are connected by another wire, the needle
will be deflected, and will remain so as long as the circuit is complete.
Now if the troughs merely act by causing a peculiar arrangement in the wire
either of its particles or its electricity, that arrangement constituting
its electrical and magnetic state, then the wire NP' should be in a similar
state of arrangement _before_ P and N' were connected, to what it is
afterwards, and should have deflected the needle, although less powerfully,
perhaps to one half the extent which would result when the communication is
complete throughout. But if the magnetic effects depend upon a current,
then it is evident why they could not be produced in _any_ degree before
the circuit was complete; because prior to that no current could exist.

283. By _current_, I mean anything progressive, whether it be a fluid of
electricity, or two fluids moving in opposite directions, or merely
vibrations, or, speaking still more generally, progressive forces. By
_arrangement_, I understand a local adjustment of particles, or fluids, or
forces, not progressive. Many other reasons might be urged in support of
the view of a _current_ rather than an _arrangement_, but I am anxious to
avoid stating unnecessarily what will occur to others at the moment.

II. _Ordinary Electricity._

284. By ordinary electricity I understand that which can be obtained from
the common machine, or from the atmosphere, or by pressure, or cleavage of
crystals, or by a multitude of other operations; its distinctive character
being that of great intensity, and the exertion of attractive and repulsive
powers, not merely at sensible but at considerable distances.

285. _Tension._ The attractions and repulsions at sensible distances,
caused by ordinary electricity, are well known to be so powerful in certain
cases, as to surpass, almost infinitely, the similar phenomena produced by
electricity, otherwise excited. But still those attractions and repulsions
are exactly of the same nature as those already referred to under the head
_Tension, Voltaic electricity_ (268.); and the difference in degree between
them is not greater than often occurs between cases of ordinary electricity
only. I think it will be unnecessary to enter minutely into the proofs of
the identity of this character in the two instances. They are abundant; are
generally admitted as good; and lie upon the surface of the subject: and
whenever in other parts of the comparison I am about to draw, a similar
case occurs, I shall content myself with a mere announcement of the
similarity, enlarging only upon those parts where the great question of
distinction or identity still exists.

286. The discharge of common electricity through heated air is a well-known
fact. The parallel case of voltaic electricity has already been described
(272, &c.).

287. _In motion._ i. _Evolution of heat._--The heating power of common
electricity, when passed through wires or other substances, is perfectly
well known. The accordance between it and voltaic electricity is in this
respect complete. Mr. Harris has constructed and described[A] a very
beautiful and sensible instrument on this principle, in which the heat
produced in a wire by the discharge of a small portion of common
electricity is readily shown, and to which I shall have occasion to refer
for experimental proof in a future part of this paper (344.).

  [A] Philosophical Transactions, 1827, p. 18. Edinburgh Transactions,
  1831. Harris on a New Electrometer, &c. &c.

288. ii. _Magnetism._--Voltaic electricity has most extraordinary and
exalted magnetic powers. If common electricity be identical with it, it
ought to have the same powers. In rendering needles or bars magnetic, it is
found to agree with voltaic electricity, and the _direction_ of the
magnetism, in both cases, is the same; but in deflecting the magnetic
needle, common electricity has been found deficient, so that sometimes its
power has been denied altogether, and at other times distinctions have been
hypothetically assumed for the purpose of avoiding the difficulty[A].

  [A] Demonferrand's Manuel d'Electricité dynamique, p. 121.

289. M. Colladon, of Geneva, considered that the difference might be due to
the use of insufficient quantities of common electricity in all the
experiments before made on this head; and in a memoir read to the Academie
des Sciences in 1826[A], describes experiments, in which, by the use of a
battery, points, and a delicate galvanometer, he succeeded in obtaining
deflections, and thus establishing identity in that respect. MM. Arago,
Ampère, and Savary, are mentioned in the paper as having witnessed a
successful repetition of the experiments. But as no other one has come
forward in confirmation, MM. Arago, Ampère, and Savary, not having
themselves published (that I am aware of) their admission of the results,
and as some have not been able to obtain them, M. Colladon's conclusions
have been occasionally doubted or denied; and an important point with me
was to establish their accuracy, or remove them entirely from the body of
received experimental research. I am happy to say that my results fully
confirm those by M. Colladon, and I should have had no occasion to describe
them, but that they are essential as proofs of the accuracy of the final
and general conclusions I am enabled to draw respecting the magnetic and
chemical action of electricity (360. 366. 367. 377. &c.).

  [A] Annales de Chimie, xxxiii. p. 62.

290. The plate electrical machine I have used is fifty inches in diameter;
it has two sets of rubbers; its prime conductor consists of two brass
cylinders connected by a third, the whole length being twelve feet, and the
surface in contact with air about 1422 square inches. When in good
excitation, one revolution of the plate will give ten or twelve sparks from
the conductors, each an inch in length. Sparks or flashes from ten to
fourteen inches in length may easily be drawn from the conductors. Each
turn of the machine, when worked moderately, occupies about 4/5ths of a
second.

291. The electric battery consisted of fifteen equal jars. They are coated
eight inches upwards from the bottom, and are twenty-three inches in
circumference, so that each contains one hundred and eighty-four square
inches of glass, coated on both sides; this is independent of the bottoms,
which are of thicker glass, and contain each about fifty square inches.

292. A good _discharging train_ was arranged by connecting metallically a
sufficiently thick wire with the metallic gas pipes of the house, with the
metallic gas pipes belonging to the public gas works of London; and also
with the metallic water pipes of London. It was so effectual in its office
as to carry off instantaneously electricity of the feeblest tension, even
that of a single voltaic trough, and was essential to many of the
experiments.

293. The galvanometer was one or the other of those formerly described (87.
205.), but the glass jar covering it and supporting the needle was coated
inside and outside with tinfoil, and the upper part (left uncoated, that
the motions of the needle might be examined,) was covered with a frame of
wire-work, having numerous sharp points projecting from it. When this frame
and the two coatings were connected with the discharging train (292.), an
insulated point or ball, connected with the machine when most active, might
be brought within an inch of any part of the galvanometer, yet without
affecting the needle within by ordinary electrical attraction or repulsion.

294. In connexion with these precautions, it may be necessary to state that
the needle of the galvanometer is very liable to have its magnetic power
deranged, diminished, or even inverted by the passage of a shock through
the instrument. If the needle be at all oblique, in the wrong direction, to
the coils of the galvanometer when the shock passes, effects of this kind
are sure to happen.

295. It was to the retarding power of bad conductors, with the intention of
diminishing its _intensity_ without altering its _quantity_, that I first
looked with the hope of being able to make common electricity assume more
of the characters and power of voltaic electricity, than it is usually
supposed to have.

296, The coating and armour of the galvanometer were first connected with
the discharging train (292.); the end B (87.) of the galvanometer wire was
connected with the outside coating of the battery, and then both these with
the discharging train; the end A of the galvanometer wire was connected
with a discharging rod by a wet thread four feet long; and finally, when
the battery (291.) had been positively charged by about forty turns of the
machine, it was discharged by the rod and the thread through the
galvanometer. The needle immediately moved.

297. During the time that the needle completed its vibration in the first
direction and returned, the machine was worked, and the battery recharged;
and when the needle in vibrating resumed its first direction, the discharge
was again made through the galvanometer. By repeating this action a few
times, the vibrations soon extended to above 40° on each side of the line
of rest.

298. This effect could be obtained at pleasure. Nor was it varied,
apparently, either in direction or degree, by using a short thick string,
or even four short thick strings in place of the long fine thread. With a
more delicate galvanometer, an excellent swing of the needle could be
obtained by one discharge of the battery.

299. On reversing the galvanometer communications so as to pass the
discharge through from B to A, the needle was equally well deflected, but
in the opposite direction.

300. The deflections were in the same direction as if a voltaic current had
been passed through the galvanometer, i.e. the positively charged surface
of the electric battery coincided with the positive end of the voltaic
apparatus (268.) and the negative surface of the former with the negative
end of the latter.

301. The battery was then thrown out of use, and the communications so
arranged that the current could be passed from the prime conductor, by the
discharging rod held against it, through the wet string, through the
galvanometer coil, and into the discharging train (292), by which it was
finally dispersed. This current could be stopped at any moment, by removing
the discharging rod, and either stopping the machine or connecting the
prime conductor by another rod with the discharging train; and could be as
instantly renewed. The needle was so adjusted, that whilst vibrating in
moderate and small arcs, it required time equal to twenty-five beats of a
watch to pass in one direction through the arc, and of course an equal time
to pass in the other direction.

302. Thus arranged, and the needle being stationary, the current, direct
from the machine, was sent through the galvanometer for twenty-five beats,
then interrupted for other twenty-five beats, renewed for twenty-five beats
more, again interrupted for an equal time, and so on continually. The
needle soon began to vibrate visibly, and after several alternations of
this kind, the vibration increased to 40° or more.

303. On changing the direction of the current through the galvanometer, the
direction of the deflection of the needle was also changed. In all cases
the motion of the needle was in direction the same as that caused either by
the use of the electric battery or a voltaic trough (300).

304. I now rejected the wet string, and substituted a copper wire, so that
the electricity of the machine passed at once into wires communicating
directly with the discharging train, the galvanometer coil being one of the
wires used for the discharge. The effects were exactly those obtained above
(302).

305. Instead of passing the electricity through the system, by bringing the
discharging rod at the end of it into contact with the conductor, four
points were fixed on to the rod; when the current was to pass, they were
held about twelve inches from the conductor, and when it was not to pass,
they were turned away. Then operating as before (302.), except with this
variation, the needle was soon powerfully deflected, and in perfect
consistency with the former results. Points afforded the means by which
Colladon, in all cases, made his discharges.

306. Finally, I passed the electricity first through an exhausted receiver,
so as to make it there resemble the aurora borealis, and then through the
galvanometer to the earth; and it was found still effective in deflecting
the needle, and apparently with the same force as before.

307. From all these experiments, it appears that a current of common
electricity, whether transmitted through water or metal, or rarefied air,
or by means of points in common air, is still able to deflect the needle;
the only requisite being, apparently, to allow time for its action: that it
is, in fact, just as magnetic in every respect as a voltaic current, and
that in this character therefore no distinction exists.

308. Imperfect conductors, as water, brine, acids, &c. &c. will be found
far more convenient for exhibiting these effects than other modes of
discharge, as by points or balls; for the former convert at once the charge
of a powerful battery into a feeble spark discharge, or rather continuous
current, and involve little or no risk of deranging the magnetism of the
needles (294.).

309. iii. _Chemical decomposition._--The chemical action of voltaic
electricity is characteristic of that agent, but not more characteristic
than are the _laws_ under which the bodies evolved by decomposition arrange
themselves at the poles. Dr. Wollaston showed[A] that common electricity
resembled it in these effects, and "that they are both essentially the
same"; but he mingled with his proofs an experiment having a resemblance,
and nothing more, to a case of voltaic decomposition, which however he
himself partly distinguished; and this has been more frequently referred to
by some, on the one hand, to prove the occurrence of electro-chemical
decomposition, like that of the pile, and by others to throw doubt upon the
whole paper, than the more numerous and decisive experiments which he has
detailed.

  [A] Philosophical Transactions, 1801, pp. 427, 434.

310. I take the liberty of describing briefly my results, and of thus
adding my testimony to that of Dr. Wollaston on the identity of voltaic and
common electricity as to chemical action, not only that I may facilitate
the repetition of the experiments, but also lead to some new consequences
respecting electrochemical decomposition (376. 377.).

311. I first repeated Wollaston's fourth experiment[A], in which the ends
of coated silver wires are immersed in a drop of sulphate of copper. By
passing the electricity of the machine through such an arrangement, that
end in the drop which received the electricity became coated with metallic
copper. One hundred turns of the machine produced an evident effect; two
hundred turns a very sensible one. The decomposing action was however very
feeble. Very little copper was precipitated, and no sensible trace of
silver from the other pole appeared in the solution.

  [A] Philosophical Transactions, 1801, p. 429.

312. A much more convenient and effectual arrangement for chemical
decompositions by common electricity, is the following. Upon a glass plate,
fig. 43, placed over, but raised above a piece of white paper, so that
shadows may not interfere, put two pieces of tinfoil _a, b_; connect one of
these by an insulated wire _c_, or wire and string (301.) with the machine,
and the other _g_, with the discharging train (292.) or the negative
conductor; provide two pieces of fine platina wire, bent as in fig. 44, so
that the part _d, f_ shall be nearly upright, whilst the whole is resting
on the three bearing points _p, e, f_ place these as in fig. 43; the points
_p, n_ then become the decomposing poles. In this way surfaces of contact,
as minute as possible, can be obtained at pleasure, and the connexion can
be broken or renewed in a moment, and the substances acted upon examined
with the utmost facility.

313. A coarse line was made on the glass with solution of sulphate of
copper, and the terminations _p_ and _n_ put into it; the foil _a_ was
connected with the positive conductor of the machine by wire and wet
string, so that no sparks passed: twenty turns of the machine caused the
precipitation of so much copper on the end _n_, that it looked like copper
wire; no apparent change took place at _p_.

314. A mixture of equal parts of muriatic acid and water was rendered deep
blue by sulphate of indigo, and a large drop put on the glass, fig. 43, so
that _p_ and _n_ were immersed at opposite sides: a single turn of the
machine showed bleaching effects round _p_, from evolved chlorine. After
twenty revolutions no effect of the kind was visible at _n_, but so much
chlorine had been set free at _p_, that when the drop was stirred the whole
became colourless.

315. A drop of solution of iodide of potassium mingled with starch was put
into the same position at _p_ and _n_; on turning the machine, iodine was
evolved at _p_, but not at _n_.

316. A still further improvement in this form of apparatus consists in
wetting a piece of filtering paper in the solution to be experimented on,
and placing that under the points _p_ and _n_, on the glass: the paper
retains the substance evolved at the point of evolution, by its whiteness
renders any change of colour visible, and allows of the point of contact
between it and the decomposing wires being contracted to the utmost degree.
A piece of paper moistened in the solution of iodide of potassium and
starch, or of the iodide alone, with certain precautions (322.), is a most
admirable test of electro-chemical action; and when thus placed and acted
upon by the electric current, will show iodine evolved at _p_ by only half
a turn of the machine. With these adjustments and the use of iodide of
potassium on paper, chemical action is sometimes a more delicate test of
electrical currents than the galvanometer (273.). Such cases occur when the
bodies traversed by the current are bad conductors, or when the quantity of
electricity evolved or transmitted in a given time is very small.

317. A piece of litmus paper moistened in solution of common salt or
sulphate of soda, was quickly reddened at _p_. A similar piece moistened in
muriatic acid was very soon bleached at _p_. No effects of a similar kind
took place at _n_.

318. A piece of turmeric paper moistened in solution of sulphate of soda
was reddened at _n_ by two or three turns of the machine, and in twenty or
thirty turns plenty of alkali was there evolved. On turning the paper
round, so that the spot came under _p_, and then working the machine, the
alkali soon disappeared, the place became yellow, and a brown alkaline spot
appeared in the new part under _n_.

319. On combining a piece of litmus with a piece of turmeric paper, wetting
both with solution of sulphate of soda, and putting the paper on the glass,
so that _p_ was on the litmus and _n_ on the turmeric, a very few turns of
the machine sufficed to show the evolution of acid at the former and alkali
at the latter, exactly in the manner effected by a volta-electric current.

320. All these decompositions took place equally well, whether the
electricity passed from the machine to the foil _a_, through water, or
through wire only; by _contact_ with the conductor, or by _sparks_ there;
provided the sparks were not so large as to cause the electricity to pass
in sparks from _p_ to _n_, or towards _n_; and I have seen no reason to
believe that in cases of true electro-chemical decomposition by the
machine, the electricity passed in sparks from the conductor, or at any
part of the current, is able to do more, because of its tension, than that
which is made to pass merely as a regular current.

321. Finally, the experiment was extended into the following form,
supplying in this case the tidiest analogy between common and voltaic
electricity. Three compound pieces of litmus and turmeric paper (319.) were
moistened in solution of sulphate of soda, and arranged on a plate of glass
with platina wires, as in fig. 45. The wire _m_ was connected with the
prime conductor of the machine, the wire _t_ with the discharging train,
and the wires _r_ and _s_ entered into the course of the electrical current
by means of the pieces of moistened paper; they were so bent as to rest
each on three points, _n, r, p; n, s, p_, the points _r_ and _s_ being
supported by the glass, and the others by the papers; the three
terminations _p, p, p_ rested on the litmus, and the other three _n, n, n_
on the turmeric paper. On working the machine for a short time only, acid
was evolved at _all_ the poles or terminations _p, p, p_, by which the
electricity entered the solution, and alkali at the other poles _n, n, n_,
by which the electricity left the solution.

322. In all experiments of electro-chemical decomposition by the common
machine and moistened papers (316.), it is necessary to be aware of and to
avoid the following important source of error. If a spark passes over
moistened litmus and turmeric paper, the litmus paper (provided it be
delicate and not too alkaline,) is reddened by it; and if several sparks
are passed, it becomes powerfully reddened. If the electricity pass a
little way from the wire over the surface of the moistened paper, before it
finds mass and moisture enough to conduct it, then the reddening extends as
far as the ramifications. If similar ramifications occur at the termination
_n_, on the turmeric paper, they _prevent_ the occurrence of the red spot
due to the alkali, which would otherwise collect there: sparks or
ramifications from the points _n_ will also redden litmus paper. If paper
moistened by a solution of iodide of potassium (which is an admirably
delicate test of electro-chemical action,) be exposed to the sparks or
ramifications, or even a feeble stream of electricity through the air from
either the point _p_ or _n_, iodine will be immediately evolved.

323. These effects must not be confounded with those due to the true
electro-chemical powers of common electricity, and must be carefully
avoided when the latter are to be observed. No sparks should be passed,
therefore, in any part of the current, nor any increase of intensity
allowed, by which the electricity may be induced to pass between the
platina wires and the moistened papers, otherwise than by conduction; for
if it burst through the air, the effect referred to above (322.) ensues.

324. The effect itself is due to the formation of nitric acid by the
combination of the oxygen and nitrogen of the air, and is, in fact, only a
delicate repetition of Cavendish's beautiful experiment. The acid so
formed, though small in quantity, is in a high state of concentration as to
water, and produces the consequent effects of reddening the litmus paper;
or preventing the exhibition of alkali on the turmeric paper; or, by acting
on the iodide of potassium, evolving iodine.

325. By moistening a very small slip of litmus paper in solution of caustic
potassa, and then passing the electric spark over its length in the air, I
gradually neutralized the alkali, and ultimately rendered the paper red; on
drying it, I found that nitrate of potassa had resulted from the operation,
and that the paper had become touch-paper.

326. Either litmus paper or white paper, moistened in a strong solution of
iodide of potassium, offers therefore a very simple, beautiful, and ready
means of illustrating Cavendish's experiment of the formation of nitric
acid from the atmosphere.

327. I have already had occasion to refer to an experiment (265. 309.) made
by Dr. Wollaston, which is insisted upon too much, both by those who oppose
and those who agree with the accuracy of his views respecting the identity
of voltaic and ordinary electricity. By covering fine wires with glass or
other insulating substances, and then removing only so much matter as to
expose the point, or a section of the wires, and by passing electricity
through two such wires, the guarded points of which were immersed in water,
Wollaston found that the water could be decomposed even by the current from
the machine, without sparks, and that two streams of gas arose from the
points, exactly resembling, in appearance, those produced by voltaic
electricity, and, like the latter, giving a mixture of oxygen and hydrogen
gases. But Dr. Wollaston himself points out that the effect is different
from that of the voltaic pile, inasmuch as both oxygen and hydrogen are
evolved from _each_ pole; he calls it "a very close _imitation_ of the
galvanic phenomena," but adds that "in fact the resemblance is not
complete," and does not trust to it to establish the principles correctly
laid down in his paper.

328. This experiment is neither more nor less than a repetition, in a
refined manner, of that made by Dr. Pearson in 1797[A], and previously by
MM. Paets Van Troostwyk and Deiman in 1789 or earlier. That the experiment
should never be quoted as proving true electro-chemical decomposition, is
sufficiently evident from the circumstance, that the _law_ which regulates
the transference and final place of the evolved bodies (278. 309.) has no
influence here. The water is decomposed at both poles independently of each
other, and the oxygen and hydrogen evolved at the wires are the elements of
the water existing the instant before in those places. That the poles, or
rather points, have no mutual decomposing dependence, may be shown by
substituting a wire, or the finger, for one of them, a change which does
not at all interfere with the other, though it stops all action at the
changed pole. This fact may be observed by turning the machine for some
time; for though bubbles will rise from the point left unaltered, in
quantity sufficient to cover entirely the wire used for the other
communication, if they could be applied to it, yet not a single bubble will
appear on that wire.

  [A] Nicholson's Journal, 4to. vol. I. pp. 311, 299. 349.

329. When electro-chemical decomposition takes place, there is great reason
to believe that the _quantity_ of matter decomposed is not proportionate to
the intensity, but to the quantity of electricity passed (320.). Of this I
shall be able to offer some proofs in a future part of this paper (375.
377.). But in the experiment under consideration, this is not the case. If,
with a constant pair of points, the electricity be passed from the machine
in sparks, a certain proportion of gas is evolved; but if the sparks be
rendered shorter, less gas is evolved; and if no sparks be passed, there is
scarcely a sensible portion of gases set free. On substituting solution of
sulphate of soda for water, scarcely a sensible quantity of gas could be
procured even with powerful sparks, and nearly none with the mere current;
yet the quantity of electricity in a given time was the same in all these
cases.

330. I do not intend to deny that with such an apparatus common electricity
can decompose water in a manner analogous to that of the voltaic pile; I
believe at present that it can. But when what I consider the true effect
only was obtained, the quantity of gas given off was so small that I could
not ascertain whether it was, as it ought to be, oxygen at one wire and
hydrogen at the other. Of the two streams one seemed more copious than the
other, and on turning the apparatus round, still the same side in relation
to the machine; gave the largest stream. On substituting solution of
sulphate of soda for pure water (329.), these minute streams were still
observed. But the quantities were so small, that on working the machine for
half an hour I could not obtain at either pole a bubble of gas larger than
a small grain of sand. If the conclusion which I have drawn (377.) relating
to the amount of chemical action be correct, this ought to be the case.

331. I have been the more anxious to assign the true value of this
experiment as a test of electro-chemical action, because I shall have
occasion to refer to it in cases of supposed chemical action by
magneto-electric and other electric currents (336. 346.) and elsewhere.
But, independent of it, there cannot be now a doubt that Dr. Wollaston was
right in his general conclusion; and that voltaic and common electricity
have powers of chemical decomposition, alike in their nature, and governed
by the same law of arrangement.

332. iv. _Physiological effects._--The power of the common electric current
to shock and convulse the animal system, and when weak to affect the tongue
and the eyes, may be considered as the same with the similar power of
voltaic electricity, account being taken of the intensity of the one
electricity and duration of the other. When a wet thread was interposed in
the course of the current of common electricity from the battery (291.)
charged by eight or ten[A] revolutions of the machine in good action
(290.), and the discharge made by platina spatulas through the tongue or
the gums, the effect upon the tongue and eyes was exactly that of a
momentary feeble voltaic circuit.

  [A] Or even from thirty to forty.

333. v. _Spark._--The beautiful flash of light attending the discharge of
common electricity is well known. It rivals in brilliancy, if it does not
even very much surpass, the light from the discharge of voltaic
electricity; but it endures for an instant only, and is attended by a sharp
noise like that of a small explosion. Still no difficulty can arise in
recognising it to be the same spark as that from the voltaic battery,
especially under certain circumstances. The eye cannot distinguish the
difference between a voltaic and a common electricity spark, if they be
taken between amalgamated surfaces of metal, at intervals only, and through
the same distance of air.

334. When the Leyden battery (291.) was discharged through a wet string
placed in some part of the circuit away from the place where the spark was
to pass, the spark was yellowish, flamy, having a duration sensibly longer
than if the water had not been interposed, was about three-fourths of an
inch in length, was accompanied by little or no noise, and whilst losing
part of its usual character had approximated in some degree to the voltaic
spark. When the electricity retarded by water was discharged between pieces
of charcoal, it was exceedingly luminous and bright upon both surfaces of
the charcoal, resembling the brightness of the voltaic discharge on such
surfaces. When the discharge of the unretarded electricity was taken upon
charcoal, it was bright upon both the surfaces, (in that respect resembling
the voltaic spark,) but the noise was loud, sharp, and ringing.

335. I have assumed, in accordance, I believe, with the opinion of every
other philosopher, that atmospheric electricity is of the same nature with
ordinary electricity (284.), and I might therefore refer to certain
published statements of chemical effects produced by the former as proofs
that the latter enjoys the power of decomposition in common with voltaic
electricity. But the comparison I am drawing is far too rigorous to allow
me to use these statements without being fully assured of their accuracy;
yet I have no right to suppress them, because, if accurate, they establish
what I am labouring to put on an undoubted foundation, and have priority to
my results.

336. M. Bonijol of Geneva[A] is said to have constructed very delicate
apparatus for the decomposition of water by common electricity. By
connecting an insulated lightning rod with his apparatus, the decomposition
of the water proceeded in a continuous and rapid manner even when the
electricity of the atmosphere was not very powerful. The apparatus is not
described; but as the diameter of the wire is mentioned as very small, it
appears to have been similar in construction to that of Wollaston (327.);
and as that does not furnish a case of true polar electro-chemical
decomposition (328.), this result of M. Bonijol does not prove the identity
in chemical action of common and voltaic electricity.

  [A] Bibliothèque Universelle, 1830, tome xlv. p. 213.

337. At the same page of the Bibliothèque Universelle, M. Bonijol is said
to have decomposed, _potash_, and also chloride of silver, by putting them
into very narrow tubes and passing electric sparks from an ordinary machine
over them. It is evident that these offer no analogy to cases of true
voltaic decomposition, where the electricity only decomposes when it is
_conducted_ by the body acted upon, and ceases to decompose, according to
its ordinary laws, when it passes in sparks. These effects are probably
partly analogous to that which takes place with water in Pearson's or
Wollaston's apparatus, and may be due to very high temperature acting on
minute portions of matter; or they may be connected with the results in air
(322.). As nitrogen can combine directly with oxygen under the influence of
the electric spark (324.), it is not impossible that it should even take it
from the potassium of the potash, especially as there would be plenty of
potassa in contact with the acting particles to combine with the nitric
acid formed. However distinct all these actions may be from true polar
electro-chemical decompositions, they are still highly important, and
well-worthy of investigation.

338. The late Mr. Barry communicated a paper to the Royal Society[A] last
year, so distinct in the details, that it would seem at once to prove the
identity in chemical action of common and voltaic electricity; but, when
examined, considerable difficulty arises in reconciling certain of the
effects with the remainder. He used two tubes, each having a wire within it
passing through the closed end, as is usual for voltaic decompositions. The
tubes were filled with solution of sulphate of soda, coloured with syrup of
violets, and connected by a portion of the same solution, in the ordinary
manner; the wire in one tube was connected by a _gilt thread_ with the
string of an insulated electrical kite, and the wire in the other tube by a
similar _gilt thread_ with the ground. Hydrogen soon appeared in the tube
connected with the kite, and oxygen in the other, and in ten minutes the
liquid in the first tube was green from the alkali evolved, and that in the
other red from free acid produced. The only indication of the strength or
intensity of the atmospheric electricity is in the expression, "the usual
shocks were felt on touching the string."

  [A] Philosophical Transactions, 1831, p. 165.

339. That the electricity in this case does not resemble that from any
ordinary source of common electricity, is shown by several circumstances.
Wollaston could not effect the decomposition of water by such an
arrangement, and obtain the gases in _separate_ vessels, using common
electricity; nor have any of the numerous philosophers, who have employed
such an apparatus, obtained any such decomposition, either of water or of a
neutral salt, by the use of the machine. I have lately tried the large
machine (290.) in full action for a quarter of an hour, during which time
seven hundred revolutions were made, without producing any sensible
effects, although the shocks that it would then give must have been far
more powerful and numerous than could have been taken, with any chance of
safety, from an electrical kite-string; and by reference to the comparison
hereafter to be made (371.), it will be seen that for common electricity to
have produced the effect, the quantity must have been awfully great, and
apparently far more than could have been conducted to the earth by a gilt
thread, and at the same time only have produced the "usual shocks."

340. That the electricity was apparently not analogous to voltaic
electricity is evident, for the "usual shocks" only were produced, and
nothing like the terrible sensation due to a voltaic battery, even when it
has a tension so feeble as not to strike through the eighth of an inch of
air.

341. It seems just possible that the air which was passing by the kite and
string, being in an electrical state sufficient to produce the "usual
shocks" only, could still, when the electricity was drawn off below, renew
the charge, and so continue the current. The string was 1500 feet long, and
contained two double threads. But when the enormous quantity which must
have been thus collected is considered (371. 376.), the explanation seems
very doubtful. I charged a voltaic battery of twenty pairs of plates four
inches square with double coppers very strongly, insulated it, connected
its positive extremity with the discharging train (292.), and its negative
pole with an apparatus like that of Mr. Barry, communicating by a wire
inserted three inches into the wet soil of the ground. This battery thus
arranged produced feeble decomposing effects, as nearly as I could judge
answering the description Mr. Barry has given. Its intensity was, of
course, far lower than the electricity of the kite-string, but the supply
of quantity from the discharging train was unlimited. It gave no shocks to
compare with the "usual shocks" of a kite-string.

342. Mr. Barry's experiment is a very important one to repeat and verify.
If confirmed, it will be, as far as I am aware, the first recorded case of
true electro-chemical decomposition of water by common electricity, and it
will supply a form of electrical current, which, both in quantity and
intensity, is exactly intermediate with those of the common electrical
machine and the voltaic pile.

       *       *       *       *       *

III. _Magneto-Electricity._

343. _Tension_.--The attractions and repulsions due to the tension of
ordinary electricity have been well observed with that evolved by
magneto-electric induction. M. Pixii, by using an apparatus, clever in its
construction and powerful in its action[A], was able to obtain great
divergence of the gold leaves of an electrometer[B].

  [A] Annales de Chimie, l. p. 322.

  [B] Ibid. li. p 77.

344. _In motion_: i. _Evolution of Heat._--The current produced by
magneto-electric induction can heat a wire in the manner of ordinary
electricity. At the British Association of Science at Oxford, in June of
the present year, I had the pleasure, in conjunction with Mr. Harris,
Professor Daniell, Mr. Duncan, and others, of making an experiment, for
which the great magnet in the museum, Mr. Harris's new electrometer (287.),
and the magneto-electric coil described in my first paper (34.), were put
in requisition. The latter had been modified in the manner I have elsewhere
described[A] so as to produce an electric spark when its contact with the
magnet was made or broken. The terminations of the spiral, adjusted so as
to have their contact with each other broken when the spark was to pass,
were connected with the wire in the electrometer, and it was found that
each time the magnetic contact was made and broken, expansion of the air
within the instrument occurred, indicating an increase, at the moment, of
the temperature of the wire.

  [A] Phil, Mag. and Annals, 1832, vol. xi. p. 405.

315. ii. _Magnetism._--These currents were discovered by their magnetic
power.

346. iii. _Chemical decomposition._--I have made many endeavours to effect
chemical decomposition by magneto-electricity, but unavailingly. In July
last I received an anonymous letter (which has since been published[A],)
describing a magneto-electric apparatus, by which the decomposition of
water was effected. As the term "guarded points" is used, I suppose the
apparatus to have been Wollaston's (327. &c.), in which case the results
did not indicate polar electro-chemical decomposition. Signor Botto has
recently published certain results which he has obtained[B]; but they are,
as at present described, inconclusive. The apparatus he used was apparently
that of Dr. Wollaston, which gives only fallacious indications (327. &c.).
As magneto-electricity can produce sparks, it would be able to show the
effects proper to this apparatus. The apparatus of M. Pixii already
referred to (343.) has however, in the hands of himself[C] and M.
Hachctte[D], given decisive chemical results, so as to complete this link
in the chain of evidence. Water was decomposed by it, and the oxygen and
hydrogen obtained in separate tubes according to the law governing
volta-electric and machine-electric decomposition.

  [A] Lond. and Edinb. Phil. Mag. and Journ., 1832, vol. i. p. 161.

  [B] Ibid. 1832. vol. i. p. 441.

  [C] Annales de Chimie, li, p. 77.

  [D] Ibid. li. p. 72

347. iv. _Physiological effects._--A frog was convulsed in the earliest
experiments on these currents (56.). The sensation upon the tongue, and the
flash before the eyes, which I at first obtained only in a feeble degree
(56.), have been since exalted by more powerful apparatus, so as to become
even disagreeable.

348. v. _Spark._--The feeble spark which I first obtained with these
currents (32.), has been varied and strengthened by Signori Nobili and
Antinori, and others, so as to leave no doubt as to its identity with the
common electric spark.

       *       *       *       *       *

IV. _Thermo-Electricity._

349. With regard to thermo-electricity, (that beautiful form of electricity
discovered by Seebeck,) the very conditions under which it is excited are
such as to give no ground for expecting that it can be raised like common
electricity to any high degree of tension; the effects, therefore, due to
that state are not to be expected. The sum of evidence respecting its
analogy to the electricities already described, is, I believe, as
follows:--_Tension._ The attractions and repulsions due to a certain degree
of tension have not been observed. _In currents_: i. _Evolution of Heat._ I
am not aware that its power of raising temperature has been observed. ii.
_Magnetism._ It was discovered, and is best recognised, by its magnetic
powers. iii. _Chemical decomposition_ has not been effected by it. iv.
_Physiological effects._ Nobili has shown[A] that these currents are able
to cause contractions in the limbs of a frog. v. _Spark._ The spark has not
yet been seen.

  [A] Bibliothèque Universelle, xxxvii. 15.

350. Only those effects are weak or deficient which depend upon a certain
high degree of intensity; and if common electricity be reduced in that
quality to a similar degree with the thermo-electricity, it can produce no
effects beyond the latter.

       *       *       *       *       *

V. _Animal Electricity._

351. After an examination of the experiments of Walsh[A] Ingenhousz[B],
Cavendish[C], Sir H. Davy[D], and Dr. Davy[E], no doubt remains on my mind
as to the identity of the electricity of the torpedo with common and
voltaic electricity; and I presume that so little will remain on the minds
of others as to justify my refraining from entering at length into the
philosophical proofs of that identity. The doubts raised by Sir H. Davy
have been removed by his brother Dr. Davy; the results of the latter being
the reverse of those of the former. At present the sum of evidence is as
follows:--

  [A] Philosophical Transactions, 1773, p. 461.

  [B] Ibid. 1775, p. 1.

  [C] Ibid. 1776, p. 196.

  [D] Ibid. 1829, p. 15.

  [E] Ibid. 1832, p. 259.

352. _Tension._--No sensible attractions or repulsions due to tension have
been observed.

353. _In motion_: i. Evolution of Heat; not yet observed; I have little or
no doubt that Harris's electrometer would show it (287. 359.).

354. ii. _Magnetism._--Perfectly distinct. According to Dr. Davy[A], the
current deflected the needle and made magnets under the same law, as to
direction, which governs currents of ordinary and voltaic electricity.

  [A] Philosophical Transactions, 1832, p. 260.

355. iii. _Chemical decomposition._--Also distinct; and though Dr. Davy
used an apparatus of similar construction with that of Dr. Wollaston
(327.), still no error in the present case is involved, for the
decompositions were polar, and in their nature truly electro-chemical. By
the direction of the magnet it was found that the under surface of the fish
was negative, and the upper positive; and in the chemical decompositions,
silver and lead were precipitated on the wire connected with the under
surface, and not on the other; and when these wires were either steel or
silver, in solution of common salt, gas (hydrogen?) rose from the negative
wire, but none from the positive.

356. Another reason for the decomposition being electrochemical is, that a
Wollaston's apparatus constructed with _wires_, coated by sealing-wax,
would most probably not have decomposed water, even in its own peculiar
way, unless the electricity had risen high enough in intensity to produce
sparks in some part of the circuit; whereas the torpedo was not able to
produce sensible sparks. A third reason is, that the purer the water in
Wollaston's apparatus, the more abundant is the decomposition; and I have
found that a machine and wire points which succeeded perfectly well with
distilled water, failed altogether when the water was rendered a good
conductor by sulphate of soda, common salt, or other saline bodies. But in
Dr. Davy's experiments with the torpedo, _strong_ solutions of salt,
nitrate of silver, and superacetate of lead were used successfully, and
there is no doubt with more success than weaker ones.

357. iv. _Physiological effects._--These are so characteristic, that by
them the peculiar powers of the torpedo and gymnotus are principally
recognised.

358. v. _Spark._--The electric spark has not yet been obtained, or at least
I think not; but perhaps I had better refer to the evidence on this point.
Humboldt, speaking of results obtained by M. Fahlberg, of Sweden, says,
"This philosopher has seen an electric spark, as Walsh and Ingenhousz had
done before him in London, by placing the gymnotus in the air, and
interrupting the conducting chain by two gold leaves pasted upon glass, and
a line distant from each other[A]." I cannot, however, find any record of
such an observation by either Walsh or Ingenhousz, and do not know where to
refer to that by M. Fahlberg. M. Humboldt could not himself perceive any
luminous effect.

  [A] Edinburgh Phil. Journal, ii. p. 249.

Again, Sir John Leslie, in his dissertation on the progress of mathematical
and physical science, prefixed to the seventh edition of the Encyclopædia
Britannica, Edinb. 1830, p. 622, says, "From a healthy specimen" of the
_Silurus electricus,_ meaning rather the _gymnotus_, "exhibited in London,
vivid sparks were drawn in a darkened room"; but he does not say he saw
them himself, nor state who did see them; nor can I find any account of
such a phenomenon; so that the statement is doubtful[A].

  [A] Mr. Brayley, who referred me to those statements, and has
  extensive knowledge of recorded facts, is unacquainted with any
  further account relating to them.

359. In concluding this summary of the powers of torpedinal electricity, I
cannot refrain from pointing out the enormous absolute quantity of
electricity which the animal must put in circulation at each effort. It is
doubtful whether any common electrical machine has as yet been able to
supply electricity sufficient in a reasonable time to cause true
electro-chemical decomposition of water (330. 339.), yet the current from
the torpedo has done it. The same high proportion is shown by the magnetic
effects (296. 371.). These circumstances indicate that the torpedo has
power (in the way probably that Cavendish describes,) to continue the
evolution for a sensible time, so that its successive discharges rather
resemble those of a voltaic arrangement, intermitting in its action, than
those of a Leyden apparatus, charged and discharged many times in
succession. In reality, however, there is _no philosophical difference_
between these two cases.

360. The _general conclusion_ which must, I think, be drawn from this
collection of facts is, that _electricity, whatever may be its source, is
identical in its nature_. The phenomena in the five kinds or species
quoted, differ, not in their character but only in degree; and in that
respect vary in proportion to the variable circumstances of _quantity_ and
_intensity_[A] which can at pleasure be made to change in almost any one of
the kinds of electricity, as much as it does between one kind and another.

  [A] The term _quantity_ in electricity is perhaps sufficiently definite
as to sense; the term _intensity_ is more difficult to define strictly.
I am using the terms in their ordinary and accepted meaning.

Table of the experimental Effects common to the Electricities derived from
different Sources[A].

Table headings

A: Physiological Effects
B: Magnetic Deflection.
C: Magnets made.
D: Spark.
E: Heating Power.
F: True chemical Action.
G: Attraction and Repulsion.
H: Discharge by Hot Air.
 _________________________________________________________
|                         |   |   |   |   |   |   |   |   |
|                         | A | B | C | D | E | F | G | H |
|_________________________|___|___|___|___|___|___|___|___|
|                         |   |   |   |   |   |   |   |   |
| 1. Voltaic electricity  | X | X | X | X | X | X | X | X |
|_________________________|___|___|___|___|___|___|___|___|
|                         |   |   |   |   |   |   |   |   |
| 2. Common electricity   | X | X | X | X | X | X | X | X |
|_________________________|___|___|___|___|___|___|___|___|
|                         |   |   |   |   |   |   |   |   |
| 3. Magneto-Electricity  | X | X | X | X | X | X | X |   |
|_________________________|___|___|___|___|___|___|___|___|
|                         |   |   |   |   |   |   |   |   |
| 4. Thermo-Electricity   | X | X | + | + | + | + |   |   |
|_________________________|___|___|___|___|___|___|___|___|
|                         |   |   |   |   |   |   |   |   |
| 5. Animal Electricity   | X | X | X | + | + | X |   |   |
|_________________________|___|___|___|___|___|___|___|___|

  [A] Many of the spaces in this table originally left blank may now be
  filled. Thus with _thermo-electricity_, Botto made magnets and
  obtained polar chemical decomposition: Antinori produced the spark;
  and if it has not been done before, Mr. Watkins has recently heated a
  wire in Harris's thermo-electrometer. In respect to _animal
  electricity_, Matteucci and Linari have obtained the spark from the
  torpedo, and I have recently procured it from the gymnotus: Dr. Davy
  has observed the heating power of the current from the torpedo. I have
  therefore filled up these spaces with crosses, in a different position
  to the others originally in the table. There remain but five spaces
  unmarked, two under _attraction_ and _repulsion_, and three under
  _discharge by hot air_; and though these effects have not yet been
  obtained, it is a necessary conclusion that they must be possible,
  since the _spark_ corresponding to them has been procured. For when a
  discharge across cold air can occur, that intensity which is the only
  essential additional requisite for the other effects must be
  present.--_Dec. 13 1838._


§ 8. _Relation by Measure of common and voltaic Electricity._[A]

  [A] In further illustration of this subject see 855-873 in Series
  VII.--_Dec. 1838._


361. Believing the point of identity to be satisfactorily established, I
next endeavoured to obtain a common measure, or a known relation as to
quantity, of the electricity excited by a machine, and that from a voltaic
pile; for the purpose not only of confirming their identity (378.), but
also of demonstrating certain general principles (366, 377, &c.), and
creating an extension of the means of investigating and applying the
chemical powers of this wonderful and subtile agent.

362. The first point to be determined was, whether the same absolute
quantity of ordinary electricity, sent through a galvanometer, under
different circumstances, would cause the same deflection of the needle. An
arbitrary scale was therefore attached to the galvanometer, each division
of which was equal to about 4°, and the instrument arranged as in former
experiments (296.). The machine (290.), battery (291.), and other parts of
the apparatus were brought into good order, and retained for the time as
nearly as possible in the same condition. The experiments were alternated
so as to indicate any change in the condition of the apparatus and supply
the necessary corrections.

363. Seven of the battery jars were removed, and eight retained for present
use. It was found that about forty turns would fully charge the eight jars.
They were then charged by thirty turns of the machine, and discharged
through the galvanometer, a thick wet string, about ten inches long, being
included in the circuit. The needle was immediately deflected five
divisions and a half, on the one side of the zero, and in vibrating passed
as nearly as possible through five divisions and a half on the other side.

364. The other seven jars were then added to the eight, and the whole
fifteen charged by thirty turns of the machine. The Henley's electrometer
stood not quite half as high as before; but when the discharge was made
through the galvanometer, previously at rest, the needle immediately
vibrated, passing _exactly_ to the same division as in the former instance.
These experiments with eight and with fifteen jars were repeated several
times alternately with the same results.

365. Other experiments were then made, in which all the battery was used,
and its charge (being fifty turns of the machine,) sent through the
galvanometer: but it was modified by being passed sometimes through a mere
wet thread, sometimes through thirty-eight inches of thin string wetted by
distilled water, and sometimes through a string of twelve times the
thickness, only twelve inches in length, and soaked in dilute acid (298.).
With the thick string the charge passed at once; with the thin string it
occupied a sensible time, and with the thread it required two or three
seconds before the electrometer fell entirely down. The current therefore
must have varied extremely in intensity in these different cases, and yet
the deflection of the needle was sensibly the same in all of them. If any
difference occurred, it was that the thin string and thread caused greatest
deflection; and if there is any lateral transmission, as M. Colladon says,
through the silk in the galvanometer coil, it ought to have been so,
because then the intensity is lower and the lateral transmission less.

366. Hence it would appear that _if the same absolute quantity of
electricity pass through the galvanometer, whatever may be its intensity,
the dejecting force upon the magnetic needle is the same._

367. The battery of fifteen jars was then charged by sixty revolutions of
the machine, and discharged, as before, through the galvanometer. The
deflection of the needle was now as nearly as possible to the eleventh
division, but the graduation was not accurate enough for me to assert that
the arc was exactly double the former arc; to the eye it appeared to be so.
The probability is, that _the deflecting force of an electric current is
directly proportional to the absolute quantity of electricity passed_, at
whatever intensity that electricity may be[A].

  [A] The great and general value of the galvanometer, as an actual
  measure of the electricity passing through it, either continuously or
  interruptedly, must be evident from a consideration of these two
  conclusions. As constructed by Professor Ritchie with glass threads
  (see Philosophical Transactions, 1830, p. 218, and Quarterly Journal
  of Science, New Series, vol. i. p.29.), it apparently seems to leave
  nothing unsupplied in its own department.

368. Dr. Ritchie has shown that in a case where the intensity of the
electricity remained the same, the deflection of the magnetic needle was
directly as the quantity of electricity passed through the galvanometer[A].
Mr. Harris has shown that the _heating_ power of common electricity on
metallic wires is the same for the same quantity of electricity whatever
its intensity might have previously been[B].

  [A] Quarterly Journal of Science, New Series, vol. i. p. 33.

  [B] Plymouth Transactions, page 22.

369. The next point was to obtain a _voltaic_ arrangement producing an
effect equal to that just described (367.). A platina and a zinc wire were
passed through the same hole of a draw-plate, being then one eighteenth of
an inch in diameter; these were fastened to a support, so that their lower
ends projected, were parallel, and five sixteenths of an inch apart. The
upper ends were well-connected with the galvanometer wires. Some acid was
diluted, and, after various preliminary experiments, that adopted as a
standard which consisted of one drop strong sulphuric acid in four ounces
distilled water. Finally, the time was noted which the needle required in
swinging either from right to left or left to right: it was equal to
seventeen beats of my watch, the latter giving one hundred and fifty in a
minute. The object of these preparations was to arrange a voltaic
apparatus, which, by immersion in a given acid for a given time, much less
than that required by the needle to swing in one direction, should give
equal deflection to the instrument with the discharge of ordinary
electricity from the battery (363. 364.); and a new part of the zinc wire
having been brought into position with the platina, the comparative
experiments were made.

370. On plunging the zinc and platina wires five eighths of an inch deep
into the acid, and retaining them there for eight beats of the watch,
(after which they were quickly withdrawn,) the needle was deflected, and
continued to advance in the same direction some time after the voltaic
apparatus had been removed from the acid. It attained the five-and-a-half
division, and then returned swinging an equal distance on the other side.
This experiment was repeated many times, and always with the same result.

371. Hence, as an approximation, and judging from _magnetic force_ only at
present (376.), it would appear that two wires, one of platina and one of
zinc, each one eighteenth of an inch in diameter, placed five sixteenths of
an inch apart and immersed to the depth of five eighths of an inch in acid,
consisting of one drop oil of vitriol and four ounces distilled water, at a
temperature about 60°, and connected at the other extremities by a copper
wire eighteen feet long and one eighteenth of an inch thick (being the wire
of the galvanometer coils), yield as much electricity in eight beats of my
watch, or in 8/150ths of a minute, as the electrical battery charged by
thirty turns of the large machine, in excellent order (363. 364.).
Notwithstanding this apparently enormous disproportion, the results are
perfectly in harmony with those effects which are known to be produced by
variations in the intensity and quantity of the electric fluid.

372. In order to procure a reference to _chemical action_, the wires were
now retained immersed in the acid to the depth of five eighths of an inch,
and the needle, when stationary, observed; it stood, as nearly as the
unassisted eye could decide, at 5-1/3 division. Hence a permanent
deflection to that extent might be considered as indicating a constant
voltaic current, which in eight beats of my watch (369.) could supply as
much electricity as the electrical battery charged by thirty turns of the
machine.

373. The following arrangements and results are selected from many that
were made and obtained relative to chemical action. A platina wire one
twelfth of an inch in diameter, weighing two hundred and sixty grains, had
the extremity rendered plain, so as to offer a definite surface equal to a
circle of the same diameter as the wire; it was then connected in turn with
the conductor of the machine, or with the voltaic apparatus (369.), so as
always to form the positive pole, and at the same time retain a
perpendicular position, that it might rest, with its whole weight, upon the
test paper to be employed. The test paper itself was supported upon a
platina spatula, connected either with the discharging train (292.), or
with the negative wire of the voltaic apparatus, and it consisted of four
thicknesses, moistened at all times to an equal degree in a standard
solution of hydriodate of potassa (316.).

374. When the platina wire was connected with the prime conductor of the
machine, and the spatula with the discharging train, ten turns of the
machine had such decomposing power as to produce a pale round spot of
iodine of the diameter of the wire; twenty turns made a much darker mark,
and thirty turns made a dark brown spot penetrating to the second thickness
of the paper. The difference in effect produced by two or three turns, more
or less, could be distinguished with facility.

375. The wire and spatula were then connected with the voltaic apparatus
(369.), the galvanometer being also included in the arrangement; and, a
stronger acid having been prepared, consisting of nitric acid and water,
the voltaic apparatus was immersed so far as to give a permanent deflection
of the needle to the 5-1/3 division (372.), the fourfold moistened paper
intervening as before[A]. Then by shifting the end of the wire from place
to place upon the test paper, the effect of the current for five, six,
seven, or any number of the beats of the watch (369.) was observed, and
compared with that of the machine. After alternating and repeating the
experiments of comparison many times, it was constantly found that this
standard current of voltaic electricity, continued for eight beats of the
watch, was equal, in chemical effect, to thirty turns of the machine;
twenty-eight revolutions of the machine were sensibly too few.

  [A] Of course the heightened power of the voltaic battery was
  necessary to compensate for the bad conductor now interposed.

376. Hence it results that both in _magnetic deflection_ (371.) and in
_chemical force_, the current of electricity of the standard voltaic
battery for eight beats of the watch was equal to that of the machine
evolved by thirty revolutions.

377. It also follows that for this case of electro-chemical decomposition,
and it is probable for all cases, that the _chemical power, like the
magnetic force_ (36.), _is in direct proportion to the absolute quantity
of electricity_ which passes.

378. Hence arises still further confirmation, if any were required, of the
identity of common and voltaic electricity, and that the differences of
intensity and quantity are quite sufficient to account for what were
supposed to be their distinctive qualities.

379. The extension which the present investigations have enabled me to make
of the facts and views constituting the theory of electro-chemical
decomposition, will, with some other points of electrical doctrine, be
almost immediately submitted to the Royal Society in another series of
these Researches.

_Royal Institution,
15th Dec. 1832._

Note.--I am anxious, and am permitted, to add to this paper a correction of
an error which I have attributed to M. Ampère the first series of these
Experimental Researches. In referring to his experiment on the induction of
electrical currents (78.), I have called that a disc which I should have
called a circle or a ring. M. Ampère used a ring, or a very short cylinder
made of a narrow plate of copper bent into a circle, and he tells me that
by such an arrangement the motion is very readily obtained. I have not
doubted that M. Ampère obtained the motion he described; but merely mistook
the kind of mobile conductor used, and so far I described his _experiment_
erroneously.

In the same paragraph I have stated that M. Ampère says the disc turned "to
take a position of equilibrium exactly as the spiral itself would have
turned had it been free to move"; and further on I have said that my
results tended to invert the sense of the proposition "stated by M. Ampère,
_that a current of electricity tends to put the electricity of conductors
near which it passes in motion in the same direction._" M. Ampère tells me
in a letter which I have just received from him, that he carefully avoided,
when describing the experiment, any reference to the direction of the
induced current; and on looking at the passages he quotes to me, I find
that to be the case. I have therefore done him injustice in the above
statements, and am anxious to correct my error.

But that it may not be supposed I lightly wrote those passages, I will
briefly refer to my reasons for understanding them in the sense I did. At
first the experiment failed. When re-made successfully about a year
afterwards, it was at Geneva in company with M.A. De la Rive: the latter
philosopher described the results[A], and says that the plate of copper
bent into a circle which was used as the mobile conductor "sometimes
advanced between the two branches of the (horse-shoe) magnet, and sometimes
was repelled, _according_ to the direction of the current in the
surrounding conductors."

  [A] Bibliothèque Universelle, xxi. p. 48.

I have been in the habit of referring to Demonferrand's _Manuel
d'Electricité Dynamique_, as a book of authority in France; containing the
general results and laws of this branch of science, up to the time of its
publication, in a well arranged form. At p. 173, the author, when
describing this experiment, says, "The mobile circle turns to take a
position of equilibrium as a conductor would do in which the current moved
in the _same direction_ as in the spiral;" and in the same paragraph he
adds, "It is therefore proved _that a current of electricity tends to put
the electricity of conductors, near which it passes, in motion in the same
direction._" These are the words I quoted in my paper (78.).

Le Lycée of 1st of January, 1832, No. 36, in an article written after the
receipt of my first unfortunate letter to M. Hachette, and before my papers
were printed, reasons upon the direction of the induced currents, and says,
that there ought to be "an elementary current produced in the same
direction as the corresponding portion of the producing current." A little
further on it says, "therefore we ought to obtain currents, moving in the
_same direction_, produced upon a metallic wire, either by a magnet or a
current. M. Ampère _was so thouroughly persuaded that such ought to be the
direction of the currents by influence_, that he neglected to assure
himself of it in his experiment at Geneva."

It was the precise statements in Demonferrand's Manuel, agreeing as they
did with the expression in M. De la Rive's paper, (which, however, I now
understand as only meaning that when the inducing current was changed, the
motion of the mobile circle changed also,) and not in discordance with
anything expressed by M. Ampère himself where he speaks of the experiment,
which made me conclude, when I wrote the paper, that what I wrote was
really his avowed opinion; and when the Number of the Lycée referred to
appeared, which was before my paper was printed, it could excite no
suspicion that I was in error.

Hence the mistake into which I unwittingly fell. I am proud to correct it
and do full justice to the acuteness and accuracy which, as far as I can
understand the subjects, M. Ampère carries into all the branches of
philosophy which he investigates.

Finally, my note to (79.) says that the Lycée, No. 36. "mistakes the
erroneous results of MM. Fresnel and Ampère for true ones," &c. &c. In
calling M. Ampère's results erroneous, I spoke of the results described in,
and referred to by the Lycée itself; but _now_ that the expression of the
direction of the induced current is to be separated, the term _erroneous_
ought no longer to be attached to them.

April 29, 1833.
M.F.]




FOURTH SERIES.


§ 9. _On a new Law of Electric Conduction._ § 10. _On Conducting Power
generally._

Received April 24,--Read May 23, 1833.


§ 9. _On a new Law of Electric Conduction._[A]

  [A] In reference to this law see further considerations at 910. 1358.
  1705.--_Dec. 1838._


380. It was during the progress of investigations relating to
electro-chemical decomposition, which I still have to submit to the Royal
Society, that I encountered effects due to a very _general law_ of electric
conduction not hitherto recognised; and though they prevented me from
obtaining the condition I sought for, they afforded abundant compensation
for the momentary disappointment, by the new and important interest which
they gave to an extensive part of electrical science.

381. I was working with ice, and the solids resulting from the freezing of
solutions, arranged either as barriers across a substance to be decomposed,
or as the actual poles of a voltaic battery, that I might trace and catch
certain elements in their transit, when I was suddenly stopped in my
progress by finding that ice was in such circumstances a non-conductor of
electricity; and that as soon as a thin film of it was interposed, in the
circuit of a very powerful voltaic battery, the transmission of electricity
was prevented, and all decomposition ceased.

382. At first the experiments were made with common ice, during the cold
freezing weather of the latter end of January 1833; but the results were
fallacious, from the imperfection of the arrangements, and the following
more unexceptionable form of experiment was adopted.

383. Tin vessels were formed, five inches deep, one inch and a quarter wide
in one direction, of different widths from three eighths to five eighths of
an inch in the other, and open at one extremity. Into these were fixed by
corks, plates of platina, so that the latter should not touch the tin
cases; and copper wires having previously been soldered to the plate, these
were easily connected, when required, with a voltaic pile. Then distilled
water, previously boiled for three hours, was poured into the vessels, and
frozen by a mixture of salt and snow, so that pure transparent solid ice
intervened between the platina and tin; and finally these metals were
connected with the opposite extremities of the voltaic apparatus, a
galvanometer being at the same time included in the circuit.

384. In the first experiment, the platina pole was three inches and a half
long, and seven eighths of an inch wide; it was wholly immersed in the
water or ice, and as the vessel was four eighths of an inch in width, the
average thickness of the intervening ice was only a quarter of an inch,
whilst the surface of contact with it at both poles was nearly fourteen
square inches. After the water was frozen, the vessel was still retained in
the frigorific mixture, whilst contact between the tin and platina
respectively was made with the extremities of a well-charged voltaic
battery, consisting of twenty pairs of four-inch plates, each with double
coppers. Not the slightest deflection of the galvanometer needle occurred.

385. On taking the frozen arrangement out of the cold mixture, and applying
warmth to the bottom of the tin case, so as to melt part of the ice, the
connexion with the battery being in the mean time retained, the needle did
not at first move; and it was only when the thawing process had extended so
far as to liquefy part of the ice touching the platina pole, that
conduction took place; but then it occurred effectually, and the
galvanometer needle was permanently deflected nearly 70°.

386. In another experiment, a platina spatula, five inches in length and
seven eighths of an inch in width, had four inches fixed in the ice, and
the latter was only three sixteenths of an inch thick between one metallic
surface and the other; yet this arrangement insulated as perfectly as the
former.

387. Upon pouring a little water in at the top of this vessel on the ice,
still the arrangement did not conduct; yet fluid water was evidently there.
This result was the consequence of the cold metals having frozen the water
where they touched it, and thus insulating the fluid part; and it well
illustrates the non-conducting power of ice, by showing how thin a film
could prevent the transmission of the battery current. Upon thawing parts
of this thin film, at _both_ metals, conduction occurred.

388. Upon warming the tin case and removing the piece of ice, it was found
that a cork having slipped, one of the edges of the platina had been all
but in contact with the inner surface of the tin vessel; yet,
notwithstanding the extreme thinness of the interfering ice in this place,
no sensible portion of electricity had passed.

389. These experiments were repeated many times with the same results. At
last a battery of fifteen troughs, or one hundred and fifty pairs of
four-inch plates, powerfully charged, was used; yet even here no sensible
quantity of electricity passed the thin barrier of ice.

390. It seemed at first as if occasional departures from these effects
occurred; but they could always be traced to some interfering
circumstances. The water should in every instance be well-frozen; for
though it is not necessary that the ice should reach from pole to pole,
since a barrier of it about one pole would be quite sufficient to prevent
conduction, yet, if part remain fluid, the mere necessary exposure of the
apparatus to the air or the approximation of the hands, is sufficient to
produce, at the _upper surface_ of the water and ice, a film of fluid,
extending from the platina to the tin; and then conduction occurs. Again,
if the corks used to block the platina in its place are damp or wet within,
it is necessary that the cold be sufficiently well applied to freeze the
water in them, or else when the surfaces of their contact with the tin
become slightly warm by handling, that part will conduct, and the interior
being ready to conduct also, the current will pass. The water should be
pure, not only that unembarrassed results may be obtained, but also that,
as the freezing proceeds, a minute portion of concentrated saline solution
may not be formed, which remaining fluid, and being interposed in the ice,
or passing into cracks resulting from contraction, may exhibit conducting
powers independent of the ice itself.

391. On one occasion I was surprised to find that after thawing much of the
ice the conducting power had not been restored; but I found that a cork
which held the wire just where it joined the platina, dipped so far into
the ice, that with the ice itself it protected the platina from contact
with the melted part long after that contact was expected.

392. This insulating power of ice is not effective with electricity of
exalted intensity. On touching a diverged gold-leaf electrometer with a
wire connected with the platina, whilst the tin case was touched by the
hand or another wire, the electrometer was instantly discharged (419.).

393. But though electricity of an intensity so low that it cannot diverge
the electrometer, can still pass (though in very limited quantities
(419.),) through ice; the comparative relation of water and ice to the
electricity of the voltaic apparatus is not less extraordinary on that
account, Or less important in its consequences.

394. As it did not seem likely that this _law of the assumption of
conducting power during liquefaction, and loss of it during congelation_,
would be peculiar to water, I immediately proceeded to ascertain its
influence in other cases, and found it to be very general. For this purpose
bodies were chosen which were solid at common temperatures, but readily
fusible; and of such composition as, for other reasons connected with
electrochemical action, led to the conclusion that they would be able when
fused to replace water as conductors. A voltaic battery of two troughs, or
twenty pairs of four-inch plates (384.), was used as the source of
electricity, and a galvanometer introduced into the circuit to indicate the
presence or absence of a current.

395. On fusing a little chloride of lead by a spirit lamp on a fragment of
a Florence flask, and introducing two platina wires connected with the
poles of the battery, there was instantly powerful action, the galvanometer
was most violently affected, and the chloride rapidly decomposed. On
removing the lamp, the instant the chloride solidified all current and
consequent effects ceased, though the platina wires remained inclosed in
the chloride not more than the one-sixteenth of an inch from each other. On
renewing the heat, as soon as the fusion had proceeded far enough to allow
liquid matter to connect the poles, the electrical current instantly
passed.

396. On fusing the chloride, with one wire introduced, and then touching
the liquid with the other, the latter being cold, caused a little knob to
concrete on its extremity, and no current passed; it was only when the wire
became so hot as to be able to admit or allow of contact with the liquid
matter, that conduction took place, and then it was very powerful.

397. When chloride of silver and chlorate of potassa were experimented
with, in a similar manner, exactly the same results occurred.

398. Whenever the current passed in these cases, there was decomposition of
the substances; but the electro-chemical part of this subject I purpose
connecting with more general views in a future paper[A].

  [A] In 1801, Sir H. Davy knew that "dry nitre, caustic potash, and
  soda are conductors of galvanism when rendered fluid by a high degree
  of heat," (Journals of the Royal Institution, 1802, p. 53,) but was
  not aware of the general law which I have been engaged in developing.
  It is remarkable, that eleven years after that, he should say, "There
  are no fluids known except such as contain water, which are capable of
  being made the medium of connexion between the metal or metals of the
  voltaic apparatus."--Elements of Chemical Philosophy, p. 169.

399. Other substances, which could not be melted on glass, were fused by
the lamp and blowpipe on platina connected with one pole of the battery,
and then a wire, connected with the other, dipped into them. In this way
chloride of sodium, sulphate of soda, protoxide of lead, mixed carbonates
of potash and soda, &c. &c., exhibited exactly the same phenomena as those
already described: whilst liquid, they conducted and were decomposed;
whilst solid, though very hot, they insulated the battery current even when
four troughs were used.

400. Occasionally the substances were contained in small bent tubes of
green glass, and when fused, the platina poles introduced, one on each
side. In such cases the same general results as those already described
were procured; but a further advantage was obtained, namely, that whilst
the substance was conducting and suffering decomposition, the final
arrangement of the elements could be observed. Thus, iodides of potassium
and lead gave iodine at the positive pole, and potassium or lead at the
negative pole. Chlorides of lead and silver gave chlorine at the positive,
and metals at the negative pole. Nitre and chlorate; of potassa gave
oxygen, &c., at the positive, and alkali, or even potassium, at the
negative pole.

[Illustration]

401. A fourth arrangement was used for substances requiring very high
temperatures for their fusion. A platina wire was connected with one pole
of the battery; its extremity bent into a small ring, in the manner
described by Berzelius, for blowpipe experiments; a little of the salt,
glass, or other substance, was melted on this ring by the ordinary
blowpipe, or even in some cases by the oxy-hydrogen blowpipe, and when the
drop, retained in its place by the ring, was thoroughly hot and fluid, a
platina wire from the opposite pole of the battery was made to touch it,
and the effects observed.

402. The following are various substances, taken from very different
classes chemically considered, which are subject to this law. The list
might, no doubt, be enormously extended; but I have not had time to do more
than confirm the law by a sufficient number of instances.

First, _water_.

Amongst _oxides_;--potassa, protoxide of lead, glass of antimony, protoxide
of antimony, oxide of bismuth.

_Chlorides_ of potassium, sodium, barium, strontium, calcium, magnesium,
manganese, zinc, copper (proto-), lead, tin (proto-), antimony, silver.

_Iodides_ of potassium, zinc and lead, protiodide of tin, periodide of
mercury; _fluoride_ of potassium; _cyanide_ of potassium; _sulpho-cyanide_
of potassium.

_Salts._ Chlorate of potassa; nitrates of potassa, soda, baryta, strontia,
lead, copper, and silver; sulphates of soda and lead, proto-sulphate of
mercury; phosphates of potassa, soda, lead, copper, phosphoric glass or
acid phosphate of lime; carbonates of potassa and soda, mingled and
separate; borax, borate of lead, per-borate of tin; chromate of potassa,
bi-chromate of potassa, chromate of lead; acetate of potassa.

_Sulphurets._ Sulphuret of antimony, sulphuret of potassium made by
reducing sulphate of potassa by hydrogen; ordinary sulphuret of potassa.

Silicated potassa; chameleon mineral.

403. It is highly interesting in the instances of those substances which
soften before they liquefy, to observe at what period the conducting power
is acquired, and to what degree it is exalted by perfect fluidity. Thus,
with the borate of lead, when heated by the lamp upon glass, it becomes as
soft as treacle, but it did not conduct, and it was only when urged by the
blowpipe and brought to a fair red heat, that it conducted. When rendered
quite liquid, it conducted with extreme facility.

404. I do not mean to deny that part of the increased conducting power in
these cases of softening was probably due to the elevation of temperature
(432. 445.); but I have no doubt that by far the greater part was due to
the influence of the general law already demonstrated, and which in these
instances came gradually, instead of suddenly, into operation.

405. The following are bodies which acquired no conducting power upon
assuming the liquid state:--

Sulphur, phosphorus; iodide of sulphur, per-iodide of tin; orpiment,
realgar; glacial acetic acid, mixed margaric and oleic acids, artificial
camphor; caffeine, sugar, adipocire, stearine of cocoa-nut oil, spermaceti,
camphor, naphthaline, resin, gum sandarach, shell lac.

406. Perchloride of tin, chloride of arsenic, and the hydrated chloride of
arsenic, being liquids, had no sensible conducting power indicated by the
galvanometer, nor were they decomposed.

407. Some of the above substances are sufficiently remarkable as exceptions
to the general law governing the former cases. These are orpiment, realgar,
acetic acid, artificial camphor, per-iodide of tin, and the chlorides of
tin and arsenic. I shall have occasion to refer to these cases in the paper
on Electro-chemical Decomposition.

408. Boracic acid was raised to the highest possible temperature by an
oxy-hydrogen flame (401.), yet it gained no conducting powers sufficient to
affect the galvanometer, and underwent no apparent voltaic decomposition.
It seemed to be quite as bad a conductor as air. Green bottle-glass, heated
in the same manner, did not gain conducting power sensible to the
galvanometer. Flint glass, when highly heated, did conduct a little and
decompose; and as the proportion of potash or oxide of lead was increased
in the glass, the effects were more powerful. Those glasses, consisting of
boracic acid on the one hand, and oxide of lead or potassa on the other,
show the assumption of conducting power upon fusion and the accompanying
decomposition very well.

409. I was very anxious to try the general experiment with sulphuric acid,
of about specific gravity 1.783, containing that proportion of water which
gives it the power of crystallizing at 40° Fahr.; but I found it impossible
to obtain it so that I could be sure the whole would congeal even at 0°
Fahr. A ten-thousandth part of water, more or less than necessary, would,
upon cooling the whole, cause a portion of uncongealable liquid to
separate, and that remaining in the interstices of the solid mass, and
moistening the planes of division, would prevent the correct observation of
the phenomena due to entire solidification and subsequent liquefaction.

410. With regard to the substances on which conducting power is thus
conferred by liquidity, the degree of power so given is generally very
great. Water is that body in which this acquired power is feeblest. In the
various oxides, chlorides, salts, &c. &c., it is given in a much higher
degree. I have not had time to measure the conducting power in these cases,
but it is apparently some hundred times that of pure water. The increased
conducting power known to be given to water by the addition of salts, would
seem to be in a great degree dependent upon the high conducting power of
these bodies when in the liquid state, that state being given them for the
time, not by heat but solution in the water[A].

  [A] See a doubt on this point at 1356.--_Dec. 1838._

411. Whether the conducting power of these liquefied bodies is a
consequence of their decomposition or not (413.), or whether the two
actions of conduction and decomposition are essentially connected or not,
would introduce no difference affecting the probable accuracy of the
preceding statement.

412. This _general assumption of conducting power_ by bodies as soon as
they pass from the solid to the liquid state, offers a new and
extraordinary character, the existence of which, as far as I know, has not
before been suspected; and it seems importantly connected with some
properties and relations of the particles of matter which I may now briefly
point out.

413. In almost all the instances, as yet observed, which are governed by
this law, the substances experimented with have been those which were not
only compound bodies, but such as contain elements known to arrange
themselves at the opposite poles; and were also such as could be
_decomposed_ by the electrical current. When conduction took place,
decomposition occurred; when decomposition ceased, conduction ceased also;
and it becomes a fair and an important question, Whether the conduction
itself may not, wherever the law holds good, be a consequence not merely of
the capability, but of the act of decomposition? And that question may be
accompanied by another, namely, Whether solidification does not prevent
conduction, merely by chaining the particles to their places, under the
influence of aggregation, and preventing their final separation in the
manner necessary for decomposition?

414. But, on the other hand, there is one substance (and others may occur),
the _per-iodide of mercury_, which, being experimented with like the others
(400.), was found to insulate when solid, and to acquire conducting power
when fluid; yet it did not seem to undergo decomposition in the latter
case.

415. Again, there are many substances which contain elements such as would
be expected to arrange themselves at the opposite poles of the pile, and
therefore in that respect fitted for decomposition, which yet do not
conduct. Amongst these are the iodide of sulphur, per-iodide of zinc,
per-chloride of tin, chloride of arsenic, hydrated chloride of arsenic,
acetic acid, orpiment, realgar, artificial camphor, &c.; and from these it
might perhaps be assumed that decomposition is dependent upon conducting
power, and not the latter upon the former. The true relation, however, of
conduction and decomposition in those bodies governed by the general law
which it is the object of this paper to establish, can only be
satisfactorily made out from a far more extensive series of observations
than those I have yet been able to supply[A].

  [A] See 673, &c. &c.--_Dec. 1838._

416. The relation, under this law, of the conducting power for electricity
to that for heat, is very remarkable, and seems to imply a natural
dependence of the two. As the solid becomes a fluid, it loses almost
entirely the power of conduction for heat, but gains in a high degree that
for electricity; but as it reverts hack to the solid state, it gains the
power of conducting heat, and loses that of conducting electricity. If,
therefore, the properties are not incompatible, still they are most
strongly contrasted, one being lost as the other is gained. We may hope,
perhaps, hereafter to understand the physical reason of this very
extraordinary relation of the two conducting powers, both of which appear
to be directly connected with the corpuscular condition of the substances
concerned.

417. The assumption of conducting power and a decomposable condition by
liquefaction, promises new opportunities of, and great facilities in,
voltaic decomposition. Thus, such bodies as the oxides, chlorides,
cyanides, sulpho-cyanides, fluorides, certain vitreous mixtures, &c. &c.,
may be submitted to the action of the voltaic battery under new
circumstances; and indeed I have already been able, with ten pairs of
plates, to decompose common salt, chloride of magnesium, borax, &c. &c.,
and to obtain sodium, magnesium, boron, &c., in their separate states.


§ 10. _On Conducting Power generally._[A]

  [A] In reference to this § refer to 983 in series viii., and the
  results connected with it.--_Dec. 1838._


418. It is not my intention here to enter into an examination of all the
circumstances connected with conducting power, but to record certain facts
and observations which have arisen during recent inquiries, as additions to
the general stock of knowledge relating to this point of electrical
science.

419. I was anxious, in the first place, to obtain some idea of the
conducting power of ice and solid salts for electricity of high tension
(392.), that a comparison might be made between it and the large accession
of the same power gained upon liquefaction. For this purpose the large
electrical machine (290.) was brought into excellent action, its conductor
connected with a delicate gold-leaf electrometer, and also with the platina
inclosed in the ice (383.), whilst the tin case was connected with the
discharging train (292.). On working the machine moderately, the gold
leaves barely separated; on working it rapidly, they could be opened nearly
two inches. In this instance the tin case was five-eighths of an inch in
width; and as, after the experiment, the platina plate was found very
nearly in the middle of the ice, the average thickness of the latter had
been five-sixteenths of an inch, and the extent of surface of contact with
tin and platina fourteen square inches (384.). Yet, under these
circumstances, it was but just able to conduct the small quantity of
electricity which this machine could evolve (371.), even when of a tension
competent to open the leaves two inches; no wonder, therefore, that it
could not conduct any sensible portion of the electricity of the troughs
(384.), which, though almost infinitely surpassing that of the machine in
quantity, had a tension so low as not to be sensible to an electrometer.

420. In another experiment, the tin case was only four-eighths of an inch
in width, and it was found afterwards that the platina had been not quite
one-eighth of an inch distant in the ice from one side of the tin vessel.
When this was introduced into the course of the electricity from the
machine (419.), the gold leaves could be opened, but not more than half an
inch; the thinness of the ice favouring the conduction of the electricity,
and permitting the same quantity to pass in the same time, though of a much
lower tension.

421. Iodide of potassium which had been fused and cooled was introduced
into the course of the electricity from the machine. There were two pieces,
each about a quarter of an inch in thickness, and exposing a surface on
each side equal to about half a square inch; these were placed upon platina
plates, one connected with the machine and electrometer (419.), and the
other with the discharging train, whilst a fine platina wire connected the
two pieces, resting upon them by its two points. On working the electrical
machine, it was possible to open the electrometer leaves about two-thirds
of an inch.

422. As the platina wire touched only by points, the facts show that this
salt is a far better conductor than ice; but as the leaves of the
electrometer opened, it is also evident with what difficulty conduction,
even of the small portion of electricity produced by the machine, is
effected by this body in the solid state, when compared to the facility
with which enormous quantities at very low tensions are transmitted by it
when in the fluid state.

423. In order to confirm these results by others, obtained from the voltaic
apparatus, a battery of one hundred and fifty plates, four inches square,
was well-charged: its action was good; the shock from it strong; the
discharge would _continue_ from copper to copper through four-tenths of an
inch of air, and the gold-leaf electrometer before used could be opened
nearly a quarter of an inch.

424. The ice vessel employed (420.) was half an inch in width; as the
extent of contact of the ice with the tin and platina was nearly fourteen
square inches, the whole was equivalent to a plate of ice having a surface
of seven square inches, of perfect contact at each side, and only one
fourth of an inch thick. It was retained in a freezing mixture during the
experiment.

425. The order of arrangement in the course of the electric current was as
follows. The positive pole of the battery was connected by a wire with the
platina plate in the ice; the plate was in contact with the ice, the ice
with the tin jacket, the jacket with a wire, which communicated with a
piece of tin foil, on which rested one end of a bent platina wire (312.),
the other or decomposing end being supported on paper moistened with
solution of iodide of potassium (316.): the paper was laid flat on a
platina spatula connected with the negative end of the battery. All that
part of the arrangement between the ice vessel and the decomposing wire
point, including both these, was insulated, so that no electricity might
pass through the latter which had not traversed the former also.

426. Under these circumstances, it was found that, a pale brown spot of
iodine was slowly formed under the decomposing platina point, thus
indicating that ice could conduct a little of the electricity evolved by a
voltaic battery charged up to the degree of intensity indicated by the
electrometer. But it is quite evident that notwithstanding the enormous
quantity of electricity which the battery could furnish, it was, under
present circumstances, a very inferior instrument to the ordinary machine;
for the latter could send as much through the ice as it could carry, being
of a far higher intensity, i.e. able to open the electrometer leaves half
an inch or more (419. 420.).

427. The decomposing wire and solution of iodide of potassium were then
removed, and replaced by a very delicate galvanometer (205.); it was so
nearly astatic, that it vibrated to and fro in about sixty-three beats of a
watch giving one hundred and fifty beats in a minute. The same feebleness
of current as before was still indicated; the galvanometer needle was
deflected, but it required to break and make contact three or four times
(297.), before the effect was decided.

428. The galvanometer being removed, two platina plates were connected with
the extremities of the wires, and the tongue placed between them, so that
the whole charge of the battery, so far as the ice would let it pass, was
free to go through the tongue. Whilst standing on the stone floor, there
was shock, &c., but when insulated, I could feel no sensation. I think a
frog would have been scarcely, if at all, affected.

429. The ice was now removed, and experiments made with other solid bodies,
for which purpose they were placed under the end of the decomposing wire
instead of the solution of iodide of potassium (125.). For instance, a
piece of dry iodide of potassium was placed on the spatula connected with
the negative pole of the battery, and the point of the decomposing wire
placed upon it, whilst the positive end of the battery communicated with
the latter. A brown spot of iodine very slowly appeared, indicating the
passage of a little electricity, and agreeing in that respect with the
results obtained by the use of the electrical machine (421.). When the
galvanometer was introduced into the circuit at the same time with the
iodide, it was with difficulty that the action of the current on it could
be rendered sensible.

430. A piece of common salt previously fused and solidified being
introduced into the circuit was sufficient almost entirely to destroy the
action on the galvanometer. Fused and cooled chloride of lead produced the
same effect. The conducting power of these bodies, _when fluid_, is very
great (395. 402.).

431. These effects, produced by using the common machine and the voltaic
battery, agree therefore with each other, and with the law laid down in
this paper (394.); and also with the opinion I have supported, in the Third
Series of these Researches, of the identity of electricity derived from
different sources (360.).

432. The effect of heat in increasing the conducting power of many
substances, especially for electricity of high tension, is well known. I
have lately met with an extraordinary case of this kind, for electricity of
low tension, or that of the voltaic pile, and which is in direct contrast
with the influence of heat upon metallic bodies, as observed and described
by Sir Humphry Davy[A].

  [A] Philosophical Transactions, 1821, p. 131.

433. The substance presenting this effect is sulphuret of silver. It was
made by fusing a mixture of precipitated silver and sublimed sulphur,
removing the film of silver by a file from the exterior of the fused mass,
pulverizing the sulphuret, mingling it with more sulphur, and fusing it
again in a green glass tube, so that no air should obtain access during the
process. The surface of the sulphuret being again removed by a file or
knife, it was considered quite free from uncombined silver.

434. When a piece of this sulphuret, half an inch in thickness, was put
between surfaces of platina, terminating the poles of a voltaic battery of
twenty pairs of four-inch plates, a galvanometer being also included in the
circuit, the needle was slightly deflected, indicating a feeble conducting
power. On pressing the platina poles and sulphuret together with the
fingers, the conducting power increased as the whole became warm. On
applying a lamp under the sulphuret between the poles, the conducting power
rose rapidly with the heat, and at last-the galvanometer needle jumped into
a fixed position, and the sulphuret was found conducting in the manner of a
metal. On removing the lamp and allowing the heat to fall, the effects were
reversed, the needle at first began to vibrate a little, then gradually
left its transverse direction, and at last returned to a position very
nearly that which it would take when no current was passing through the
galvanometer.

435. Occasionally, when the contact of the sulphuret with the platina poles
was good, the battery freshly charged, and the commencing temperature not
too low, the mere current of electricity from the battery was sufficient to
raise the temperature of the sulphuret; and then, without any application
of extraneous heat, it went on increasing conjointly in temperature and
conducting power, until the cooling influence of the air limited the
effects. In such cases it was generally necessary to cool the whole
purposely, to show the returning series of phenomena.

436. Occasionally, also, the effects would sink of themselves, and could
not be renewed until a fresh surface of the sulphuret had been applied to
the positive pole. This was in consequence of peculiar results of
decomposition, to which I shall have occasion to revert in the section on
Electro-chemical Decomposition, and was conveniently avoided by inserting
the ends of two pieces of platina wire into the opposite extremities of a
portion of sulphuret fused in a glass tube, and placing this arrangement
between the poles of the battery.

437. The hot sulphuret of silver conducts sufficiently well to give a
bright spark with charcoal, &c. &c., in the manner of a metal.

438. The native grey sulphuret of silver, and the ruby silver ore, both
presented the same phenomena. The native malleable sulphuret of silver
presented precisely the same appearances as the artificial sulphuret.

439. There is no other body with which I am acquainted, that, like
sulphuret of silver, can compare with metals in conducting power for
electricity of low tension when hot, but which, unlike them, during
cooling, loses in power, whilst they, on the contrary, gain. Probably,
however, many others may, when sought for, be found[A].

  [A] See now on this subject, 1340, 1341.--_Dec. 1838._

440. The proto-sulphuret of iron, the native per-sulphuret of iron,
arsenical sulphuret of iron, native yellow sulphuret of copper and iron,
grey artificial sulphuret of copper, artificial sulphuret of bismuth, and
artificial grey sulphuret of tin, all conduct the voltaic battery current
when cold, more or less, some giving sparks like the metals, others not
being sufficient for that high effect. They did not seem to conduct better
when heated, than before; but I had not time to enter accurately into the
investigation of this point. Almost all of them became much heated by the
transmission of the current, and present some very interesting phenomena in
that respect. The sulphuret of antimony does not conduct the same current
sensibly either hot or cold, but is amongst those bodies acquiring
conducting power when fused (402.). The sulphuret of silver and perhaps
some others decompose whilst in the solid state; but the phenomena of this
decomposition will be reserved for its proper place in the next series of
these Researches.

441. Notwithstanding the extreme dissimilarity between sulphuret of silver
and gases or vapours, I cannot help suspecting the action of heat upon them
to be the same, bringing them all into the same class as conductors of
electricity, although with those great differences in degree, which are
found to exist under common circumstances. When gases are heated, they
increase in conducting power, both for common and voltaic electricity
(271.); and it is probable that if we could compress and condense them at
the same time, we should still further increase their conducting power.
Cagniard de la Tour has shown that a substance, for instance water, may be
so expanded by heat whilst in the liquid state, or condensed whilst in the
vaporous state, that the two states shall coincide at one point, and the
transition from one to the other be so gradual that no line of demarcation
can be pointed out[A]; that, in fact, the two states shall become
one;--which one state presents us at different times with differences in
degree as to certain properties and relations; and which differences are,
under ordinary circumstances, so great as to be equivalent to two different
states.

  [A] Annales de Chimie, xxi. pp. 127, 178.

442. I cannot but suppose at present that at that point where the liquid
and the gaseous state coincide, the conducting properties are the same for
both; but that they diminish as the expansion of the matter into a rarer
form takes place by the removal of the necessary pressure; still, however,
retaining, as might be expected, the capability of having what feeble
conducting power remains, increased by the action of heat.

443. I venture to give the following summary of the conditions of electric
conduction in bodies, not however without fearing that I may have omitted
some important points[A].

  [A] See now in relation to this subject, 1320--1242.--_Dec. 1838._

444. All bodies conduct electricity in the same manner from metals to lac
and gases, but in very different degrees.

445. Conducting power is in some bodies powerfully increased by heat, and
in others diminished, yet without our perceiving any accompanying essential
electrical difference, either in the bodies or in the changes occasioned by
the electricity conducted.

446. A numerous class of bodies, insulating electricity of low intensity,
when solid, conduct it very freely when fluid, and are then decomposed by
it.

447. But there are many fluid bodies which do not sensibly conduct
electricity of this low intensity; there are some which conduct it and are
not decomposed; nor is fluidity essential to decomposition[A].

  [A] See the next series of these Experimental Researches.

448. There is but one body yet discovered[A] which, insulating a voltaic
current when solid, and conducting it when fluid, is not decomposed in the
latter case (414.).

  [A] It is just possible that this case may, by more delicate
  experiment, hereafter disappear. (See now, 1340, 1341, in relation to
  this note.--_Dec. 1838._)

449. There is no strict electrical distinction of conduction which can, as
yet, be drawn between bodies supposed to be elementary, and those known to
be compounds.

_Royal Institution,
April 15, 1833_.




FIFTH SERIES.


§ 11. _On Electro-chemical Decomposition._ ¶ i. _New conditions of
Electro-chemical Decomposition._ ¶ ii. _Influence of Water in
Electro-chemical Decomposition._ ¶ iii. _Theory of Electro-chemical
Decomposition._

Received June 18,--Read June 20, 1833.


§ 11. _On Electro-chemical Decomposition._[A]

  [A] Refer to the note after 1047, Series viii.--_Dec. 1838._


450. I have in a recent series of these Researches (265.) proved (to my own
satisfaction, at least,) the identity of electricities derived from
different sources, and have especially dwelt upon the proofs of the
sameness of those obtained by the use of the common electrical machine and
the voltaic battery.

451. The great distinction of the electricities obtained from these two
sources is the very high tension to which the small quantity obtained by
aid of the machine may be raised, and the enormous quantity (371. 376.) in
which that of comparatively low tension, supplied by the voltaic battery,
may be procured; but as their actions, whether magnetical, chemical, or of
any other nature, are essentially the same (360.), it appeared evident that
we might reason from the former as to the manner of action of the latter;
and it was, to me, a probable consequence, that the use of electricity of
such intensity as that afforded by the machine, would, when applied to
effect and elucidate electro-chemical decomposition, show some new
conditions of that action, evolve new views of the internal arrangements
and changes of the substances under decomposition, and perhaps give
efficient powers over matter as yet undecomposed.

452. For the purpose of rendering the bearings of the different parts of
this series of researches more distinct, I shall divide it into several
heads.


¶ i. _New conditions of Electro-chemical Decomposition._

453. The tension of machine electricity causes it, however small in
quantity, to pass through any length of water, solutions, or other
substances classing with these as conductors, as fast as it can be
produced, and therefore, in relation to quantity, as fast as it could have
passed through much shorter portions of the same conducting substance. With
the voltaic battery the case is very different, and the passing current of
electricity supplied by it suffers serious diminution in any substance, by
considerable extension of its length, but especially in such bodies as
those mentioned above.

454. I endeavoured to apply this facility of transmitting the current of
electricity through any length of a conductor, to an investigation of the
transfer of the elements in a decomposing body, in contrary directions,
towards the poles. The general form of apparatus used in these experiments
has been already described (312. 316); and also a particular experiment
(319.), in which, when a piece of litmus paper and a piece of turmeric
paper were combined and moistened in solution of sulphate of soda, the
point of the wire from the machine (representing the positive pole) put
upon the litmus paper, and the receiving point from the discharging train
(292. 316.), representing the negative pole, upon the turmeric paper, a
very few turns of the machine sufficed to show the evolution of acid at the
former, and alkali at the latter, exactly in the manner effected by a
volta-electric current.

455. The pieces of litmus and turmeric paper were _now_ placed each upon a
separate plate of glass, and connected by an insulated string four feet
long, moistened in the same solution of sulphate of soda: the terminal
decomposing wire points were placed upon the papers as before. On working
the machine, the same evolution of acid and alkali appeared as in the
former instance, and with equal readiness, notwithstanding that the places
of their appearance were four feet apart from each other. Finally, a piece
of string, seventy feet long, was used. It was insulated in the air by
suspenders of silk, so that the electricity passed through its entire
length: decomposition took place exactly as in former cases, alkali and
acid appearing at the two extremities in their proper places.

456. Experiments were then made both with sulphate of soda and iodide of
potassium, to ascertain if any diminution of decomposing effect was
produced by such great extension as those just described of the moist
conductor or body under decomposition; but whether the contact of the
decomposing point connected with the discharging train was made with
turmeric paper touching the prime conductor, or with other turmeric paper
connected with it through the seventy feet of string, the spot of alkali
for an equal number of turns of the machine had equal intensity of colour.
The same results occurred at the other decomposing wire, whether the salt
or the iodide were used; and it was fully proved that this great extension
of the distance between the poles produced no effect whatever on the amount
of decomposition, provided the same _quantity_ of electricity were passed
in both cases (377.).

457. The negative point of the discharging train, the turmeric paper, and
the string were then removed; the positive point was left resting upon the
litmus paper, and the latter touched by a piece of moistened string held in
the hand. A few turns of the machine evolved acid at the positive point as
freely as before.

458. The end of the moistened string, instead of being held in the hand,
was suspended by glass in the air. On working the machine the electricity
proceeded from the conductor through the wire point to the litmus paper,
and thence away by the intervention of the string to the air, so that there
was (as in the last experiment) but one metallic pole; still acid was
evolved there as freely as in any former case.

459. When any of these experiments were repeated with electricity from the
negative conductor, corresponding effects were produced whether one or two
decomposing wires were used. The results were always constant, considered
in relation to the _direction_ of the electric current.

460. These experiments were varied so as to include the action of only one
metallic pole, but that not the pole connected with the machine. Turmeric
paper was moistened in solution of sulphate of soda, placed upon glass, and
connected with the discharging train (292.) by a decomposing wire (312.); a
piece of wet string was hung from it, the lower extremity of which was
brought opposite a point connected with the positive prime conductor of the
machine. The machine was then worked for a few turns, and alkali
immediately appeared at the point of the discharging train which rested on
the turmeric paper. Corresponding effects took place at the negative
conductor of a machine.

461. These cases are abundantly sufficient to show that electrochemical
decomposition does not depend upon the simultaneous action of two metallic
poles, since a single pole might be used, decomposition ensue, and one or
other of the elements liberated, pass to the pole, according as it was
positive or negative. In considering the course taken by, and the final
arrangement of, the other element, I had little doubt that I should find it
had receded towards the other extremity, and that the air itself had acted
as a pole, an expectation which was fully confirmed in the following
manner.

462. A piece of turmeric paper, not more than 0.4 of an inch in length and
0.5 of an inch in width, was moistened with sulphate of soda and placed
upon the edge of a glass plate opposite to, and about two inches from, a
point connected with the discharging train (Plate IV. fig. 47.); a piece of
tinfoil, resting upon the same glass plate, was connected with the machine,
and also with the turmeric paper, by a decomposing wire _a_ (312.). The
machine was then worked, the positive electricity passing into the turmeric
paper at the point _p_, and out at the extremity _n_. After forty or fifty
turns of the machine, the extremity _n_ was examined, and the two points or
angles found deeply coloured by the presence of free alkali (fig. 48.).

463. A similar piece of litmus paper, dipped in solution of sulphate of
soda _n_, fig. 49, was now supported upon the end of the discharging train
_a_, and its extremity brought opposite to a point _p_, connected with the
conductor of the machine. After working the machine for a short time, acid
was developed at both the corners towards the point, i.e. at both the
corners receiving the electricities from the air. Every precaution was
taken to prevent this acid from being formed by sparks or brushes passing
through the air (322.); and these, with the accompanying general facts, are
sufficient to show that the acid was really the result of electro-chemical
decomposition (466.).

464. Then a long piece of turmeric paper, large at one end and pointed at
the other, was moistened in the saline solution, and immediately connected
with the conductor of the machine, so that its pointed extremity was
opposite a point upon the discharging train. When the machine was worked,
alkali was evolved at that point; and even when the discharging train was
removed, and the electricity left to be diffused and carried off altogether
by the air, still alkali was evolved where the electricity left the
turmeric paper.

465. Arrangements were then made in which no metallic communication with
the decomposing matter was allowed, but both poles (if they might now be
called by that name) formed of air only. A piece of turmeric paper _a_ fig.
50, and a piece of litmus paper _b_, were dipped in solution of sulphate of
soda, put together so as to form one moist pointed conductor, and supported
on wax between two needle points, one, _p_, connected by a wire with the
conductor of the machine, and the other, _n_, with the discharging train.
The interval in each case between the points was about half an inch; the
positive point _p_ was opposite the litmus paper; the negative point _n_
opposite the turmeric. The machine was then worked for a time, upon which
evidence of decomposition quickly appeared, for the point of the litmus _b_
became reddened from acid evolved there, and the point of the turmeric _a_
red from a similar and simultaneous evolution of alkali.

466. Upon turning the paper conductor round, so that the litmus point
should now give off the positive electricity, and the turmeric point
receive it, and working the machine for a short time, both the red spots
disappeared, and as on continuing the action of the machine no red spot was
re-formed at the litmus extremity, it proved that in the first instance
(463.) the effect was not due to the action of brushes or mere electric
discharges causing the formation of nitric acid from the air (322.).

467. If the combined litmus and turmeric paper in this experiment be
considered as constituting a conductor independent of the machine or the
discharging train, and the final places of the elements evolved be
considered in relation to this conductor, then it will be found that the
acid collects at the _negative_ or receiving end or pole of the
arrangement, and the alkali at the _positive_ or delivering extremity.

468. Similar litmus and turmeric paper points were now placed upon glass
plates, and connected by a string six feet long, both string and paper
being moistened in solution of sulphate of soda; a needle point connected
with the machine was brought opposite the litmus paper point, and another
needle point connected with the discharging train brought opposite the
turmeric paper. On working the machine, acid appeared on the litmus, and
alkali on the turmeric paper; but the latter was not so abundant as in
former cases, for much of the electricity passed off from the string into
the air, and diminished the quantity discharged at the turmeric point.

469. Finally, a series of four small compound conductors, consisting of
litmus and turmeric paper (fig. 51.) moistened in solution of sulphate of
soda, were supported on glass rods, in a line at a little distance from
each other, between the points _p_ and _n_ of the machine and discharging
train, so that the electricity might pass in succession through them,
entering in at the litmus points _b, b_, and passing out at the turmeric
points _a, a_. On working the machine carefully, so as to avoid sparks and
brushes (322.), I soon obtained evidence of decomposition in each of the
moist conductors, for all the litmus points exhibited free acid, and the
turmeric points equally showed free alkali.

470. On using solutions of iodide of potassium, acetate of lead, &c.,
similar effects were obtained; but as they were all consistent with the
results above described, I refrain from describing the appearances
minutely.

471. These cases of electro-chemical decomposition are in their nature
exactly of the same kind as those affected under ordinary circumstances by
the voltaic battery, notwithstanding the great differences as to the
presence or absence, or at least as to the nature of the parts usually
called poles; and also of the final situation of the elements eliminated at
the electrified boundary surfaces (467.). They indicate at once an internal
action of the parts suffering decomposition, and appear to show that the
power which is effectual in separating the elements is exerted there, and
not at the poles. But I shall defer the consideration of this point for a
short time (493. 518.), that I may previously consider another supposed
condition of electro-chemical decomposition[A].

  [A] I find (since making and describing these results,) from a note to
  Sir Humphry Davy's paper in the Philosophical Transactions, 1807, p.
  31, that that philosopher, in repeating Wollaston's experiment of the
  decomposition of water by common electricity (327. 330.) used an
  arrangement somewhat like some of those I have described. He immersed
  a guarded platina point connected with the machine in distilled water,
  and dissipated the electricity from the water into the air by
  moistened filaments of cotton. In this way he states that he obtained
  oxygen and hydrogen _separately_ from each other. This experiment, had
  I known of it, ought to have been quoted in an earlier series of these
  Researches (342.); but it does not remove any of the objections I have
  made to the use of Wollaston's apparatus as a test of true chemical
  action (331.).


¶ ii. _Influence of Water in Electro-chemical Decomposition._

472. It is the opinion of several philosophers, that the presence of water
is essential in electro-chemical decomposition, and also for the evolution
of electricity in the voltaic battery itself. As the decomposing cell is
merely one of the cells of the battery, into which particular substances
are introduced for the purpose of experiment, it is probable that what is
an essential condition in the one case is more or less so in the other. The
opinion, therefore, that water is necessary to decomposition, may have been
founded on the statement made by Sir Humphry Davy, that "there are no
fluids known, except such as contain water, which are capable of being made
the medium of connexion between the metals or metal of the voltaic
apparatus[A]:" and again, "when any substance rendered fluid by heat,
consisting of _water_, oxygen, and inflammable or metallic matter, is
exposed to those wires, similar phenomena (of decomposition) occur[B]."

  [A] Elements of Chemical Philosophy, p. 160, &c.

  [B] Ibid. pp. 144, 145.

473. This opinion has, I think, been shown by other philosophers not to be
accurate, though I do not know where to refer for a contradiction of it.
Sir Humphry Davy himself said in 1801[A], that dry nitre, caustic potash
and soda are conductors of galvanism when rendered fluid by a high degree
of heat, but he must have considered them, or the nitre at least, as not
suffering decomposition, for the statements above were made by him eleven
years subsequently. In 1826 he also pointed out, that bodies not containing
water, as _fused litharge_ and _chlorate of potassa_, were sufficient to
form, with platina and zinc, powerful electromotive circles[B]; but he is
here speaking of the _production_ of electricity in the pile, and not of
its effects when evolved; nor do his words at all imply that any correction
of his former distinct statements relative to _decomposition_ was required.

  [A] Journal of the Royal Institution, 1802, p. 53.

  [B] Philosophical Transactions, 1826, p. 406.

474. I may refer to the last series of these Experimental Researches (380.
402.) as setting the matter at rest, by proving that there are hundreds of
bodies equally influential with water in this respect; that amongst binary
compounds, oxides, chlorides, iodides, and even sulphurets (402.) were
effective; and that amongst more complicated compounds, cyanides and salts,
of equal efficacy, occurred in great numbers (402.).

475. Water, therefore, is in this respect merely one of a very numerous
class of substances, instead of being the _only one_ and _essential_; and
it is of that class one of the _worst_ as to its capability of facilitating
conduction and suffering decomposition. The reasons why it obtained for a
time an exclusive character which it so little deserved are evident, and
consist, in the general necessity of a fluid condition (394.); in its being
the _only one_ of this class of bodies existing in the fluid state at
common temperatures; its abundant supply as the great natural solvent; and
its constant use in that character in philosophical investigations, because
of its having a smaller interfering, injurious, or complicating action upon
the bodies, either dissolved or evolved, than any other substance.

476. The analogy of the decomposing or experimental cell to the other cells
of the voltaic battery renders it nearly certain that any of those
substances which are decomposable when fluid, as described in my last paper
(402.), would, if they could be introduced between the metallic plates of
the pile, be equally effectual with water, if not more so. Sir Humphry Davy
found that litharge and chlorate of potassa were thus effectual[A]. I have
constructed various voltaic arrangements, and found the above conclusion to
hold good. When any of the following substances in a fused state were
interposed between copper and platina, voltaic action more or less powerful
was produced. Nitre; chlorate of potassa; carbonate of potassa; sulphate of
soda; chloride of lead, of sodium, of bismuth, of calcium; iodide of lead;
oxide of bismuth; oxide of lead: the electric current was in the same
direction as if acids had acted upon the metals. When any of the same
substances, or phosphate of soda, were made to act on platina and iron,
still more powerful voltaic combinations of the same kind were produced.
When either nitrate of silver or chloride of silver was the fluid substance
interposed, there was voltaic action, but the electric current was in the
reverse direction.

  [A] Philosophical Transactions, 1826, p. 406.

iii. _Theory of Electro-chemical Decomposition._

477. The extreme beauty and value of electro-chemical decompositions have
given to that power which the voltaic pile possesses of causing their
occurrence an interest surpassing that of any other of its properties; for
the power is not only intimately connected with the continuance, if not
with the production, of the electrical phenomena, but it has furnished us
with the most beautiful demonstrations of the nature of many compound
bodies; has in the hands of Becquerel been employed in compounding
substances; has given us several new combinations, and sustains us with the
hope that when thoroughly understood it will produce many more.

478. What may be considered as the general facts of electrochemical
decomposition are agreed to by nearly all who have written on the subject.
They consist in the separation of the decomposable substance acted upon
into its proximate or sometimes ultimate principles, whenever both poles of
the pile are in contact with that substance in a proper condition; in the
evolution of these principles at distant points, i.e. at the poles of the
pile, where they are either finally set free or enter into union with the
substance of the poles; and in the constant determination of the evolved
elements or principles to particular poles according to certain
well-ascertained laws.

479. But the views of men of science vary much as to the nature of the
action by which these effects are produced; and as it is certain that we
shall be better able to apply the power when we really understand the
manner in which it operates, this difference of opinion is a strong
inducement to further inquiry. I have been led to hope that the following
investigations might be considered, not as an increase of that which is
doubtful, but a real addition to this branch of knowledge.

480. It will be needful that I briefly state the views of electro-chemical
decomposition already put forth, that their present contradictory and
unsatisfactory state may be seen before I give that which seems to me more
accurately to agree with facts; and I have ventured to discuss them freely,
trusting that I should give no offence to their high-minded authors; for I
felt convinced that if I were right, they would be pleased that their views
should serve as stepping-stones for the advance of science; and that if I
were wrong, they would excuse the zeal which misled me, since it was
exerted for the service of that great cause whose prosperity and progress
they have desired.

481. Grotthuss, in the year 1805, wrote expressly on the decomposition of
liquids by voltaic electricity[A]. He considers the pile as an electric
magnet, i.e. as an attractive and repulsive agent; the poles having
_attractive_ and _repelling_ powers. The pole from whence resinous
electricity issues attracts hydrogen and repels oxygen, whilst that from
which vitreous electricity proceeds attracts oxygen and repels hydrogen; so
that each of the elements of a particle of water, for instance, is subject
to an attractive and a repulsive force, acting in contrary directions, the
centres of action of which are reciprocally opposed. The action of each
force in relation to a molecule of water situated in the course of the
electric current is in the inverse ratio of the square of the distance at
which it is exerted, thus giving (it is stated) for such a molecule a
_constant force_[B]. He explains the appearance of the elements at a
distance from each other by referring to a succession of decompositions and
recompositions occurring amongst the intervening particles[C], and he
thinks it probable that those which are about to separate at the poles
unite to the two electricities there, and in consequence become gases[D].

  [A] Annales de Chimie, 1806, tom, lviii. p. 64.

  [B] Ibid. pp. 66, 67, also tom. lxiii. p. 20.

  [C] Ibid. tom. lviii. p. 68, tom, lxiii. p. 20.

  [D] Ibid. tom. lxiii. p. 34.

482. Sir Humphry Davy's celebrated Bakerian Lecture on some chemical
agencies of electricity was read in November 1806, and is almost entirely
occupied in the consideration of _electro-chemical decompositions_. The
facts are of the utmost value, and, with the general points established,
are universally known. The _mode of action_ by which the effects take place
is stated very generally, so generally, indeed, that probably a dozen
precise schemes of electro-chemical action might be drawn up, differing
essentially from each other, yet all agreeing with the statement there
given.

483. When Sir Humphry Davy uses more particular expressions, he seems to
refer the decomposing effects to the attractions of the poles. This is the
case in the "general expression of facts" given at pp. 28 and 29 of the
Philosophical Transactions for 1807, also at p. 30. Again at p. 160 of the
Elements of Chemical Philosophy, he speaks of the great attracting powers
of the surfaces of the poles. He mentions the probability of a succession
of decompositions and recompositions throughout the fluid,--agreeing in
that respect with Grotthuss[A]; and supposes that the attractive and
repellent agencies may be communicated from the metallic surfaces
throughout the whole of the menstruum[B], being communicated from _one
particle to another particle of the same kind_[C], and diminishing in
strength from the place of the poles to the middle point, which is
necessarily neutral[D]. In reference to this diminution of power at
increased distances from the poles, he states that in a circuit of ten
inches of water, solution of sulphate of potassa placed four inches from
the positive pole, did not decompose; whereas when only two inches from
that pole, it did render up its elements[E].

  [A] Philosophical Transactions, 1807, pp. 29, 30.

  [B] Ibid. p. 39.

  [C] Ibid. p. 29.

  [D] Ibid. p. 42.

  [E] Ibid. p. 42.

484. When in 1826 Sir Humphry Davy wrote again on this subject, he stated
that he found nothing to alter in the fundamental theory laid down in the
original communication[A], and uses the terms attraction and repulsion
apparently in the same sense as before[B].

  [A] Philosophical Transactions, 1826, p. 383.

  [B] Ibid. pp. 389, 407, 115.

485. Messrs. Riffault and Chompré experimented on this subject in 1807.
They came to the conclusion that the voltaic current caused decompositions
throughout its whole course in the humid conductor, not merely as
preliminary to the recompositions spoken of by Grotthuss and Davy, but
producing final separation of the elements in the _course_ of the current,
and elsewhere than at the poles. They considered the _negative_ current as
collecting and carrying the acids, &c. to the _positive_ pole, and the
_positive_ current as doing the same duty with the bases, and collecting
them at the _negative_ pole. They likewise consider the currents as _more
powerful_ the nearer they are to their respective poles, and state that the
positive current is _superior_ in power to the negative current[A].

  [A] Annales de Chimie, 1807, tom. lxiii. p. 83, &c.

486. M. Biot is very cautious in expressing an opinion as to the cause of
the separation of the elements of a compound body[A]. But as far as the
effects can be understood, he refers them to the opposite electrical states
of the portions of the decomposing substance in the neighbourhood of the
two poles. The fluid is most positive at the positive pole; that state
gradually diminishes to the middle distance, where the fluid is neutral or
not electrical; but from thence to the negative pole it becomes more and
more negative[B]. When a particle of salt is decomposed at the negative
pole, the acid particle is considered as acquiring a negative electrical
state from the pole, stronger than that of the surrounding _undecomposed_
particles, and is therefore repelled from amongst them, and from out of
that portion of the liquid towards the positive pole, towards which also it
is drawn by the attraction of the pole itself and the particles of positive
_undecomposed_ fluid around it[C].

  [A] Précis Elémentaire de Physique, 3me édition, 1824, tom. i. p. 641.

  [B] Ibid. p. 637.

  [C] Ibid. pp. 641, 642.

487. M. Biot does not appear to admit the successive decompositions and
recompositions spoken of by Grotthuss, Davy, &c. &c.; but seems to consider
the substance whilst in transit as combined with, or rather attached to,
the electricity for the time[A], and though it communicates this
electricity to the surrounding undecomposed matter with which it is in
contact, yet it retains during the transit a little superiority with
respect to that kind which it first received from the pole, and is, by
virtue of that difference, carried forward through the fluid to the
opposite pole[B].

  [A] Précis Elémentaire de Physique, 3me édition, 1824, tom. i. p. 636.

  [B] Ibid. p, 642.

488. This theory implies that decomposition takes place at both poles upon
distinct portions of fluid, and not at all in the intervening parts. The
latter serve merely as imperfect conductors, which, assuming an electric
state, urge particles electrified more highly at the poles through them in
opposite directions, by virtue of a series of ordinary electrical
attractions and repulsions[A].

  [A] Précis Elémentaire de Physique, 3me édition, 1824, tom. i. pp.
  638, 642.

489. M.A. de la Rive investigated this subject particularly, and published
a paper on it in 1825[A]. He thinks those who have referred the phenomena
to the attractive powers of the poles, rather express the general fact than
give any explication of it. He considers the results as due to an actual
combination of the elements, or rather of half of them, with the
electricities passing from the poles in consequence of a kind of play of
affinities between the matter and electricity[B]. The current from the
positive pole combining with the hydrogen, or the bases it finds there,
leaves the oxygen and acids at liberty, but carries the substances it is
united with across to the negative pole, where, because of the peculiar
character of the metal as a conductor[C], it is separated from them,
entering the metal and leaving the hydrogen or bases upon its surface. In
the same manner the electricity from the negative pole sets the hydrogen
and bases which it finds there, free, but combines with the oxygen and
acids, carries them across to the positive pole, and there deposits
them[D]. In this respect M. de la Rive's hypothesis accords in part with
that of MM. Riffault and Chompré (485.).

  [A] Annales de Chimie, tom, xxviii. p. 190.

  [B] Ibid. pp. 200, 202.

  [C] Ibid. p. 202.

  [D] Ibid. p. 201.

490. M. de la Rive considers the portions of matter which are decomposed to
be those contiguous to _both_ poles[A]. He does not admit with others the
successive decompositions and recompositions in the whole course of the
electricity through the humid conductor[B], but thinks the middle parts are
in themselves unaltered, or at least serve only to conduct the two contrary
currents of electricity and matter which set off from the opposite
poles[C]. The decomposition, therefore, of a particle of water, or a
particle of salt, may take place at either pole, and when once effected, it
is final for the time, no recombination taking place, except the momentary
union of the transferred particle with the electricity be so considered.

  [A] Annales de Chimie, tom, xxviii. pp. 197, 198.

  [B] Ibid. pp. 192, 199.

  [C] Ibid. p. 200.

491. The latest communication that I am aware of on the subject is by M.
Hachette: its date is October 1832[A]. It is incidental to the description
of the decomposition of water by the magneto-electric currents (346.). One
of the results of the experiment is, that "it is not necessary, as has been
supposed, that for the chemical decomposition of water, the action of the
two electricities, positive and negative, should be simultaneous."

  [A] Annales de Chimie, tom, xxviii. tom. li. p. 73.

492. It is more than probable that many other views of electro-chemical
decomposition may have been published, and perhaps amongst them some which,
differing from those above, might, even in my own opinion, were I
acquainted with them, obviate the necessity for the publication of my
views. If such be the case, I have to regret my ignorance of them, and
apologize to the authors.

       *       *       *       *       *

493. That electro-chemical decomposition does not depend upon any direct
attraction and repulsion of the poles (meaning thereby the metallic
terminations either of the voltaic battery, or ordinary electrical machine
arrangements (312.),) upon the elements in contact with or near to them,
appeared very evident from the experiments made in air (462, 465, &c.),
when the substances evolved did not collect about any poles, but, in
obedience to the direction of the current, were evolved, and I would say
ejected, at the extremities of the decomposing substance. But
notwithstanding the extreme dissimilarity in the character of air and
metals, and the almost total difference existing between them as to their
mode of conducting electricity, and becoming charged with it, it might
perhaps still be contended, although quite hypothetically, that the
bounding portions of air were now the surfaces or places of attraction, as
the metals had been supposed to be before. In illustration of this and
other points, I endeavoured to devise an arrangement by which I could
decompose a body against a surface of water, as well as against air or
metal, and succeeded in doing so unexceptionably in the following manner.
As the experiment for very natural reasons requires many precautions, to be
successful, and will be referred to hereafter in illustration of the views
I shall venture to give, I must describe it minutely.

494. A glass basin (fig. 52.), four inches in diameter and four inches
deep, had a division of mica _a_, fixed across the upper part so as to
descend one inch and a half below the edge, and be perfectly water-tight at
the sides: a plate of platina _b_, three inches wide, was put into the
basin on one side of the division _a_, and retained there by a glass block
below, so that any gas produced by it in a future stage of the experiment
should not ascend beyond the mica, and cause currents in the liquid on that
side. A strong solution of sulphate of magnesia was carefully poured
without splashing into the basin, until it rose a little above the lower
edge of the mica division _a_, great care being taken that the glass or
mica on the unoccupied or _c_ side of the division in the figure, should
not be moistened by agitation of the solution above the level to which it
rose. A thin piece of clean cork, well-wetted in distilled water, was then
carefully and lightly placed on the solution at the _c_ side, and distilled
water poured gently on to it until a stratum the eighth of an inch in
thickness appeared over the sulphate of magnesia; all was then left for a
few minutes, that any solution adhering to the cork might sink away from
it, or be removed by the water on which it now floated; and then more
distilled water was added in a similar manner, until it reached nearly to
the top of the glass. In this way solution of the sulphate occupied the
lower part of the glass, and also the upper on the right-hand side of the
mica; but on the left-hand side of the division a stratum of water from _c_
to _d_, one inch and a half in depth, reposed upon it, the two presenting,
when looked through horizontally, a comparatively definite plane of
contact. A second platina pole _e_, was arranged so as to be just under the
surface of the water, in a position nearly horizontal, a little inclination
being given to it, that gas evolved during decomposition might escape: the
part immersed was three inches and a half long by one inch wide, and about
seven-eighths of an inch of water intervened between it and the solution of
sulphate of magnesia.

495. The latter pole _e_ was now connected with the negative end of a
voltaic battery, of forty pairs of plates four inches square, whilst the
former pole _b_ was connected with the positive end. There was action and
gas evolved at both poles; but from the intervention of the pure water, the
decomposition was very feeble compared to what the battery would have
effected in a uniform solution. After a little while (less than a minute,)
magnesia also appeared at the negative side: _it did not make its
appearance at the negative metallic pole, but in the water_, at the plane
where the solution and the water met; and on looking at it horizontally, it
could be there perceived lying in the water upon the solution, not rising
more than the fourth of an inch above the latter, whilst the water between
it and the negative pole was perfectly clear. On continuing the action, the
bubbles of hydrogen rising upwards from the negative pole impressed a
circulatory movement on the stratum of water, upwards in the middle, and
downwards at the side, which gradually gave an ascending form to the cloud
of magnesia in the part just under the pole, having an appearance as if it
were there attracted to it; but this was altogether an effect of the
currents, and did not occur until long after the phenomena looked for were
satisfactorily ascertained.

496. After a little while the voltaic communication was broken, and the
platina poles removed with as little agitation as possible from the water
and solution, for the purpose of examining the liquid adhering to them. The
pole _c_, when touched by turmeric paper, gave no traces of alkali, nor
could anything but pure water be found upon it. The pole _b_, though drawn
through a much greater depth and quantity of fluid, was found so acid as to
give abundant evidence to litmus paper, the tongue, and other tests. Hence
there had been no interference of alkaline salts in any way, undergoing
first decomposition, and then causing the separation of the magnesia at a
distance from the pole by mere chemical agencies. This experiment was
repeated again and again, and always successfully.

497. As, therefore, the substances evolved in cases of electrochemical
decomposition may be made to appear against air (465. 469.),--which,
according to common language, is not a conductor, nor is decomposed, or
against water (495.), which is a conductor, and can be decomposed,--as well
as against the metal poles, which are excellent conductors, but
undecomposable, there appears but little reason to consider the phenomena
generally, as due to the _attraction_ or attractive powers of the latter,
when used in the ordinary way, since similar attractions can hardly be
imagined in the former instances.

498. It may be said that the surfaces of air or of water in these cases
become the poles, and exert attractive powers; but what proof is there of
that, except the fact that the matters evolved collect there, which is the
point to be explained, and cannot be justly quoted as its own explanation?
Or it may be said, that any section of the humid conductor, as that in the
present case, where the solution and the water meet, may be considered as
representing the pole. But such does not appear to me to be the view of
those who have written on the subject, certainly not of some of them, and
is inconsistent with the supposed laws which they have assumed, as
governing the diminution of power at increased distances from the poles.

499. Grotthuss, for instance, describes the poles as centres of attractive
and repulsive forces (481.), these forces varying inversely as the squares
of the distances, and says, therefore, that a particle placed anywhere
between the poles will be acted upon by a constant force. But the compound
force, resulting from such a combination as he supposes, would be anything
but a constant force; it would evidently be a force greatest at the poles,
and diminishing to the middle distance. Grotthuss is right, however, _in
the fact_, according to my experiments (502. 505.), that the particles are
acted upon by equal force everywhere in the circuit, when the conditions of
the experiment are the simplest possible; but the fact is against his
theory, and is also, I think, against all theories that place the
decomposing effect in the attractive power of the poles.

500. Sir Humphry Davy, who also speaks of the _diminution_ of power with
increase of distance from the poles[A] (483.), supposes, that when both
poles are acting on substances to decompose them, still the power of
decomposition _diminishes_ to the middle distance. In this statement of
fact he is opposed to Grotthuss, and quotes an experiment in which sulphate
of potassa, placed at different distances from the poles in a humid
conductor of constant length, decomposed when near the pole, but not when
at a distance. Such a consequence would necessarily result theoretically
from considering the poles as centres of attraction and repulsion; but I
have not found the statement borne out by other experiments (505.); and in
the one quoted by him the effect was doubtless due to some of the many
interfering causes of variation which attend such investigations.

  [A] Philosophical Transactions, 1807, p. 42.

501. A glass vessel had a platina plate fixed perpendicularly across it, so
as to divide it into two cells: a head of mica was fixed over it, so as to
collect the gas it might evolve during experiments; then each cell, and the
space beneath the mica, was filled with dilute sulphuric acid. Two poles
were provided, consisting each of a platina wire terminated by a plate of
the same metal; each was fixed into a tube passing through its upper end by
an air-tight joint, that it might be moveable, and yet that the gas evolved
at it might be collected. The tubes were filled with the acid, and one
immersed in each cell. Each platina pole was equal in surface to one side
of the dividing plate in the middle glass vessel, and the whole might be
considered as an arrangement between the poles of the battery of a humid
decomposable conductor divided in the middle by the interposed platina
diaphragm. It was easy, when required, to draw one of the poles further up
the tube, and then the platina diaphragm was no longer in the middle of the
humid conductor. But whether it were thus arranged at the middle, or
towards one side, it always evolved a quantity of oxygen and hydrogen equal
to that evolved by both the extreme plates[A].

  [A] There are certain precautions, in this and such experiments, which
  can only be understood and guarded against by a knowledge of the
  phenomena to be described in the first part of the Sixth Series of
  these Researches.

502. If the wires of a galvanometer be terminated by plates, and these be
immersed in dilute acid, contained in a regularly formed rectangular glass
trough, connected at each end with a voltaic battery by poles equal to the
section of the fluid, a part of the electricity will pass through the
instrument and cause a certain deflection. And if the plates are always
retained at the _same distance from each other_ and from the sides of the
trough, are always parallel to each other, and uniformly placed relative to
the fluid, then, whether they are immersed near the middle of the
decomposing solution, or at one end, still the instrument will indicate the
same deflection, and consequently the same electric influence.

503. It is very evident, that when the width of the decomposing conductor
varies, as is always the case when mere wires or plates, as poles, are
dipped into or are surrounded by solution, no constant expression can be
given as to the action upon a single particle placed in the course of the
current, nor any conclusion of use, relative to the supposed attractive or
repulsive force of the poles, be drawn. The force will vary as the distance
from the pole varies; as the particle is directly between the poles, or
more or less on one side; and even as it is nearer to or further from the
sides of the containing vessels, or as the shape of the vessel itself
varies; and, in fact, by making variations in the form of the arrangement,
the force upon any single particle may be made to increase, or diminish, or
remain constant, whilst the distance between the particle and the pole
shall remain the same; or the force may be made to increase, or diminish,
or remain constant, either as the distance increases or as it diminishes.

504. From numerous experiments, I am led to believe the following general
expression to be correct; but I purpose examining it much further, and
would therefore wish not to be considered at present as pledged to its
accuracy. The _sum of chemical decomposition is constant_ for any section
taken across a decomposing conductor, uniform in its nature, at whatever
distance the poles may be from each other or from the section; or however
that section may intersect the currents, whether directly across them, or
so oblique as to reach almost from pole to pole, or whether it be plane, or
curved, or irregular in the utmost degree; provided the current of
electricity be retained constant in quantity (377.), and that the section
passes through every part of the current through the decomposing conductor.

505. I have reason to believe that the statement might be made still more
general, and expressed thus: That _for a constant quantity of electricity,
whatever the decomposing conductor may be, whether water, saline solutions,
acids, fused bodies, &c., the amount of electro-chemical action is also a
constant quantity, i.e. would always be equivalent to a standard chemical
effect founded upon ordinary chemical affinity_. I have this investigation
in hand, with several others, and shall be prepared to give it in the next
series but one of these Researches.

506. Many other arguments might be adduced against the hypotheses of the
attraction of the poles being the cause of electro-chemical decomposition;
but I would rather pass on to the view I have thought more consistent with
facts, with this single remark; that if decomposition by the voltaic
battery depended upon the attraction of the poles, or the parts about them,
being stronger than the mutual attraction of the particles separated, it
would follow that the weakest _electrical_ attraction was stronger than, if
not the strongest, yet very strong _chemical_ attraction, namely, such as
exists between oxygen and hydrogen, potassium and oxygen, chlorine and
sodium, acid and alkali, &c., a consequence which, although perhaps not
impossible, seems in the present state of the subject very unlikely.

507. The view which M. de la Rive has taken (489.), and also MM. Riffault
and Chompré (485.), of the manner in which electro-chemical decomposition
is effected, is very different to that already considered, and is not
affected by either the arguments or facts urged against the latter.
Considering it as stated by the former philosopher, it appears to me to be
incompetent to account for the experiments of decomposition against
surfaces of air (462. 469.) and water (495.), which I have described; for
if the physical differences between metals and humid conductors, which M.
de la Rive supposes to account for the transmission of the compound of
matter and electricity in the latter, and the transmission of the
electricity only with the rejection of the matter in the former, be allowed
for a moment, still the analogy of air to metal is, electrically
considered, so small, that instead of the former replacing the latter
(462.), an effect the very reverse might have been expected. Or if even
that were allowed, the experiment with water (495.), at once sets the
matter at rest, the decomposing pole being now of a substance which is
admitted as competent to transmit the assumed compound of electricity and
matter.

508. With regard to the views of MM. Riffault and Chompré (485.), the
occurrence of decomposition alone in the _course_ of the current is so
contrary to the well-known effects obtained in the forms of experiment
adopted up to this time, that it must be proved before the hypothesis
depending on it need be considered.

509. The consideration of the various theories of electro-chemical
decomposition, whilst it has made me diffident, has also given me
confidence to add another to the number; for it is because the one I have
to propose appears, after the most attentive consideration, to explain and
agree with the immense collection of facts belonging to this branch of
science, and to remain uncontradicted by, or unopposed to, any of them,
that I have been encouraged to give it.

510. Electro-chemical decomposition is well known to depend essentially
upon the _current_ of electricity. I have shown that in certain cases
(375.) the decomposition is proportionate to the quantity of electricity
passing, whatever may be its intensity or its source, and that the same is
probably true for all cases (377.), even when the utmost generality is
taken on the one hand, and great precision of expression on the other
(505.).

511. In speaking of the current, I find myself obliged to be still more
particular than on a former occasion (283.), in consequence of the variety
of views taken by philosophers, all agreeing in the effect of the current
itself. Some philosophers, with Franklin, assume but one electric fluid;
and such must agree together in the general uniformity and character of the
electric current. Others assume two electric fluids; and here singular
differences have arisen.

512. MM. Riffault and Chompré, for instance, consider the positive and
negative currents each as causing decomposition, and state that the
positive current is _more powerful_ than the negative current[A], the
nitrate of soda being, under similar circumstances, decomposed by the
former, but not by the latter.

  [A] Annales de Chimie, 1807, tom, lxiii. p. 84.

513. M. Hachette states[A] that "it is not necessary, as has been believed,
that the action of the two electricities, positive and negative, should be
simultaneous for the decomposition of water." The passage implying, if I
have caught the meaning aright, that one electricity can be obtained, and
can be applied in effecting decompositions, independent of the other.

  [A] Annales de Chimie, 1832, tom. li. p. 73.

514. The view of M. de la Rive to a certain extent agrees with that of M.
Hachette, for he considers that the two electricities decompose separate
portions of water (490.)[A]. In one passage he speaks of the two
electricities as two influences, wishing perhaps to avoid offering a
decided opinion upon the independent existence of electric fluids; but as
these influences are considered as combining with the elements set free as
by a species of chemical affinity, and for the time entirely masking their
character, great vagueness of idea is thus introduced, inasmuch as such a
species of combination can only be conceived to take place between things
having independent existences. The two elementary electric currents, moving
in opposite directions, from pole to pole, constitute the ordinary _voltaic
current._

  [A] Annales de Chimie, 1825, tom, xxviii. pp. 197, 201.

515. M. Grotthuss is inclined to believe that the elements of water, when
about to separate at the poles, combine with the electricities, and so
become gases. M. de la Rive's view is the exact reverse of this: whilst
passing through the fluid, they are, according to him, compounds with the
electricities; when evolved at the poles, they are de-electrified.

516. I have sought amongst the various experiments quoted in support of
these views, or connected with electro-chemical decompositions or electric
currents, for any which might be considered as sustaining the theory of two
electricities rather than that of one, but have not been able to perceive a
single fact which could be brought forward for such a purpose: or,
admitting the hypothesis of two electricities, much less have I been able
to perceive the slightest grounds for believing that one electricity in a
current can be more powerful than the other, or that it can be present
without the other, or that one can be varied or in the slightest degree
affected, without a corresponding variation in the other[A]. If, upon the
supposition of two electricities, a current of one can be obtained without
the other, or the current of one be exalted or diminished more than the
other, we might surely expect some variation either of the chemical or
magnetical effects, or of both; but no such variations have been observed.
If a current be so directed that it may act chemically in one part of its
course, and magnetically in another, the two actions are always found to
take place together. A current has not, to my knowledge, been produced
which could act chemically and not magnetically, nor any which can act on
the magnet, and not _at the same time_ chemically[B].

  [A] See now in relation to this subject, 1627-1645.--_Dec. 1838._

  [B] Thermo-electric currents are of course no exception, because when
  they fail to act chemically they also fail to be currents.

517. _Judging from facts only_, there is not as yet the slightest reason
for considering the influence which is present in what we call the electric
current,--whether in metals or fused bodies or humid conductors, or even in
air, flame, and rarefied elastic media,--as a compound or complicated
influence. It has never been resolved into simpler or elementary
influences, and may perhaps best be conceived of as _an axis of power
having contrary forces, exactly equal in amount, in contrary directions_.

       *       *       *       *       *

518. Passing to the consideration of electro-chemical decomposition, it
appears to me that the effect is produced by an _internal corpuscular
action_, exerted according to the direction of the electric current, and
that it is due to a force either _super to_, or _giving direction to the
ordinary chemical affinity_ of the bodies present. The body under
decomposition may be considered as a mass of acting particles, all those
which are included in the course of the electric current contributing to
the final effect; and it is because the ordinary chemical affinity is
relieved, weakened, or partly neutralized by the influence of the electric
current in one direction parallel to the course of the latter, and
strengthened or added to in the opposite direction, that the combining
particles have a tendency to pass in opposite courses.

519. In this view the effect is considered as _essentially dependent_ upon
the _mutual chemical affinity_ of the particles of opposite kinds.
Particles _aa_, fig. 53, could not be transferred or travel from one pole N
towards the other P, unless they found particles of the opposite kind _bb_,
ready to pass in the contrary direction: for it is by virtue of their
increased affinity for those particles, combined with their diminished
affinity for such as are behind them in their course, that they are urged
forward: and when any one particle _a_, fig. 54, arrives at the pole, it is
excluded or set free, because the particle _b_ of the opposite kind, with
which it was the moment before in combination, has, under the superinducing
influence of the current, a greater attraction for the particle _a'_, which
is before it in its course, than for the particle _a_, towards which its
affinity has been weakened.

520. As far as regards any single compound particle, the case may be
considered as analogous to one of ordinary decomposition, for in fig. 54,
_a_ may be conceived to be expelled from the compound _ab_ by the superior
attraction of _a'_ for _b_, that superior attraction belonging to it in
consequence of the relative position of _a'b_ and _a_ to the direction of
the axis of electric power (517.) superinduced by the current. But as all
the compound particles in the course of the current, except those actually
in contact with the poles, act conjointly, and consist of elementary
particles, which, whilst they are in one direction expelling, are in the
other being expelled, the case becomes more complicated, but not more
difficult of comprehension.

521. It is not here assumed that the acting particles must be in a right
line between the poles. The lines of action which may be supposed to
represent the electric currents passing through a decomposing liquid, have
in many experiments very irregular forms; and even in the simplest case of
two wires or points immersed as poles in a drop or larger single portion of
fluid, these lines must diverge rapidly from the poles; and the direction
in which the chemical affinity between particles is most powerfully
modified (519. 520.) will vary with the direction of these lines, according
constantly with them. But even in reference to these lines or currents, it
is not supposed that the particles which mutually affect each other must of
necessity be parallel to them, but only that they shall accord generally
with their direction. Two particles, placed in a line perpendicular to the
electric current passing in any particular place, are not supposed to have
their ordinary chemical relations towards each other affected; but as the
line joining them is inclined one way to the current their mutual affinity
is increased; as it is inclined in the other direction it is diminished;
and the effect is a maximum, when that line is parallel to the current[A].

  [A] In reference to this subject see now electrolytic induction and
  discharge, Series XII. ¶ viii. 1343-1351, &c.--_Dec. 1838._

522. That the actions, of whatever kind they may be, take place frequently
in oblique directions is evident from the circumstance of those particles
being included which in numerous cases are not in a line between the poles.
Thus, when wires are used as poles in a glass of solution, the
decompositions and recompositions occur to the right or left of the direct
line between the poles, and indeed in every part to which the currents
extend, as is proved by many experiments, and must therefore often occur
between particles obliquely placed as respects the current itself; and when
a metallic vessel containing the solution is made one pole, whilst a mere
point or wire is used for the other, the decompositions and recompositions
must frequently be still more oblique to the course of the currents.

523. The theory which I have ventured to put forth (almost) requires an
admission, that in a compound body capable of electro-chemical
decomposition the elementary particles have a mutual relation to, and
influence upon each other, extending beyond those with which they are
immediately combined. Thus in water, a particle of hydrogen in combination
with oxygen is considered as not altogether indifferent to other particles
of oxygen, although they are combined with other particles of hydrogen; but
to have an affinity or attraction towards them, which, though it does not
at all approach in force, under ordinary circumstances, to that by which it
is combined with its own particle, can, under the electric influence,
exerted in a definite direction, be made even to surpass it. This general
relation of particles already in combination to other particles with which
they are not combined, is sufficiently distinct in numerous results of a
purely chemical character; especially in those where partial decompositions
only take place, and in Berthollet's experiments on the effects of quantity
upon affinity: and it probably has a direct relation to, and connexion
with, attraction of aggregation, both in solids and fluids. It is a
remarkable circumstance, that in gases and vapours, where the attraction of
aggregation ceases, there likewise the decomposing powers of electricity
apparently cease, and there also the chemical action of quantity is no
longer evident. It seems not unlikely, that the inability to suffer
decomposition in these cases may be dependent upon the absence of that
mutual attractive relation of the particles which is the cause of
aggregation.

524. I hope I have now distinctly stated, although in general terms, the
view I entertain of the cause of electro-chemical decomposition, _as far as
that cause can at present be traced and understood_. I conceive the effects
to arise from forces which are _internal_, relative to the matter under
decomposition--and _not external_, as they might be considered, if directly
dependent upon the poles. I suppose that the effects are due to a
modification, by the electric current, of the chemical affinity of the
particles through or by which that current is passing, giving them the
power of acting more forcibly in one direction than in another, and
consequently making them travel by a series of successive decompositions
and recompositions in opposite directions, and finally causing their
expulsion or exclusion at the boundaries of the body under decomposition,
in the direction of the current, _and that_ in larger or smaller
quantities, according as the current is more or less powerful (377.). I
think, therefore, it would be more philosophical, and more directly
expressive of the facts, to speak of such a body, in relation to the
current passing through it, rather than to the poles, as they are usually
called, in contact with it; and say that whilst under decomposition,
oxygen, chlorine, iodine, acids, &c., are rendered at its negative
extremity, and combustibles, metals, alkalies, bases, &c., at its positive
extremity (467.), I do not believe that a substance can be transferred in
the electric current beyond the point where it ceases to find particles
with which it can combine; and I may refer to the experiments made in air
(465.) and in water (495.), already quoted, for facts illustrating these
views in the first instance; to which I will now add others.

525. In order to show the dependence of the decomposition and transfer of
elements upon the chemical affinity of the substances present, experiments
were made upon sulphuric acid in the following manner. Dilute sulphuric
acid was prepared: its specific gravity was 1.0212. A solution of sulphate
of soda was also prepared, of such strength that a measure of it contained
exactly as much sulphuric acid as an equal measure of the diluted acid just
referred to. A solution of pure soda, and another of pure ammonia, were
likewise prepared, of such strengths that a measure of either should be
exactly neutralized by a measure of the prepared sulphuric acid.

526. Four glass cups were then arranged, as in fig. 55; seventeen measures
of the free sulphuric acid (525.) were put into each of the vessels _a_ and
_b_, and seventeen measures of the solution of sulphate of soda into each
of the vessels A and B. Asbestus, which had been well-washed in acid, acted
upon by the voltaic pile, well-washed in water, and dried by pressure, was
used to connect _a_ with _b_ and A with B, the portions being as equal as
they could be made in quantity, and cut as short as was consistent with
their performing the part of effectual communications, _b_ and A were
connected by two platina plates or poles soldered to the extremities of one
wire, and the cups _a_ and B were by similar platina plates connected with
a voltaic battery of forty pairs of plates four inches square, that in _a_
being connected with the negative, and that in B with the positive pole.
The battery, which was not powerfully charged, was retained in
communication above half an hour. In this manner it was certain that the
same electric current had passed through _a b_ and A B, and that in each
instance the same quantity and strength of acid had been submitted to its
action, but in one case merely dissolved in water, and in the other
dissolved and also combined with an alkali.

527. On breaking the connexion with the battery, the portions of asbestus
were lifted out, and the drops hanging at the ends allowed to fall each
into its respective vessel. The acids in _a_ and _b_ were then first
compared, for which purpose two evaporating dishes were balanced, and the
acid from _a_ put into one, and that from _b_ into the other; but as one
was a little heavier than the other, a small drop was transferred from the
heavier to the lighter, and the two rendered equal in weight. Being
neutralized by the addition of the soda solution (525.), that from _a_, or
the negative vessel, required 15 parts of the soda solution, and that from
_b_, or the positive vessel, required 16.3 parts. That the sum of these is
not 34 parts is principally due to the acid removed with the asbestus; but
taking the mean of 15.65 parts, it would appear that a twenty-fourth part
of the acid originally in the vessel _a_ had passed, through the influence
of the electric current, from _a_ into _b_.

528. In comparing the difference of acid in A and B, the necessary equality
of weight was considered as of no consequence, because the solution was at
first neutral, and would not, therefore, affect the test liquids, and all
the evolved acid would be in B, and the free alkali in A. The solution in A
required 3.2 measures of the prepared acid (525.) to neutralize it, and the
solution in B required also 3.2 measures of the soda solution (525.) to
neutralize it. As the asbestus must have removed a little acid and alkali
from the glasses, these quantities are by so much too small; and therefore
it would appear that about a tenth of the acid originally in the vessel A
had been transferred into B during the continuance of the electric action.

529. In another similar experiment, whilst a thirty-fifth part of the acid
passed from _a_ to _b_; in the free acid vessels, between a tenth and an
eleventh passed from A to B in the combined acid vessels. Other experiments
of the same kind gave similar results.

530. The variation of electro-chemical decomposition, the transfer of
elements and their accumulation at the poles, according as the substance
submitted to action consists of particles opposed more or less in their
chemical affinity, together with the consequent influence of the latter
circumstances, are sufficiently obvious in these cases, where sulphuric
acid is acted upon in the _same quantity_ by the _same_ electric current,
but in one case opposed to the comparatively weak affinity of water for it,
and in the other to the stronger one of soda. In the latter case the
quantity transferred is from two and a half to three times what it is in
the former; and it appears therefore very evident that the transfer is
greatly dependent upon the mutual action of the particles of the
decomposing bodies[A].

  [A] See the note to (675.),--_Dec. 1838._

531. In some of the experiments the acid from the vessels _a_ and _b_ was
neutralized by ammonia, then evaporated to dryness, heated to redness, and
the residue examined for sulphates. In these cases more sulphate was always
obtained from _a_ than from _b_; showing that it had been impossible to
exclude saline bases (derived from the asbestus, the glass, or perhaps
impurities originally in the acid,) and that they had helped in
transferring the acid into _b_. But the quantity was small, and the acid
was principally transferred by relation to the water present.

532. I endeavoured to arrange certain experiments by which saline solutions
should be decomposed against surfaces of water; and at first worked with
the electric machine upon a piece of bibulous paper, or asbestus moistened
in the solution, and in contact at its two extremities with pointed pieces
of paper moistened in pure water, which served to carry the electric
current to and from the solution in the middle piece. But I found numerous
interfering difficulties. Thus, the water and solutions in the pieces of
paper could not be prevented from mingling at the point where they touched.
Again, sufficient acid could be derived from the paper connected with the
discharging train, or it may be even from the air itself, under the
influence of electric action, to neutralize the alkali developed at the
positive extremity of the decomposing solution, and so not merely prevent
its appearance, but actually transfer it on to the metal termination: and,
in fact, when the paper points were not allowed to touch there, and the
machine was worked until alkali was evolved at the delivering or positive
end of the turmeric paper, containing the sulphate of soda solution, it was
merely necessary to place the opposite receiving point of the paper
connected with the discharging train, which had been moistened by distilled
water, upon the brown turmeric point and press them together, when the
alkaline effect immediately disappeared.

533. The experiment with sulphate of magnesia already described (495.) is a
case in point, however, and shows most clearly that the sulphuric acid and
magnesia contributed to each other's transfer and final evolution, exactly
as the same acid and soda affected each other in the results just given
(527, &c.); and that so soon as the magnesia advanced beyond the reach of
the acid, and found no other substance with which it could combine, it
appeared in its proper character, and was no longer able to continue its
progress towards the negative pole.

       *       *       *       *       *

534. The theory I have ventured to put forth appears to me to explain all
the prominent features of electro-chemical decomposition in a satisfactory
manner.

535. In the first place, it explains why, in all ordinary cases, the
evolved substances _appear only at the poles_; for the poles are the
limiting surfaces of the decomposing substance, and except at them, every
particle finds other particles having a contrary tendency with which it can
combine.

536. Then it explains why, in numerous cases, the elements or evolved
substances are not _retained_ by the poles; and this is no small difficulty
in those theories which refer the decomposing effect directly to the
attractive power of the poles. If, in accordance with the usual theory, a
piece of platina be supposed to have sufficient power to attract a particle
of hydrogen from the particle of oxygen with which it was the instant
before combined, there seems no sufficient reason, nor any fact, except
those to be explained, which show why it should not, according to analogy
with all ordinary attractive forces, as those of gravitation, magnetism,
cohesion, chemical affinity, &c. _retain_ that particle which it had just
before taken from a distance and from previous combination. Yet it does not
do so, but allows it to escape freely. Nor does this depend upon its
assuming the gaseous state, for acids and alkalies, &c. are left equally at
liberty to diffuse themselves through the fluid surrounding the pole, and
show no particular tendency to combine with or adhere to the latter. And
though there are plenty of cases where combination with the pole does take
place, they do not at all explain the instances of non-combination, and do
not therefore in their particular action reveal the general principle of
decomposition.

537. But in the theory that I have just given, the effect appears to be a
natural consequence of the action: the evolved substances are _expelled_
from the decomposing mass (518. 519.), not _drawn out by an attraction_
which ceases to act on one particle without any assignable reason, while it
continues to act on another of the same kind: and whether the poles be
metal, water, or air, still the substances are evolved, and are sometimes
set free, whilst at others they unite to the matter of the poles, according
to the chemical nature of the latter, i.e. their chemical relation to those
particles which are leaving the substance under operation.

538. The theory accounts for the _transfer of elements_ in a manner which
seems to me at present to leave nothing unexplained; and it was, indeed,
the phenomena of transfer in the numerous cases of decomposition of bodies
rendered fluid by heat (380. 402.), which, in conjunction with the
experiments in air, led to its construction. Such cases as the former where
binary compounds of easy decomposability are acted upon, are perhaps the
best to illustrate the theory.

539. Chloride of lead, for instance, fused in a bent tube (400.), and
decomposed by platina wires, evolves lead, passing to what is usually
called the negative pole, and chlorine, which being evolved at the positive
pole, is in part set free, and in part combines with the platina. The
chloride of platina formed, being soluble in the chloride of lead, is
subject to decomposition, and the platina itself is gradually transferred
across the decomposing matter, and found with the lead at the negative
pole.

540. Iodide of lead evolves abundance of lead at the negative pole, and
abundance of iodine at the positive pole.

541. Chloride of silver furnishes a beautiful instance, especially when
decomposed by silver wire poles. Upon fusing a portion of it on a piece of
glass, and bringing the poles into contact with it, there is abundance of
silver evolved at the negative pole, and an equal abundance absorbed at the
positive pole, for no chlorine is set free: and by careful management, the
negative wire may be withdrawn from the fused globule as the silver is
reduced there, the latter serving as the continuation of the pole, until a
wire or thread of revived silver, five or six inches in length, is
produced; at the same time the silver at the positive pole is as rapidly
dissolved by the chlorine, which seizes upon it, so that the wire has to be
continually advanced as it is melted away. The whole experiment includes
the action of only two elements, silver and chlorine, and illustrates in a
beautiful manner their progress in opposite directions, parallel to the
electric current, which is for the time giving a uniform general direction
to their mutual affinities (524.).

542. According to my theory, an element or a substance not decomposable
under the circumstances of the experiment, (as for instance, a dilute acid
or alkali,) should not be transferred, or pass from pole to pole, unless it
be in chemical relation to some other element or substance tending to pass
in the opposite direction, for the effect is considered as essentially due
to the mutual relation of such particles. But the theories attributing the
determination of the elements to the attractions and repulsions of the
poles require no such condition, i.e. there is no reason apparent why the
attraction of the positive pole, and the repulsion of the negative pole,
upon a particle of free acid, placed in water between them, should not
(with equal currents of electricity) be as strong as if that particle were
previously combined with alkali; but, on the contrary, as they have not a
powerful chemical affinity to overcome, there is every reason to suppose
they would be stronger, and would sooner bring the acid to rest at the
positive pole[A]. Yet such is not the case, as has been shown by the
experiments on free and combined acid (526. 528.).

  [A] Even Sir Humphry Davy considered the attraction of the pole as
  being communicated from one particle to another of the _same_ kind
  (483.).

543. Neither does M. de la Rive's theory, as I understand it, _require_
that the particles should be in combination: it does not even admit, where
there are two sets of particles capable of combining with and passing by
each other, that they do combine, but supposes that they travel as separate
compounds of matter and electricity. Yet in fact the free substance
_cannot_ travel, the combined one _can_.

544. It is very difficult to find cases amongst solutions or fluids which
shall illustrate this point, because of the difficulty of finding two
fluids which shall conduct, shall not mingle, and in which an element
evolved from one shall not find a combinable element in the other.
_Solutions_ of acids or alkalies will not answer, because they exist by
virtue of an attraction; and increasing the solubility of a body in one
direction, and diminishing it in the opposite, is just as good a reason for
transfer, as modifying the affinity between the acids and alkalies
themselves[A]. Nevertheless the case of sulphate of magnesia is in point
(494. 495.), and shows that _one element or principle only_ has no power of
transference or of passing towards either pole.

  [A] See the note to (670.).--_Dec. 1838._

545. Many of the metals, however, in their solid state, offer very fair
instances of the kind required. Thus, if a plate of platina be used as the
positive pole in a solution of sulphuric acid, oxygen will pass towards it,
and so will acid; but these are not substances having such chemical
relation to the platina as, even under the favourable condition
superinduced by the current (518. 524.), to combine with it; the platina
therefore remains where it was first placed, and has no tendency to pass
towards the negative pole. But if a plate of iron, zinc or copper, be
substituted for the platina, then the oxygen and acid can combine with
these, and the metal immediately begins to travel (as an oxide) to the
opposite pole, and is finally deposited there. Or if, retaining the platina
pole, a fused chloride, as of lead, zinc, silver, &c., be substituted for
the sulphuric acid, then, as the platina finds an element it can combine
with, it enters into union, acts as other elements do in cases of voltaic
decomposition, is rapidly transferred across the melted matter, and
expelled at the negative pole.

546. I can see but little reason in the theories referring the
electro-chemical decomposition to the attractions and repulsions of the
poles, and I can perceive none in M. de la Rive's theory, why the metal of
the positive pole should not be transferred across the intervening
conductor, and deposited at the negative pole, even when it cannot act
chemically upon the element of the fluid surrounding it. It cannot be
referred to the attraction of cohesion preventing such an effect; for if
the pole be made of the lightest spongy platina, the effect is the same. Or
if gold precipitated by sulphate of iron be diffused through the solution,
still accumulation of it at the negative pole will not take place; and yet
the attraction of cohesion is almost perfectly overcome, the particles are
in it so small as to remain for hours in suspension, and are perfectly free
to move by the slightest impulse towards either pole; and _if in relation_
by chemical affinity to any substance present, are powerfully determined to
the negative pole[A].

  [A] In making this experiment, care must be taken that no substance be
  present that can act chemically on the gold. Although I used the metal
  very carefully washed, and diffused through dilute sulphuric acid, yet
  in the first instance I obtained gold at the negative pole, and the
  effect was repeated when the platina poles were changed. But on
  examining the clear liquor in the cell, after subsidence of the
  metallic gold, I found a little of that metal in solution, and a
  little chlorine was also present. I therefore well washed the gold
  which had thus been subjected to voltaic action, diffused it through
  other pure dilute sulphuric acid, and then found, that on subjecting
  it to the action of the pile, not the slightest tendency to the
  negative pole could be perceived.

547. In support of these arguments, it may be observed, that as yet no
determination of a substance to a pole, or tendency to obey the electric
current, has been observed (that I am aware of,) in cases of mere mixture;
i.e. a substance diffused through a fluid, but having no sensible chemical
affinity with it, or with substances that may be evolved from it during the
action, does not in any case seem to be affected by the electric current.
Pulverised charcoal was diffused through dilute sulphuric acid, and
subjected with the solution to the action of a voltaic battery, terminated
by platina poles; but not the slightest tendency of the charcoal to the
negative pole could be observed, Sublimed sulphur was diffused through
similar acid, and submitted to the same action, a silver plate being used
as the negative pole; but the sulphur had no tendency to pass to that pole,
the silver was not tarnished, nor did any sulphuretted hydrogen appear. The
case of magnesia and water (495. 533.), with those of comminuted metals in
certain solutions (546.), are also of this kind; and, in fact, substances
which have the instant before been powerfully determined towards the pole,
as magnesia from sulphate of magnesia, become entirely _indifferent to it_
the moment they assume their independent state, and pass away, diffusing
themselves through the surrounding fluid.

548. There are, it is true, many instances of insoluble bodies being acted
upon, as glass, sulphate of baryta, marble, slate, basalt, &c., but they
form no exception; for the substances they give up are in direct and strong
relation as to chemical affinity with those which they find in the
surrounding solution, so that these decompositions enter into the class of
ordinary effects.

549. It may be expressed as a general consequence, that the more directly
bodies are opposed to each other in chemical affinity, the more _ready_ is
their separation from each other in cases of electro-chemical
decomposition, i.e. provided other circumstances, as insolubility,
deficient conducting power, proportions, &c., do not interfere. This is
well known to be the case with water and saline solutions; and I have found
it to be equally true with _dry_ chlorides, iodides, salts, &c., rendered
subject to electro-chemical decomposition by fusion (402.). So that in
applying the voltaic battery for the purpose of decomposing bodies not yet
resolved into forms of matter simpler than their own, it must be
remembered, that success may depend not upon the weakness, or failure upon
the strength, of the affinity by which the elements sought for are held
together, but contrariwise; and then modes of application may be devised,
by which, in _association_ with ordinary chemical powers, and the
assistance of fusion (394. 417.), we may be able to penetrate much further
than at present into the constitution of our chemical elements.

550. Some of the most beautiful and surprising cases of electro-chemical
decomposition and _transfer_ which Sir Humphry Davy described in his
celebrated paper[A], were those in which acids were passed through
alkalies, and alkalies or earths through acids[B]; and the way in which
substances having the most powerful attractions for each other were thus
prevented from combining, or, as it is said, had their natural affinity
destroyed or suspended throughout the whole of the circuit, excited the
utmost astonishment. But if I be right in the view I have taken of the
effects, it will appear, that that which made the _wonder_, is in fact the
_essential condition_ of transfer and decomposition, and that the more
alkali there is in the course of an acid, the more will the transfer of
that acid be facilitated from pole to pole; and perhaps a better
illustration of the difference between the theory I have ventured, and
those previously existing, cannot be offered than the views they
respectively give of such facts as these.

  [A] Philosophical Transactions, 1807, p. 1.

  [B] Ibid. p, 24, &c.

551. The instances in which sulphuric acid could not be passed though
baryta, or baryta through sulphuric acid[A], because of the precipitation
of sulphate of baryta, enter within the pale of the law already described
(380. 412.), by which liquidity is so generally required for conduction and
decomposition. In assuming the solid state of sulphate of baryta, these
bodies became virtually non-conductors to electricity of so low a tension
as that of the voltaic battery, and the power of the latter over them was
almost infinitely diminished.

  [A] Philosophical Transactions, 1807, p. 25, &c.

552. The theory I have advanced accords in a most satisfactory manner with
the fact of an element or substance finding its place of rest, or rather of
evolution, sometimes at one pole and sometimes at the other. Sulphur
illustrates this effect very well[A]. When sulphuric acid is decomposed by
the pile, sulphur is evolved at the negative pole; but when sulphuret of
silver is decomposed in a similar way (436.), then the sulphur appears at
the positive pole; and if a hot platina pole be used so as to vaporize the
sulphur evolved in the latter case, then the relation of that pole to the
sulphur is exactly the same as the relation of the same pole to oxygen upon
its immersion in water. In both cases the element evolved is liberated at
the pole, but not retained by it; but by virtue of its elastic,
uncombinable, and immiscible condition passes away into the surrounding
medium. The sulphur is evidently determined in these opposite directions by
its opposite chemical relations to oxygen and silver; and it is to such
relations generally that I have referred all electro-chemical phenomena.
Where they do not exist, no electro-chemical action can take place. Where
they are strongest, it is most powerful; where they are reversed, the
direction of transfer of the substance is reversed with them.

  [A] At 681 and 757 of Series VII, will be found corrections of the
  statement here made respecting sulphur and sulphuric acid. At present
  there is no well-ascertained fact which proves that the same body can
  go directly to _either_ of the two poles at pleasure.--_Dec. 1838._

553. _Water_ may be considered as one of those substances which can be made
to pass to _either_ pole. When the poles are immersed in dilute sulphuric
acid (527.), acid passes towards the positive pole, and water towards the
negative pole; but when they are immersed in dilute alkali, the alkali
passes towards the negative pole, and water towards the positive pole.

554. Nitrogen is another substance which is considered as determinable to
either pole; but in consequence of the numerous compounds which it forms,
some of which pass to one pole, and some to the other, I have not always
found it easy to determine the true circumstances of its appearance. A pure
strong solution of ammonia is so bad a conductor of electricity that it is
scarcely more decomposable than pure water; but if sulphate of ammonia be
dissolved in it, then decomposition takes place very well; nitrogen almost
pure, and in some cases quite, is evolved at the positive pole, and
hydrogen at the negative pole.

555. On the other hand, if a strong solution of nitrate of ammonia be
decomposed, oxygen appears at the positive pole, and hydrogen, with
sometimes nitrogen, at the negative pole. If fused nitrate of ammonia be
employed, hydrogen appears at the negative pole, mingled with a little
nitrogen. Strong nitric acid yields plenty of oxygen at the positive pole,
but no gas (only nitrous acid) at the negative pole. Weak nitric acid
yields the oxygen and hydrogen of the water present, the acid apparently
remaining unchanged. Strong nitric acid with nitrate of ammonia dissolved
in it, yields a gas at the negative pole, of which the greater part is
hydrogen, but apparently a little nitrogen is present. I believe, that in
some of these cases a little nitrogen appeared at the negative pole. I
suspect, however, that in all these, and in all former cases, the
appearance of the nitrogen at the positive or negative pole is entirely a
secondary effect, and not an immediate consequence of the decomposing power
of the electric current[A].

  [A] Refer for proof of the truth of this supposition to 748, 752,
  &c.--_Dec. 1838._

556. A few observations on what are called the _poles_ of the voltaic
battery now seem necessary. The poles are merely the surfaces or doors by
which the electricity enters into or passes out of the substance suffering
decomposition. They limit the extent of that substance in the course of the
electric current, being its _terminations_ in that direction: Hence the
elements evolved pass so far and no further.

557. Metals make admirable poles, in consequence of their high conducting
power, their immiscibility with the substances generally acted upon, their
solid form, and the opportunity afforded of selecting such as are not
chemically acted upon by ordinary substances.

558. Water makes a pole of difficult application, except in a few cases
(494.), because of its small conducting power, its miscibility with most of
the substances acted upon, and its general relation to them in respect to
chemical affinity. It consists of elements, which in their electrical and
chemical relations are directly and powerfully opposed, yet combining to
produce a body more neutral in its character than any other. So that there
are but few substances which do not come into relation, by chemical
affinity, with water or one of its elements; and therefore either the water
or its elements are transferred and assist in transferring the infinite
variety of bodies which, in association with it, can be placed in the
course of the electric current. Hence the reason why it so rarely happens
that the evolved substances rest at the first surface of the water, and why
it therefore does not exhibit the ordinary action of a pole.

559. Air, however, and some gases are free from the latter objection, and
may be used as poles in many cases (461, &c.); but, in consequence of the
extremely low degree of conducting power belonging to them, they cannot be
employed with the voltaic apparatus. This limits their use; for the voltaic
apparatus is the only one as yet discovered which supplies sufficient
quantity of electricity (371. 376.) to effect electro-chemical
decomposition with facility.

560. When the poles are liable to the chemical action of the substances
evolved, either simply in consequence of their natural relation to them, or
of that relation aided by the influence of the current (518.), then they
suffer corrosion, and the parts dissolved are subject to transference, in
the same manner as the particles of the body originally under
decomposition. An immense series of phenomena of this kind might be quoted
in support of the view I have taken of the cause of electro-chemical
decomposition, and the transfer and evolution of the elements. Thus platina
being made the positive and negative poles in a solution of sulphate of
soda, has no affinity or attraction for the oxygen, hydrogen, acid, or
alkali evolved, and refuses to combine with or retain them. Zinc can
combine with the oxygen and acid; at the positive pole it does combine, and
immediately begins to travel as oxide towards the negative pole. Charcoal,
which cannot combine with the metals, if made the negative pole in a
metallic solution, refuses to unite to the bodies which are ejected from
the solution upon its surface; but if made the positive pole in a dilute
solution of sulphuric acid, it is capable of combining with the oxygen
evolved there, and consequently unites with it, producing both carbonic
acid and carbonic oxide in abundance.

561. A great advantage is frequently supplied, by the opportunity afforded
amongst the metals of selecting a substance for the pole, which shall or
shall not be acted upon by the elements to be evolved. The consequent use
of platina is notorious. In the decomposition of sulphuret of silver and
other sulphurets, a positive silver pole is superior to a platina one,
because in the former case the sulphur evolved there combines with the
silver, and the decomposition of the original sulphuret is rendered
evident; whereas in the latter case it is dissipated, and the assurance of
its separation at the pole not easily obtained.

562. The effects which take place when a succession of conducting
decomposable and undecomposable substances are placed in the electric
circuit, as, for instance, of wires and solutions, or of air and solutions
(465, 469.), are explained in the simplest possible manner by the
theoretical view I have given. In consequence of the reaction of the
constituents of each portion of decomposable matter, affected as they are
by the supervention of the electric current (524.), portions of the
proximate or ultimate elements proceed in the direction of the current as
far as they find matter of a contrary kind capable of effecting their
transfer, and being equally affected by them; and where they cease to find
such matter, they are evolved in their free state, i.e. upon the surfaces
of metal or air bounding the extent of decomposable matter in the direction
of the current.

563. Having thus given my theory of the mode in which electro-chemical
decomposition is effected, I will refrain for the present from entering
upon the numerous general considerations which it suggests, wishing first
to submit it to the test of publication and discussion.

_Royal Institution,
June 1833._




SIXTH SERIES.


§ 12. _On the power of Metals and other Solids to induce the Combination
of Gaseous Bodies._

Received November 30, 1833,--Read January 11, 1834.


564. The conclusion at which I have arrived in the present communication
may seem to render the whole of it unfit to form part of a series of
researches in electricity; since, remarkable as the phenomena are, the
power which produces them is not to be considered as of an electric origin,
otherwise than as all attraction of particles may have this subtile agent
for their common cause. But as the effects investigated arose out of
electrical researches, as they are directly connected with other effects
which are of an electric nature, and must of necessity be understood and
guarded against in a very extensive series of electro-chemical
decompositions (707.), I have felt myself fully justified in describing
them in this place.

565. Believing that I had proved (by experiments hereafter to be described
(705.),) the constant and definite chemical action of a certain quantity of
electricity, whatever its intensity might be, or however the circumstances
of its transmission through either the body under decomposition or the more
perfect conductors were varied, I endeavoured upon that result to construct
a new measuring instrument, which from its use might be called, at least
provisionally, a _Volta-electrometer_ (739.)[A].

  [A] Or Voltameter.--_Dec. 1838._

566. During the course of the experiments made to render the instrument
efficient, I was occasionally surprised at observing a deficiency of the
gases resulting from the decompositions of water, and at last an actual
disappearance of portions which had been evolved, collected, and measured.
The circumstances of the disappearance were these. A glass tube, about
twelve inches in length and 3/4ths of an inch in diameter, had two platina
poles fixed into its upper, hermetically sealed, extremity: the poles,
where they passed through the glass, were of wire; but terminated below in
plates, which were soldered to the wires with gold (Plate V. fig. 56.). The
tube was filled with dilute sulphuric acid, and inverted in a cup of the
same fluid; a voltaic battery was connected with the two wires, and
sufficient oxygen and hydrogen evolved to occupy 4/5ths of the tube, or by
the graduation, 116 parts. On separating the tube from the voltaic battery
the volume of gas immediately began to diminish, and in about five hours
only 13-1/2 parts remained, and these ultimately disappeared.

567. It was found by various experiments, that this effect was not due to
the escape or solution of the gas, nor to recombination of the oxygen or
hydrogen in consequence of any peculiar condition _they_ might be supposed
to possess under the circumstances; but to be occasioned by the action of
one or both of the poles within the tube upon the gas around them. On
disuniting the poles from the pile after they had acted upon dilute
sulphuric acid, and introducing them into separate tubes containing mixed
oxygen and hydrogen, it was found that the _positive_ pole effected the
union of the gases, but the negative pole apparently not (588.). It was
ascertained also that no action of a sensible kind took place between the
positive pole with oxygen or hydrogen alone.

568. These experiments reduced the phenomena to the consequence of a power
possessed by the platina, after it had been the positive pole of a voltaic
pile, of causing the combination of oxygen and hydrogen at common, or even
at low, temperatures. This effect is, as far as I am aware, altogether new,
and was immediately followed out to ascertain whether it was really of an
electric nature, and how far it would interfere with the determination of
the quantities evolved in the cases of electro-chemical decomposition
required in the fourteenth section of these Researches.

569. Several platina plates were prepared (fig. 57.). They were nearly half
an inch wide, and two inches and a half long: some were 1/200dth of an
inch, others not more than 1/600dth, whilst some were as much as 1/70th of
an inch in thickness. Each had a piece of platina wire, about seven inches
long, soldered to it by pure gold. Then a number of glass tubes were
prepared: they were about nine or ten inches in length, 5/8ths of an inch
in internal diameter, were sealed hermetically at one extremity, and were
graduated. Into these tubes was put a mixture of two volumes of hydrogen
and one of oxygen, at the water pneumatic trough, and when one of the
plates described had been connected with the positive or negative pole of
the voltaic battery for a given time, or had been otherwise prepared, it
was introduced through the water into the gas within the tube; the whole
set aside in a test-glass (fig. 58.), and left for a longer or shorter
period, that the action might be observed.

570. The following result may be given as an illustration of the phenomenon
to be investigated. Diluted sulphuric acid, of the specific gravity 1.336,
was put into a glass jar, in which was placed also a large platina plate,
connected with the negative end of a voltaic battery of forty pairs of
four-inch plates, with double coppers, and moderately charged. One of the
plates above described (569.) was then connected with the positive
extremity, and immersed in the same jar of acid for five minutes, after
which it was separated from the battery, washed in distilled water, and
introduced through the water of the pneumatic trough into a tube containing
the mixture of oxygen and hydrogen (569.). The volume of gases immediately
began to lessen, the diminution proceeding more and more rapidly until
about 3/4ths of the mixture had disappeared. The upper end of the tube
became quite warm, the plate itself so hot that the water boiled as it rose
over it; and in less than a minute a cubical inch and a half of the gases
were gone, having been combined by the power of the platina, and converted
into water.

571. This extraordinary influence acquired by the platina at the positive
pole of the pile, is exerted far more readily and effectively on oxygen and
hydrogen than on any other mixture of gases that I have tried. One volume
of nitrous gas was mixed with a volume of hydrogen, and introduced into a
tube with a plate which had been made positive in the dilute sulphuric acid
for four minutes (570.). There was no sensible action in an hour: being
left for thirty-six hours, there was a diminution of about one-eighth of
the whole volume. Action had taken place, but it had been very feeble.

572. A mixture of two volumes of nitrous oxide with one volume of hydrogen
was put with a plate similarly prepared into a tube (569. 570.). This also
showed no action immediately; but in thirty-six hours nearly a fourth of
the whole had disappeared, i.e. about half of a cubic inch. By comparison
with another tube containing the same mixture without a plate, it appeared
that a part of the diminution was due to solution, and the other part to
the power of the platina; but the action had been very slow and feeble.

573. A mixture of one volume olefiant gas and three volumes oxygen was not
affected by such a platina plate, even though left together for several
days (640. 641.).

574. A mixture of two volumes carbonic oxide and one volume oxygen was also
unaffected by the prepared platina plate in several days (645, &c.).

575. A mixture of equal volumes of chlorine and hydrogen was used in
several experiments, with plates prepared in a similar manner (570.).
Diminution of bulk soon took place; but when after thirty-six hours the
experiments were examined, it was found that nearly all the chlorine had
disappeared, having been absorbed, principally by the water, and that the
original volume of hydrogen remained unchanged. No combination of the
gases, therefore, had here taken place.

576. Reverting to the action of the prepared plates on mixtures of oxygen
and hydrogen (570.), I found that the power, though gradually diminishing
in all cases, could still be retained for a period, varying in its length
with circumstances. When tubes containing plates (569.) were supplied with
fresh portions of mixed oxygen and hydrogen as the previous portions were
condensed, the action was found to continue for above thirty hours, and in
some cases slow combination could be observed even after eighty hours; but
the continuance of the action greatly depended upon the purity of the gases
used (638.).

577. Some plates (569.) were made positive for four minutes in dilute
sulphuric acid of specific gravity 1.336: they were rinsed in distilled
water, after which two were put into a small bottle and closed up, whilst
others were left exposed to the air. The plates preserved in the limited
portion of air were found to retain their power after eight days, but those
exposed to the atmosphere had lost their force almost entirely in twelve
hours, and in some situations, where currents existed, in a much shorter
time.

578. Plates were made positive for five minutes in sulphuric acid, specific
gravity 1.336. One of these was retained in similar acid for eight minutes
after separation from the battery: it then acted on mixed oxygen and
hydrogen with apparently undiminished vigour. Others were left in similar
acid for forty hours, and some even for eight days, after the
electrization, and then acted as well in combining oxygen and hydrogen gas
as those which were used immediately after electrization.

579. The effect of a solution of caustic potassa in preserving the platina
plates was tried in a similar manner. After being retained in such a
solution for forty hours, they acted exceedingly well on oxygen and
hydrogen, and one caused such rapid condensation of the gases, that the
plate became much heated, and I expected the temperature would have risen
to ignition.

580. When similarly prepared plates (569.) had been put into distilled
water for forty hours, and then introduced into mixed oxygen and hydrogen,
they were found to act but very slowly and feebly as compared with those
which had been preserved in acid or alkali. When, however, the quantity of
water was but small, the power was very little impaired after three or four
days. As the water had been retained in a wooden vessel, portions of it
were redistilled in glass, and this was found to preserve prepared plates
for a great length of time. Prepared plates were put into tubes with this
water and closed up; some of them, taken out at the end of twenty-four
days, were found very active on mixed oxygen and hydrogen; others, which
were left in the water for fifty-three days, were still found to cause the
combination of the gases. The tubes had been closed only by corks.

581. The act of combination always seemed to diminish, or apparently
exhaust, the power of the platina plate. It is true, that in most, if not
all instances, the combination of the gases, at first insensible, gradually
increased in rapidity, and sometimes reached to explosion; but when the
latter did not happen, the rapidity of combination diminished; and although
fresh portions of gas were introduced into the tubes, the combination went
on more and more slowly, and at last ceased altogether. The first effect of
an increase in the rapidity of combination depended in part upon the water
flowing off from the platina plate, and allowing a better contact with the
gas, and in part upon the heat evolved during the progress of the
combination (630.). But notwithstanding the effect of these causes,
diminution, and at last cessation of the power, always occurred. It must
not, however, be unnoticed, that the purer the gases subjected to the
action of the plate, the longer was its combining power retained. With the
mixture evolved at the poles of the voltaic pile, in pure dilute sulphuric
acid, it continued longest; and with oxygen and hydrogen, of perfect
purity, it probably would not be diminished at all.

582. Different modes of treatment applied to the platina plate, after it
had ceased to be the positive pole of the pile, affected its power very
curiously. A plate which had been a positive pole in diluted sulphuric acid
of specific gravity 1.336 for four or five minutes, if rinsed in water and
put into mixed oxygen and hydrogen, would act very well, and condense
perhaps one cubic inch and a half of gas in six or seven minutes; but if
that same plate, instead of being merely rinsed, had been left in distilled
water for twelve or fifteen minutes, or more, it would rarely fail, when
put into the oxygen and hydrogen, of becoming, in the course of a minute or
two, ignited, and would generally explode the gases. Occasionally the time
occupied in bringing on the action extended to eight or nine minutes, and
sometimes even to forty minutes, and yet ignition and explosion would
result. This effect is due to the removal of a portion of acid which
otherwise adheres firmly to the plate [A].

  [A] In proof that this is the case, refer to 1038.--_Dec. 1838._

583. Occasionally the platina plates (569.), after being made the positive
pole of the battery, were washed, wiped with filtering-paper or a cloth,
and washed and wiped again. Being then introduced into mixed oxygen and
hydrogen, they acted apparently as if they had been unaffected by the
treatment. Sometimes the tubes containing the gas were opened in the air
for an instant, and the plates put in dry; but no sensible difference in
action was perceived, except that it commenced sooner.

584. The power of heat in altering the action of the prepared platina
plates was also tried (595.). Plates which had been rendered positive in
dilute sulphuric acid for four minutes were well-washed in water, and
heated to redness in the flame of a spirit-lamp: after this they acted very
well on mixed oxygen and hydrogen. Others, which had been heated more
powerfully by the blowpipe, acted afterwards on the gases, though not so
powerfully as the former. Hence it appears that heat does not take away the
power acquired by the platina at the positive pole of the pile: the
occasional diminution of force seemed always referable to other causes than
the mere heat. If, for instance, the plate had not been well-washed from
the acid, or if the flame used was carbonaceous, or was that of an alcohol
lamp trimmed with spirit containing a little acid, or having a wick on
which salt, or other extraneous matter, had been placed, then the power of
the plate was quickly and greatly diminished (634. 636.).

585. This remarkable property was conferred upon platina when it was made
the positive pole in sulphuric acid of specific gravity 1.336, or when it
was considerably weaker, or when stronger, even up to the strength of oil
of vitriol. Strong and dilute nitric acid, dilute acetic acid, solutions of
tartaric, citric, and oxalic acids, were used with equal success. When
muriatic acid was used, the plates acquired the power of condensing the
oxygen and hydrogen, but in a much inferior degree.

586. Plates which were made positive in solution of caustic potassa did not
show any sensible action upon the mixed oxygen and hydrogen. Other plates
made positive in solutions of carbonates of potassa and soda exhibited the
action, but only in a feeble degree.

587. When a neutral solution of sulphate of soda, or of nitre, or of
chlorate of potassa, or of phosphate of potassa, or acetate of potassa, or
sulphate of copper, was used, the plates, rendered positive in them for
four minutes, and then washed in water, acted very readily and powerfully
on the mixed oxygen and hydrogen.

588. It became a very important point, in reference to the _cause_ of this
action of the platina, to determine whether the _positive_ pole _only_
could confer it (567.), or whether, notwithstanding the numerous contrary
cases, the _negative_ pole might not have the power when such circumstances
as could interfere with or prevent the action were avoided. Three plates
were therefore rendered negative, for four minutes in diluted sulphuric
acid of specific gravity 1.336, washed in distilled water, and put into
mixed oxygen and hydrogen. _All_ of them _acted_, though not so strongly as
they would have done if they had been rendered positive. Each combined
about a cubical inch and a quarter of the gases in twenty-five minutes. On
every repetition of the experiment the same result was obtained; and when
the plates were retained in distilled water for ten or twelve minutes,
before being introduced into the gas (582.), the action was very much
quickened.

589. But when there was any metallic or other substance present in the
acid, which could be precipitated on the negative plate, then that plate
ceased to act upon the mixed oxygen and hydrogen.

590. These experiments led to the expectation that the power of causing
oxygen and hydrogen to combine, which could be conferred upon any piece of
platina by making it the positive pole of a voltaic pile, was not
essentially dependent upon the action of the pile, or upon any structure or
arrangement of parts it might receive whilst in association with it, but
belonged to the platina _at all times_, and was _always effective_ when the
surface was _perfectly clean_. And though, when made the _positive_ pole of
the pile in acids, the circumstances might well be considered as those
which would cleanse the surface of the platina in the most effectual
manner, it did not seem impossible that ordinary operations should produce
the same result, although in a less eminent degree.

591. Accordingly, a platina plate (569.) was cleaned by being rubbed with a
cork, a little water, and some coal-fire ashes upon a glass plate: being
washed, it was put into mixed oxygen and hydrogen, and was found to act at
first slowly, and then more rapidly. In an hour, a cubical inch and a half
had disappeared.

592. Other plates were cleaned with ordinary sand-paper and water; others
with chalk and water; others with emery and water; others, again, with
black oxide of manganese and water; and others with a piece of charcoal and
water. All of these acted in tubes of oxygen and hydrogen, causing
combination of the gases. The action was by no means so powerful as that
produced by plates having been in communication with the battery; but from
one to two cubical inches of the gases disappeared, in periods extending
from twenty-five to eighty or ninety minutes.

593. Upon cleaning the plates with a cork, ground emery, and dilute
sulphuric acid, they were found to act still better. In order to simplify
the conditions, the cork was dismissed, and a piece of platina foil used
instead; still the effect took place. Then the acid was dismissed, and a
solution of _potassa_ used, but the effect occurred as before.

594. These results are abundantly sufficient to show that the mere
mechanical cleansing of the surface of the platina is sufficient to enable
it to exert its combining power over oxygen and hydrogen at common
temperatures.

595. I now tried the effect of heat in conferring this property upon
platina (584.). Plates which had no action on the mixture of oxygen and
hydrogen were heated by the flame of a freshly trimmed spirit-lamp, urged
by a mouth blowpipe, and when cold were put into tubes of the mixed gases:
they acted slowly at first, but after two or three hours condensed nearly
all the gases.

596. A plate of platina, which was about one inch wide and two and
three-quarters in length, and which had not been used in any of the
preceding experiments, was curved a little so as to enter a tube, and left
in a mixture of oxygen and hydrogen for thirteen hours: not the slightest
action or combination of the gases occurred. It was withdrawn at the
pneumatic trough from the gas through the water, heated red-hot by the
spirit-lamp and blowpipe, and then returned when cold into the _same_
portion of gas. In the course of a few minutes diminution of the gases
could be observed, and in forty-five minutes about one cubical inch and a
quarter had disappeared. In many other experiments platina plates when
heated were found to acquire the power of combining oxygen and hydrogen.

597. But it happened not infrequently that plates, after being heated,
showed no power of combining oxygen and hydrogen gases, though left
undisturbed in them for two hours. Sometimes also it would happen that a
plate which, having been heated to dull redness, acted feebly, upon being
heated to whiteness ceased to act; and at other times a plate which, having
been slightly heated, did not act, was rendered active by a more powerful
ignition.

598. Though thus uncertain in its action, and though often diminishing the
power given to the plates at the positive pole of the pile (584.), still it
is evident that heat can render platina active, which before was inert
(595.). The cause of its occasional failure appears to be due to the
surface of the metal becoming soiled, either from something previously
adhering to it, which is made to adhere more closely by the action of the
heat, or from matter communicated from the flame of the lamp, or from the
air itself. It often happens that a polished plate of platina, when heated
by the spirit-lamp and a blowpipe, becomes dulled and clouded on its
surface by something either formed or deposited there; and this, and much
less than this, is sufficient to prevent it from exhibiting the curious
power now under consideration (634. 636.). Platina also has been said to
combine with carbon; and it is not at all unlikely that in processes of
heating, where carbon or its compounds are present, a film of such a
compound may be thus formed, and thus prevent the exhibition of the
properties belonging to _pure_ platina[A].

  [A] When heat does confer the property it is only by the destruction
  or dissipation of organic or other matter which had previously soiled
  the plate (632. 633. 634.).--_Dec. 1838._

599. The action of alkalies and acids in giving platina this property was
now experimentally examined. Platina plates (569.) having no action on
mixed oxygen and hydrogen, being boiled in a solution of caustic potassa,
washed, and then put into the gases, were found occasionally to act pretty
well, but at other times to fail. In the latter case I concluded that the
impurity upon the surface of the platina was of a nature not to be removed
by the mere solvent action of the alkali, for when the plates were rubbed
with a little emery, and the same solution of alkali (592.), they became
active.

600. The action of acids was far more constant and satisfactory. A platina
plate was boiled in dilute nitric acid: being washed and put into mixed
oxygen and hydrogen gases, it acted well. Other plates were boiled in
strong nitric acid for periods extending from half a minute to four
minutes, and then being washed in distilled water, were found to act very
well, condensing one cubic inch and a half of gas in the space of eight or
nine minutes, and rendering the tube warm (570.).

601. Strong sulphuric acid was very effectual in rendering the platina
active. A plate (569.) was heated in it for a minute, then washed and put
into the mixed oxygen and hydrogen, upon which it acted as well as if it
had been made the positive pole of a voltaic pile (570.).

602. Plates which, after being heated or electrized in alkali, or after
other treatment, were found inert, immediately received power by being
dipped for a minute or two, or even only for an instant, into hot oil of
vitriol, and then into water.

603. When the plate was dipped into the oil of vitriol, taken out, and then
heated so as to drive off the acid, it did not act, in consequence of the
impurity left by the acid upon its surface.

604. Vegetable acids, as acetic and tartaric, sometimes rendered inert
platina active, at other times not. This, I believe, depended upon the
character of the matter previously soiling the plates, and which may easily
be supposed to be sometimes of such a nature as to be removed by these
acids, and at other times not. Weak sulphuric acid showed the same
difference, but strong sulphuric acid (601.) never failed in its action.

605. The most favourable treatment, except that of making the plate a
positive pole in strong acid, was as follows. The plate was held over a
spirit-lamp flame, and when hot, rubbed with a piece of potassa fusa
(caustic potash), which melting, covered the metal with a coat of very
strong alkali, and this was retained fused upon the surface for a second or
two[A]: it was then put into water for four or five minutes to wash off the
alkali, shaken, and immersed for about a minute in hot strong oil of
vitriol; from this it was removed into distilled water, where it was
allowed to remain ten or fifteen minutes to remove the last traces of acid
(582.). Being then put into a mixture of oxygen and hydrogen, combination
immediately began, and proceeded rapidly; the tube became warm, the platina
became red-hot, and the residue of the gases was inflamed. This effect
could be repeated at pleasure, and thus the maximum phenomenon could be
produced without the aid of the voltaic battery.

  [A] The heat need not be raised so much as to make the alkali tarnish
  the platina, although if that effect does take place it does not
  prevent the ultimate action.

606. When a solution of tartaric or acetic acid was substituted, in this
mode of preparation, for the sulphuric acid, still the plate was found to
acquire the same power, and would often produce explosion in the mixed
gases; but the strong sulphuric acid was most certain and powerful.

607. If borax, or a mixture of the carbonates of potash and soda, be fused
on the surface of a platina plate, and that plate be well-washed in water,
it will be found to have acquired the power of combining oxygen and
hydrogen, but only in a moderate degree; but if, after the fusion and
washing, it be dipped in the hot sulphuric acid (601.), it will become very
active.

608. Other metals than platina were then experimented with. Gold and
palladium exhibited the power either when made the positive pole of the
voltaic battery (570.), or when acted on by hot oil of vitriol (601.). When
palladium is used, the action of the battery or acid should be moderated,
as that metal is soon acted upon under such circumstances. Silver and
copper could not be made to show any effect at common temperatures.

       *       *       *       *       *

609. There can remain no doubt that the property of inducing combination,
which can thus be conferred upon masses of platina and other metals by
connecting them with the poles of the battery, or by cleansing processes
either of a mechanical or chemical nature, is the same as that which was
discovered by Döbereiner[A], in 1823, to belong in so eminent a degree to
spongy platina, and which was afterwards so well experimented upon and
illustrated by MM. Dulong and Thenard[B], in 1823. The latter philosophers
even quote experiments in which a very fine platina wire, which had been
coiled up and digested in nitric, sulphuric, or muriatic acid, became
ignited when put into a jet of hydrogen gas[C]. This effect I can now
produce at pleasure with either wires or plates by the processes described
(570. 601. 605.); and by using a smaller plate cut so that it shall rest
against the glass by a few points, and yet allow the water to flow off
(fig. 59.), the loss of heat is less, the metal is assimilated somewhat to
the spongy state, and the probability of failure almost entirely removed.

  [A] Annales de Chimie, tom. xxiv. p. 93.

  [B] Ibid. tom. xxiii. p. 440; tom. xxiv. p. 380.

  [C] Ibid. tom. xxiv. p. 383.

610. M. Döbereiner refers the effect entirely to an electric action. He
considers the platina and hydrogen as forming a voltaic element of the
ordinary kind, in which the hydrogen, being very highly positive,
represents the zinc of the usual arrangement, and like it, therefore,
attracts oxygen and combines with it[A].

  [A] tom. xxiv. pp. 94, 95. Also Bibliothèque Universelle, tom. xxiv.
  p. 54.

611. In the two excellent experimental papers by MM. Dulong and Thenard[A],
those philosophers show that elevation of temperature favours the action,
but does not alter its character; Sir Humphry Davy's incandescent platina
wire being the same phenomenon with Döbereiner's spongy platina. They show
that _all_ metals have this power in a greater or smaller degree, and that
it is even possessed by such bodies as charcoal, pumice, porcelain, glass,
rock crystal, &c., when their temperatures are raised; and that another of
Davy's effects, in which oxygen and hydrogen had combined slowly together
at a heat below ignition, was really dependent upon the property of the
heated glass, which it has in common with the bodies named above. They
state that liquids do not show this effect, at least that mercury, at or
below the boiling point, has not the power; that it is not due to porosity;
that the same body varies very much in its action, according to its state;
and that many other gaseous mixtures besides oxygen and hydrogen are
affected, and made to act chemically, when the temperature is raised. They
think it probable that spongy platina acquires its power from contact with
the acid evolved during its reduction, or from the heat itself to which it
is then submitted.

  [A] Annales de Chimie, tom. xxiii. p. 440; tom. xxiv. p, 380.

612. MM. Dulong and Thenard express themselves with great caution on the
theory of this action; but, referring to the decomposing power of metals on
ammonia when heated to temperatures not sufficient alone to affect the
alkali, they remark that those metals which in this case are most
efficacious, are the least so in causing the combination of oxygen and
hydrogen; whilst platina, gold, &c., which have least power of decomposing
ammonia, have most power of combining the elements of water:--from which
they are led to believe, that amongst gases, some tend to _unite_ under the
influence of metals, whilst others tend to _separate_, and that this
property varies in opposite directions with the different metals. At the
close of their second paper they observe, that the action is of a kind that
cannot be connected with any known theory; and though it is very remarkable
that the effects are transient, like those of most electrical actions, yet
they state that the greater number of the results observed by them are
inexplicable, by supposing them to be of a purely electric origin.

613. Dr. Fusinieri has also written on this subject, and given a theory
which he considers as sufficient to account for the phenomena[A]. He
expresses the immediate cause thus: "The platina determines upon its
surface a continual renovation of _concrete laminæ_ of the combustible
substance of the gases or vapours, which flowing over it are burnt, pass
away, and are renewed: this combustion at the surface raises and sustains
the temperature of the metal." The combustible substance, thus reduced into
imperceptible laminæ, of which the concrete parts are in contact with the
oxygen, is presumed to be in a state combinable with the oxygen at a much
lower temperature than when it is in the gaseous state, and more in analogy
with what is called the nascent condition. That combustible gases should
lose their elastic state, and become concrete, assuming the form of
exceedingly attenuated but solid strata, is considered as proved by facts,
some of which are quoted in the Giornale di Fisica for 1824[B]; and though
the theory requires that they should assume this state at high
temperatures, and though the _similar_ films of aqueous and other matter
are dissipated by the action of heat, still the facts are considered as
justifying the conclusion against all opposition of reasoning.

  [A] Giornale di Fisica, &c., 1825, tom. viii. p. 259.

  [B] pp. 138, 371.

614. The power or force which makes combustible gas or vapour abandon its
elastic state in contact with a solid, that it may cover the latter with a
thin stratum of its own proper substance, is considered as being neither
attraction nor affinity. It is able also to extend liquids and solids in
concrete laminæ over the surface of the acting solid body, and consists in
a _repulsion_, which is developed from the parts of the solid body by the
simple fact of attenuation, and is highest when the attenuation is most
complete. The force has a progressive development, and acts most
powerfully, or at first, in the direction in which the dimensions of the
attenuated mass decrease, and then in the direction of the angles or
corners which from any cause may exist on the surface. This force not only
causes spontaneous diffusion of gases and other substances over the
surface, but is considered as very elementary in its nature, and competent
to account for all the phenomena of capillarity, chemical affinity,
attraction of aggregation, rarefaction, ebullition, volatilization,
explosion, and other thermometric effects, as well as inflammation,
detonation, &c. &c. It is considered as a form of heat to which the term
_native calorie_ is given, and is still further viewed as the principle of
the two electricities and the two magnetisms.

615. I have been the more anxious to give a correct abstract of Dr.
Fusinieri's view, both because I cannot form a distinct idea of the power
to which he refers the phenomena, and because of my imperfect knowledge of
the language in which the memoir is written. I would therefore beg to refer
those who pursue the subject to the memoir itself.

616. Not feeling, however, that the problem has yet been solved, I venture
to give the view which seems to me sufficient, upon _known principles_, to
account for the effect.

617. It may be observed of this action, that, with regard to platina, it
cannot be due to any peculiar, temporary condition, either of an electric
or of any other nature: the activity of plates rendered either positive or
negative by the pole, or cleaned with such different substances as acids,
alkalies, or water; charcoal, emery, ashes, or glass; or merely heated, is
sufficient to negative such an opinion. Neither does it depend upon the
spongy and porous, or upon the compact and burnished, or upon the massive
or the attenuated state of the metal, for in any of these states it may be
rendered effective, or its action may be taken away. The only essential
condition appears to be a _perfectly clean_ and _metallic surface_, for
whenever that is present the platina acts, whatever its form and condition
in other respects may be; and though variations in the latter points will
very much affect the rapidity, and therefore the visible appearances and
secondary effects, of the action, i.e. the ignition of the metal and the
inflammation of the gases, they, even in their most favourable state,
cannot produce any effect unless the condition of a clean, pure, metallic
surface be also fulfilled.

618. The effect is evidently produced by most, if not all, solid bodies,
weakly perhaps by many of them, but rising to a high degree in platina.
Dulong and Thenard have very philosophically extended our knowledge of the
property to its possession by all the metals, and by earths, glass, stones,
&c. (611.); and every idea of its being a known and recognised electric
action is in this way removed.

619. All the phenomena connected with this subject press upon my mind the
conviction that the effects in question are entirely incidental and of a
secondary nature; that they are dependent upon the _natural conditions_ of
gaseous elasticity, combined with the exertion of that attractive force
possessed by many bodies, especially those which are solid, in an eminent
degree, and probably belonging to all; by which they are drawn into
association more or less close, without at the same time undergoing
chemical combination, though often assuming the condition of adhesion; and
which occasionally leads, under very favourable circumstances, as in the
present instance, to the combination of bodies simultaneously subjected to
this attraction. I am prepared myself to admit (and probably many others
are of the same opinion), both with respect to the attraction of
aggregation and of chemical affinity, that the sphere of action of
particles extends beyond those other particles with which they are
immediately and evidently in union (523.), and in many cases produces
effects rising into considerable importance: and I think that this kind of
attraction is a determining cause of Döbereiner's effect, and of the many
others of a similar nature.

620. Bodies which become wetted by fluids with which they do not combine
chemically, or in which they do not dissolve, are simple and well-known
instances of this kind of attraction.

621. All those cases of bodies which being insoluble in water and not
combining with it are hygrometric, and condense its vapour around or upon
their surface, are stronger instances of the same power, and approach a
little nearer to the cases under investigation. If pulverized clay,
protoxide or peroxide of iron, oxide of manganese, charcoal, or even
metals, as spongy platina or precipitated silver, be put into an atmosphere
containing vapour of water, they soon become moist by virtue of an
attraction which is able to condense the vapour upon, although not to
combine it with, the substances; and if, as is well known, these bodies so
damped be put into a dry atmosphere, as, for instance, one confined over
sulphuric acid, or if they be heated, then they yield up this water again
almost entirely, it not being in direct or permanent combination[A].

  [A] I met at Edinburgh with a case, remarkable as to its extent, of
  hygrometric action, assisted a little perhaps by very slight solvent
  power. Some turf had been well-dried by long exposure in a covered
  place to the atmosphere, but being then submitted to the action of a
  hydrostatic press, it yielded, _by the mere influence of the
  pressure_, 54 per cent. of water.

622. Still better instances of the power I refer to, because they are more
analogous to the cases to be explained, are furnished by the attraction
existing between glass and air, so well known to barometer and thermometer
makers, for here the adhesion or attraction is exerted between a solid and
gases, bodies having very different physical conditions, having no power of
combination with each other, and each retaining, during the time of action,
its physical state unchanged[A]. When mercury is poured into a barometer
tube, a film of air will remain between the metal and glass for months, or,
as far as is known, for years, for it has never been displaced except by
the action of means especially fitted for the purpose. These consist in
boiling the mercury, or in other words, of forming an abundance of vapour,
which coming in contact with every part of the glass and every portion of
surface of the mercury, gradually mingles with, dilutes, and carries off
the air attracted by, and adhering to, those surfaces, replacing it by
other vapour, subject to an equal or perhaps greater attraction, but which
when cooled condenses into the same liquid as that with which the tube is
filled.

  [A] Fusinieri and Bellani consider the air as forming solid concrete
  films in these cases.--Giornale di Fisica, tom. viii, p. 262. 1825.

623. Extraneous bodies, which, acting as nuclei in crystallizing or
depositing solutions, cause deposition of substances on them, when it does
not occur elsewhere in the liquid, seem to produce their effects by a power
of the same kind, i.e. a power of attraction extending to neighbouring
particles, and causing them to become attached to the nuclei, although it
is not strong enough to make them combine chemically with their substance.

624. It would appear from many cases of nuclei in solutions, and from the
effects of bodies put into atmospheres containing the vapours of water, or
camphor, or iodine, &c., as if this attraction were in part elective,
partaking in its characters both of the attraction of aggregation and
chemical affinity: nor is this inconsistent with, but agreeable to, the
idea entertained, that it is the power of particles acting, not upon others
with which they can immediately and intimately combine, but upon such as
are either more distantly situated with respect to them, or which, from
previous condition, physical constitution, or feeble relation, are unable
to enter into decided union with them.

625. Then, of all bodies, the gases are those which might be expected to
show some _mutual_ action whilst _jointly_ under the attractive influence
of the platina or other solid acting substance. Liquids, such as water,
alcohol, &c., are in so dense and comparatively incompressible a state, as
to favour no expectation that their particles should approach much closer
to each other by the attraction of the body to which they adhere, and yet
that attraction must (according to its effects) place their particles as
near to those of the solid wetted body as they are to each other, and in
many cases it is evident that the former attraction is the stronger. But
gases and vapours are bodies competent to suffer very great changes in the
relative distances of their particles by external agencies; and where they
are in immediate contact with the platina, the approximation of the
particles to those of the metal may be very great. In the case of the
hygrometric bodies referred to (621.), it is sufficient to reduce the
vapour to the fluid state, frequently from atmospheres so rare that without
this influence it would be needful to compress them by mechanical force
into a bulk not more than 1/10th or even 1/20th of their original volume
before the vapours would become liquids.

626. Another most important consideration in relation to this action of
bodies, and which, as far as I am aware, has not hitherto been noticed, is
the condition of elasticity under which the gases are placed against the
acting surface. We have but very imperfect notions of the real and intimate
conditions of the particles of a body existing in the solid, the liquid,
and the gaseous state; but when we speak of the gaseous state as being due
to the mutual repulsions of the particles or of their atmospheres, although
we may err in imagining each particle to be a little nucleus to an
atmosphere of heat, or electricity, or any other agent, we are still not
likely to be in error in considering the elasticity as dependent on
_mutuality_ of action. Now this mutual relation fails altogether on the
side of the gaseous particles next to the platina, and we might be led to
expect _à priori_ a deficiency of elastic force there to at least one half;
for if, as Dalton has shown, the elastic force of the particles of one gas
cannot act against the elastic force of the particles of another, the two
being as vacua to each other, so is it far less likely that the particles
of the platina can exert any influence on those of the gas against it, such
as would be exerted by gaseous particles of its own kind.

627. But the diminution of power to one-half on the side of the gaseous
body towards the metal is only a slight result of what seems to me to flow
as a necessary consequence of the known constitution of gases. An
atmosphere of one gas or vapour, however dense or compressed, is in effect
as a vacuum to another: thus, if a little water were put into a vessel
containing a dry gas, as air, of the pressure of one hundred atmospheres,
as much vapour of the water would _rise_ as if it were in a perfect vacuum.
Here the particles of watery vapour appear to have no difficulty in
approaching within any distance of the particles of air, being influenced
solely by relation to particles of their own kind; and if it be so with
respect to a body having the same elastic powers as itself, how much more
surely must it be so with particles, like those of the platina, or other
limiting body, which at the same time that they have not these elastic
powers, are also unlike it in nature! Hence it would seem to result that
the particles of hydrogen or any other gas or vapour which are next to the
platina, &c., must be in such contact with it as if they were in the liquid
state, and therefore almost infinitely closer to it than they are to each
other, even though the metal be supposed to exert no attractive influence
over them.

628. A third and very important consideration in favour of the mutual
action of gases under these circumstances is their perfect miscibility. If
fluid bodies capable of combining together are also capable of mixture,
_they do combine_ when they are mingled, not waiting for any other
determining circumstance; but if two such gases as oxygen and hydrogen are
put together, though they are elements having such powerful affinity as to
unite naturally under a thousand different circumstances, they do not
combine by mere mixture. Still it is evident that, from their perfect
association, the particles are in the most favourable state possible for
combination upon the supervention of any determining cause, such either as
the negative action of the platina in suppressing or annihilating, as it
were, their elasticity on its side; or the positive action of the metal in
condensing them against its surface by an attractive force; or the
influence of both together.

629. Although there are not many distinct cases of combination under the
influence of forces external to the combining particles, yet there are
sufficient to remove any difficulty which might arise on that ground. Sir
James Hull found carbonic acid and lime to remain combined under pressure
at temperatures at which they would not have remained combined if the
pressure had been removed; and I have had occasion to observe a case of
direct combination in chlorine[A], which being compressed at common
temperatures will combine with water, and form a definite crystalline
hydrate, incapable either of being formed or of existing if that pressure
be removed.

  [A] Philosophical Transactions, 1823, p. 161.

630. The course of events when platina acts upon, and combines oxygen and
hydrogen, may be stated, according to these principles, as follows. From
the influence of the circumstances mentioned (619. &c.), i.e. the
deficiency of elastic power and the attraction of the metal for the gases,
the latter, when they are in association with the former, are so far
condensed as to be brought within the action of their mutual affinities at
the existing temperature; the deficiency of elastic power, not merely
subjecting them more closely to the attractive influence of the metal, but
also bringing them into a more favourable state for union, by abstracting a
part of that power (upon which depends their elasticity,) which elsewhere
in the mass of gases is opposing their combination. The consequence of
their combination is the production of the vapour of water and an elevation
of temperature. But as the attraction of the platina for the water formed
is not greater than for the gases, if so great, (for the metal is scarcely
hygrometric,) the vapour is quickly diffused through the remaining gases;
fresh portions of this latter, therefore, come into juxtaposition with the
metal, combine, and the fresh vapour formed is also diffused, allowing new
portions of gas to be acted upon. In this way the process advances, but is
accelerated by the evolution of heat, which is known by experiment to
facilitate the combination in proportion to its intensity, and the
temperature is thus gradually exalted until ignition results.

631. The dissipation of the vapour produced at the surface of the platina,
and the contact of fresh oxygen and hydrogen with the metal, form no
difficulty in this explication. The platina is not considered as causing
the combination of any particles with itself, but only associating them
closely around it; and the compressed particles are as free to move from
the platina, being replaced by other particles, as a portion of dense air
upon the surface of the globe, or at the bottom of a deep mine, is free to
move by the slightest impulse, into the upper and rarer parts of the
atmosphere.

632. It can hardly be necessary to give any reasons why platina does not
show this effect under ordinary circumstances. It is then not sufficiently
clean (617.), and the gases are prevented from touching it, and suffering
that degree of effect which is needful to commence their combination at
common temperatures, and which they can only experience at its surface. In
fact, the very power which causes the combination of oxygen and hydrogen,
is competent, under the usual casual exposure of platina, to condense
extraneous matters upon its surface, which soiling it, take away for the
time its power of combining oxygen and hydrogen, by preventing their
contact with it (598.).

633. Clean platina, by which I mean such as has been made the positive pole
of a pile (570.), or has been treated with acid (605.), and has then been
put into distilled water for twelve or fifteen minutes, has a _peculiar
friction_ when one piece is rubbed against another. It wets freely with
pure water, even after it has been shaken and dried by the heat of a
spirit-lamp; and if made the pole of a voltaic pile in a dilute acid, it
evolves minute bubbles from every part of its surface. But platina in its
common state wants that peculiar friction: it will not wet freely with
water as the clean platina does; and when made the positive pole of a pile,
it for a time gives off large bubbles, which seem to cling or adhere to the
metal, and are evolved at distinct and separate points of the surface.
These appearances and effects, as well as its want of power on oxygen and
hydrogen, are the consequences, and the indications, of a soiled surface.

634. I found also that platina plates which had been cleaned perfectly soon
became soiled by mere exposure to the air; for after twenty-four hours they
no longer moistened freely with water, but the fluid ran up into portions,
leaving part of the surface bare, whilst other plates which had been
retained in water for the same time, when they were dried (580.) did
moisten, and gave the other indications of a clean surface.

635. Nor was this the case with platina or metals only, but also with
earthy bodies, Rock crystal and obsidian would not wet freely upon the
surface, but being moistened with strong oil of vitriol, then washed, and
left in distilled water to remove all the acid, they did freely become
moistened, whether they were previously dry or whether they were left wet;
but being dried and left exposed to the air for twenty-four hours, their
surface became so soiled that water would not then adhere freely to it, but
ran up into partial portions. Wiping with a cloth (even the cleanest) was
still worse than exposure to air; the surface either of the minerals or
metals immediately became as if it were slightly greasy. The floating upon
water of small particles of metals under ordinary circumstances is a
consequence of this kind of soiled surface. The extreme difficulty of
cleaning the surface of mercury when it has once been soiled or greased, is
due to the same cause.

636. The same reasons explain why the power of the platina plates in some
circumstances soon disappear, and especially upon use: MM. Dulong and
Thenard have observed the same effect with the spongy metal[A], as indeed
have all those who have used Döbereiner's instantaneous light machines. If
left in the air, if put into ordinary distilled water, if made to act upon
ordinary oxygen and hydrogen, they can still find in all these cases _that_
minute portion of impurity which, when once in contact with the surface of
the platina, is retained there, and is sufficient to prevent its full
action upon oxygen and hydrogen at common temperatures: a slight elevation
of temperature is again sufficient to compensate this effect, and cause
combination.

  [A] Annales de Chimie, tom. xxiv. p. 386.

637. No state of a solid body can be conceived more favourable for the
production of the effect than that which is possessed by platina obtained
from the ammonio-muriate by heat. Its surface is most extensive and pure,
yet very accessible to the gases brought in contact with it: if placed in
impurity, the interior, as Thenard and Dulong have observed, is preserved
clean by the exterior; and as regards temperature, it is so bad a conductor
of heat, because of its divided condition, that almost all which is evolved
by the combination of the first portions of gas is retained within the
mass, exalting the tendency of the succeeding portions to combine.

       *       *       *       *       *

638. I have now to notice some very extraordinary interferences with this
phenomenon, dependent, not upon the nature or condition of the metal or
other acting solid, but upon the presence of certain substances mingled
with the gases acted upon; and as I shall have occasion to speak frequently
of a mixture of oxygen and hydrogen, I wish it always to be understood that
I mean a mixture composed of one volume of oxygen to two volumes of
hydrogen, being the proportions that form water. Unless otherwise
expressed, the hydrogen was always that obtained by the action of dilute
sulphuric acid on pure zinc, and the oxygen that obtained by the action of
heat from the chlorate of potassa.

639. Mixtures of oxygen and hydrogen with _air_, containing one-fourth,
one-half, and even two-thirds of the latter, being introduced with prepared
platina plates (570. 605.) into tubes, were acted upon almost as well as if
no air were present: the retardation was far less than might have been
expected from the mere dilution and consequent obstruction to the contact
of the gases with the plates. In two hours and a half nearly all the oxygen
and hydrogen introduced as mixture was gone.

640. But when similar experiments were made with _olefiant gas_ (the
platina plates having been made the positive poles of a voltaic pile (570.)
in acid), very different results occurred. A mixture was made of 29.2
volumes hydrogen and 14.6 volumes oxygen, being the proportions for water;
and to this was added another mixture of 3 volumes oxygen and one volume
olefiant gas, so that the olefiant gas formed but 1/40th part of the whole;
yet in this mixture the platina plate would not act in forty-five hours.
The failure was not for want of any power in the plate, for when after that
time it was taken out of this mixture and put into one of oxygen and
hydrogen, it immediately acted, and in seven minutes caused explosion of
the gas. This result was obtained several times, and when larger
proportions of olefiant gas were used, the action seemed still more
hopeless.

641. A mixture of forty-nine volumes oxygen and hydrogen (638.) with one
volume of olefiant gas had a well-prepared platina plate introduced. The
diminution of gas was scarcely sensible at the end of two hours, during
which it was watched; but on examination twenty-four hours afterwards, the
tube was found blown to pieces. The action, therefore, though it had been
very much retarded, had occurred at last, and risen to a maximum.

642. With a mixture of ninety-nine volumes of oxygen and hydrogen (638.)
with one of olefiant gas, a feeble action was evident at the end of fifty
minutes; it went on accelerating (630.) until the eighty-fifth minute, and
then became so intense that the gas exploded. Here also the retarding
effect of the olefiant gas was very beautifully illustrated.

643. Plates prepared by alkali and acid (605.) produced effects
corresponding to those just described.

644. It is perfectly clear from these experiments, that _olefiant gas_,
even in small quantities, has a very remarkable influence in preventing the
combination of oxygen and hydrogen under these circumstances, and yet
without at all injuring or affecting the power of the platina.

645. Another striking illustration of similar interference may be shown in
_carbonic oxide_; especially if contrasted with _carbonic acid_. A mixture
of one volume oxygen and hydrogen (638.) with four volumes of carbonic acid
was affected at once by a platina plate prepared with acid, &c. (605.); and
in one hour and a quarter nearly all the oxygen and hydrogen was gone.
Mixtures containing less carbonic acid were still more readily affected.

646. But when carbonic oxide was substituted for the carbonic acid, not the
slightest effect of combination was produced; and when the carbonic oxide
was only one-eighth of the whole volume, no action occurred in forty and
fifty hours. Yet the plates had not lost their power; for being taken out
and put into pure oxygen and hydrogen, they acted well and at once.

647. Two volumes of carbonic oxide and one of oxygen were mingled with nine
volumes of oxygen and hydrogen (638.). This mixture was not affected by a
plate which had been made positive in acid, though it remained in it
fifteen hours. But when to the same volumes of carbonic oxide and oxygen
were added thirty-three volumes of oxygen and hydrogen, the carbonic oxide
being then only 1/18th part of the whole, the plate acted, slowly at first,
and at the end of forty-two minutes the gases exploded.

648. These experiments were extended to various gases and vapours, the
general results of which may be given as follow. Oxygen, hydrogen,
nitrogen, and nitrous oxide, when used to dilute the mixture of oxygen and
hydrogen, did not prevent the action of the plates even when they made
four-fifths of the whole volume of gas acted upon. Nor was the retardation
so great in any case as might have been expected from the mere dilution of
the oxygen and hydrogen, and the consequent mechanical obstruction to its
contact with the platina. The order in which carbonic acid and these
substances seemed to stand was as follows, the first interfering least with
the action; _nitrous oxide, hydrogen, carbonic acid, nitrogen, oxygen_: but
it is possible the plates were not equally well prepared in all the cases,
and that other circumstances also were unequal; consequently more numerous
experiments would be required to establish the order accurately.

649. As to cases of _retardation_, the powers of olefiant gas and carbonic
oxide have been already described. Mixtures of oxygen and hydrogen,
containing from 1/16th to 1/20th of sulphuretted hydrogen or phosphuretted
hydrogen, seemed to show a little action at first, but were not further
affected by the prepared plates, though in contact with them for seventy
hours. When the plates were removed they had lost all power over pure
oxygen and hydrogen, and the interference of these gases was therefore of a
different nature from that of the two former, having permanently affected
the plate.

650. A small piece of cork was dipped in sulphuret of carbon and passed up
through water into a tube containing oxygen and hydrogen (638.), so as to
diffuse a portion of its vapour through the gases. A plate being introduced
appeared at first to act a little, but after sixty-one hours the diminution
was very small. Upon putting the same plate into a pure mixture of oxygen
and hydrogen, it acted at once and powerfully, having apparently suffered
no diminution of its force.

651. A little vapour of ether being mixed with the oxygen and hydrogen
retarded the action of the plate, but did not prevent it altogether. A
little of the vapour of the condensed oil-gas liquor[A] retarded the action
still more, but not nearly so much as an equal volume of olefiant gas would
have done. In both these cases it was the original oxygen and hydrogen
which combined together, the ether and the oil-gas vapour remaining
unaffected, and in both cases the plates retained the power of acting on
fresh oxygen and hydrogen.

  [A] Philosophical Transactions, 1825, p.440.

652. Spongy platina was then used in place of the plates, and jets of
hydrogen mingled with the different gases thrown against it in air. The
results were exactly of the same kind, although presented occasionally in a
more imposing form. Thus, mixtures of one volume of olefiant gas or
carbonic oxide with three of hydrogen could not heat the spongy platina
when the experiments were commenced at common temperatures; but a mixture
of equal volumes of nitrogen and hydrogen acted very well, causing
ignition. With carbonic acid the results were still more striking. A
mixture of three volumes of that gas with one of hydrogen caused _ignition_
of the platina, yet that mixture would not continue to burn from the jet
when attempts were made to light it by a taper. A mixture even of _seven_
volumes of carbonic acid and _one_ of hydrogen will thus cause the ignition
of cold spongy platina, and yet, as if to supply a contrast, than which
none can be greater, _it cannot burn at a taper_, but causes the extinction
of the latter. On the other hand, the mixtures of carbonic oxide or
olefiant gas, which can do nothing with the platina, are _inflamed_ by the
taper, burning well.

653. Hydrogen mingled with the vapour of ether or oil-gas liquor causes the
ignition of the spongy platina. The mixture with oil-gas burns with a flame
far brighter than that of the mixture of hydrogen and olefiant gas already
referred to, so that it would appear that the retarding action of the
hydrocarbons is not at all in proportion merely to the quantity of carbon
present.

654. In connexion with these interferences, I must state, that hydrogen
itself, prepared from steam passed over ignited iron, was found when
mingled with oxygen to resist the action of platina. It had stood over
water seven days, and had lost all fetid smell; but a jet of it would not
cause the ignition of spongy platina, commencing at common temperatures;
nor would it combine with oxygen in a tube either under the influence of a
prepared plate or of spongy platina. A mixture of one volume of this gas
with three of pure hydrogen, and the due proportion of oxygen, was not
affected by plates after fifty hours. I am inclined to refer the effect to
carbonic oxide present in the gas, but have not had time to verify the
suspicion. The power of the plates was not destroyed (640. 646.).

655. Such are the general facts of these remarkable interferences. Whether
the effect produced by such small quantities of certain gases depends upon
any direct action which they may exert upon the particles of oxygen and
hydrogen, by which the latter are rendered less inclined to combine, or
whether it depends upon their modifying the action of the plate temporarily
(for they produce no real change on it), by investing it through the agency
of a stronger attraction than that of the hydrogen, or otherwise, remains
to be decided by more extended experiments.

       *       *       *       *       *

656. The theory of action which I have given for the original phenomena
appears to me quite sufficient to account for all the effects by reference
to known properties, and dispenses with the assumption of any new power of
matter. I have pursued this subject at some length, as one of great
consequence, because I am convinced that the superficial actions of matter,
whether between two bodies, or of one piece of the same body, and the
actions of particles not directly or strongly in combination, are becoming
daily more and more important to our theories of chemical as well as
mechanical philosophy[A]. In all ordinary cases of combustion it is evident
that an action of the kind considered, occurring upon the surface of the
carbon in the fire, and also in the bright part of a flame, must have great
influence over the combinations there taking place.

  [A] As a curious illustration of the influence of mechanical forces
  over chemical affinity, I will quote the refusal of certain substances
  to effloresce when their surfaces are perfect, which yield immediately
  upon the surface being broken, If crystals of carbonate of soda, or
  phosphate of soda, or sulphate of soda, having no part of their
  surfaces broken, be preserved from external violence, they will not
  effloresce. I have thus retained crystals of carbonate of soda
  perfectly transparent and unchanged from September 1827 to January
  1833; and crystals of sulphate of soda from May 1832 to the present
  time, November 1833. If any part of the surface were scratched or
  broken, then efflorescence began at that part, and covered the whole.
  The crystals were merely placed in evaporating basins and covered with
  paper.

657. The condition of elasticity upon the exterior of the gaseous or
vaporous mass already referred to (626. 627.), must be connected directly
with the action of solid bodies, as nuclei, on vapours, causing
condensation upon them in preference to any condensation in the vapours
themselves; and in the well-known effect of nuclei on solutions a similar
condition may have existence (623.), for an analogy in condition exists
between the parts of a body in solution, and those of a body in the
vaporous or gaseous state. This thought leads us to the consideration of
what are the respective conditions at the surfaces of contact of two
portions of the same substance at the same temperature, one in the solid or
liquid, and the other in the vaporous state; as, for instance, steam and
water. It would seem that the particles of vapour next to the particles of
liquid are in a different relation to the latter to what they would be with
respect to any other liquid or solid substance; as, for instance, mercury
or platina, if they were made to replace the water, i.e. if the view of
independent action which I have taken (626. 627.) as a consequence of
Dalton's principles, be correct. It would also seem that the mutual
relation of similar particles, and the indifference of dissimilar particles
which Dalton has established as a matter of fact amongst gases and vapours,
extends to a certain degree amongst solids and fluids, that is, when they
are in relation by contact with vapours, either of their own substance or
of other bodies. But though I view these points as of great importance with
respect to the relations existing between different substances and their
physical constitution in the solid, liquid, or gaseous state, I have not
sufficiently considered them to venture any strong opinions or statements
here[A].

  [A] In reference to this paragraph and also 626, see a correction by
  Dr. C. Henry, in his valuable paper on this curious subject.
  Philosophical Magazine, 1835. vol. vi. p. 305.--_Dec. 1838._

658. There are numerous well-known cases, in which substances, such as
oxygen and hydrogen, act readily in their _nascent_ state, and produce
chemical changes which they are not able to effect if once they have
assumed the gaseous condition. Such instances are very common at the poles
of the voltaic pile, and are, I think, easily accounted for, if it be
considered that at the moment of separation of any such particle it is
entirely surrounded by other particles of a _different_ kind with which it
is in close contact, and has not yet assumed those relations and conditions
which it has in its fully developed state, and which it can only assume by
association with other particles of its own kind. For, at the moment, its
elasticity is absent, and it is in the same relation to particles with
which it is in contact, and for which it has an affinity, as the particles
of oxygen and hydrogen are to each other on the surface of clean platina
(626. 627.).

659. The singular effects of retardation produced by very small quantities
of some gases, and not by large quantities of others (640. 645. 652.), if
dependent upon any relation of the added gas to the surface of the solid,
will then probably be found immediately connected with the curious
phenomena which are presented by different gases when passing through
narrow tubes at low pressures, which I observed many years ago[A]; and this
action of surfaces must, I think, influence the highly interesting
phenomena of the diffusion of gases, at least in the form in which it has
been experimented upon by Mr. Graham in 1829 and 1831[B], and also by Dr.
Mitchell of Philadelphia[C] in 1830. It seems very probable that if such a
substance as spongy platina were used, another law for the diffusion of
gases under the circumstances would come out than that obtained by the use
of plaster of Paris.

  [A] Quarterly Journal of Science, 1819, vol. vii. p. 106.

  [B] Quarterly Journal of Science, vol. xxviii. p. 74, and Edinburgh
  Transactions, 1831.

  [C] Journal of the Royal Institution for 1831, p. 101.

660. I intended to have followed this section by one on the secondary piles
of Ritter, and the peculiar properties of the poles of the pile, or of
metals through which electricity has passed, which have been observed by
Ritter, Van Marum, Yelin, De la Rive, Marianini, Berzelius, and others. It
appears to me that all these phenomena bear a satisfactory explanation on
known principles, connected with the investigation just terminated, and do
not require the assumption of any new state or new property. But as the
experiments advanced, especially those of Marianini, require very careful
repetition and examination, the necessity of pursuing the subject of
electro-chemical decomposition obliges me for a time to defer the
researches to which I have just referred.

_Royal Institution,
November 30, 1833._




SEVENTH SERIES.


§ 11. _On Electro-chemical Decomposition, continued._[A] ¶ iv. _On some
general conditions of Electro-decomposition._ ¶ v. _On a new Measurer of
Volta-electricity._ ¶ vi. _On the primary or secondary character of bodies
evolved in Electro-decomposition._ ¶ vii. _On the definite nature and
extent of Electro-chemical Decompositions._ § 13. _On the absolute quantity
of Electricity associated with the particles or atoms of Matter._

  [A] Refer to the note after 1047, Series VIII.--_Dec. 1838._

Received January 9,--Read January 23, February 6 and 13, 1834.

_Preliminary._


661. The theory which I believe to be a true expression of the facts of
electro-chemical decomposition, and which I have therefore detailed in a
former series of these Researches, is so much at variance with those
previously advanced, that I find the greatest difficulty in stating
results, as I think, correctly, whilst limited to the use of terms which
are current with a certain accepted meaning. Of this kind is the term
_pole_, with its prefixes of positive and negative, and the attached ideas
of attraction and repulsion. The general phraseology is that the positive
pole _attracts_ oxygen, acids, &c., or more cautiously, that it
_determines_ their evolution upon its surface; and that the negative pole
acts in an equal manner upon hydrogen, combustibles, metals, and bases.
According to my view, the determining force is _not_ at the poles, but
_within_ the body under decomposition; and the oxygen and acids are
rendered at the _negative_ extremity of that body, whilst hydrogen, metals,
&c., are evolved at the _positive_ extremity (518. 524.).

662. To avoid, therefore, confusion and circumlocution, and for the sake of
greater precision of expression than I can otherwise obtain, I have
deliberately considered the subject with two friends, and with their
assistance and concurrence in framing them, I purpose henceforward using
certain other terms, which I will now define. The _poles_, as they are
usually called, are only the doors or ways by which the electric current
passes into and out of the decomposing body (556.); and they of course,
when in contact with that body, are the limits of its extent in the
direction of the current. The term has been generally applied to the metal
surfaces in contact with the decomposing substance; but whether
philosophers generally would also apply it to the surfaces of air (465.
471.) and water (493.), against which I have effected electro-chemical
decomposition, is subject to doubt. In place of the term pole, I propose
using that of _Electrode_[A], and I mean thereby that substance, or rather
surface, whether of air, water, metal, or any other body, which bounds the
extent of the decomposing matter in the direction of the electric current.

  [A] [Greek: elektron], and [Greek: -odos] _a way_.

663. The surfaces at which, according to common phraseology, the electric
current enters and leaves a decomposing body, are most important places of
action, and require to be distinguished apart from the poles, with which
they are mostly, and the electrodes, with which they are always, in
contact. Wishing for a natural standard of electric direction to which I
might refer these, expressive of their difference and at the same time free
from all theory, I have thought it might be found in the earth. If the
magnetism of the earth be due to electric currents passing round it, the
latter must be in a constant direction, which, according to present usage
of speech, would be from east to west, or, which will strengthen this help
to the memory, that in which the sun appears to move. If in any case of
electro-decomposition we consider the decomposing body as placed so that
the current passing through it shall be in the same direction, and parallel
to that supposed to exist in the earth, then the surfaces at which the
electricity is passing into and out of the substance would have an
invariable reference, and exhibit constantly the same relations of powers.
Upon this notion we purpose calling that towards the east the _anode_[A],
and that towards the west the _cathode_[B]; and whatever changes may take
place in our views of the nature of electricity and electrical action, as
they must affect the _natural standard_ referred to, in the same direction,
and to an equal amount with any decomposing substances to which these terms
may at any time be applied, there seems no reason to expect that they will
lead to confusion, or tend in any way to support false views. The _anode_
is therefore that surface at which the electric current, according to our
present expression, enters: it is the _negative_ extremity of the
decomposing body; is where oxygen, chlorine, acids, &c., are evolved; and
is against or opposite the positive electrode. The _cathode_ is that
surface at which the current leaves the decomposing body, and is its
_positive_ extremity; the combustible bodies, metals, alkalies, and bases,
are evolved there, and it is in contact with the negative electrode.

  [A] [Greek: ano] _upwards_, and [Greek: -odos] _a way_; the way which
the sun rises.

  [B] [Greek: kata] _downwards_, and [Greek: -odos] _a way_; the way
  which the sun sets.

664. I shall have occasion in these Researches, also, to class bodies
together according to certain relations derived from their electrical
actions (822.); and wishing to express those relations without at the same
time involving the expression of any hypothetical views, I intend using the
following names and terms. Many bodies are decomposed directly by the
electric current, their elements being set free; these I propose to call
_electrolytes_.[A] Water, therefore, is an electrolyte. The bodies which,
like nitric or sulphuric acids, are decomposed in a secondary manner (752.
757.), are not included under this term. Then for _electro-chemically
decomposed_, I shall often use the term _electrolyzed_, derived in the same
way, and implying that the body spoken of is separated into its components
under the influence of electricity: it is analogous in its sense and sound
to _analyse_, which is derived in a similar manner. The term
_electrolytical_ will be understood at once: muriatic acid is
electrolytical, boracic acid is not.

  [A] [Greek: elektron], and [Greek: lyo], _soluo_. N. Electrolyte, V.
  Electrolyze.

665. Finally, I require a term to express those bodies which can pass to
the _electrodes_, or, as they are usually called, the poles. Substances are
frequently spoken of as being _electro-negative_, or _electro-positive_,
according as they go under the supposed influence of a direct attraction to
the positive or negative pole. But these terms are much too significant for
the use to which I should have to put them; for though the meanings are
perhaps right, they are only hypothetical, and may be wrong; and then,
through a very imperceptible, but still very dangerous, because continual,
influence, they do great injury to science, by contracting and limiting the
habitual views of those engaged in pursuing it. I propose to distinguish
such bodies by calling those _anions_[A] which go to the _anode_ of the
decomposing body; and those passing to the _cathode, cations_[B]; and when
I have occasion to speak of these together, I shall call them _ions_. Thus
the chloride of lead is an _electrolyte_, and when _electrolyzed_ evolves
the two _ions_, chlorine and lead, the former being an _anion_, and the
latter a _cation_.

  [A] [Greek: aniôn] _that which goes up._ (Neuter participle.)

  [B] [Greek: katiôn] _that which goes down._

666. These terms being once well-defined, will, I hope, in their use enable
me to avoid much periphrasis and ambiguity of expression. I do not mean to
press them into service more frequently than will be required, for I am
fully aware that names are one thing and science another.

667. It will be well understood that I am giving no opinion respecting the
nature of the electric current now, beyond what I have done on former
occasions (283. 517.); and that though I speak of the current as proceeding
from the parts which are positive to those which are negative (663.), it is
merely in accordance with the conventional, though in some degree tacit,
agreement entered into by scientific men, that they may have a constant,
certain, and definite means of referring to the direction of the forces of
that current.

  [Since this paper was read, I have changed some of the terms which were
first proposed, that I might employ only such as were at the same time
simple in their nature, clear in their reference, and free from
hypothesis.


¶ iv. _On some general conditions of Electro-chemical Decomposition._

669. From the period when electro-chemical decomposition was first effected
to the present time, it has been a remark, that those elements which, in
the ordinary phenomena of chemical affinity, were the most directly opposed
to each other, and combined with the greatest attractive force, were those
which were the most readily evolved at the opposite extremities of the
decomposing bodies (549.).

670. If this result was evident when water was supposed to be essential to,
and was present in, almost every case of such decomposition (472.), it is
far more evident now that it has been shown and proved that water is not
necessarily concerned in the phenomena (474.), and that other bodies much
surpass it in some of the effects supposed to be peculiar to that
substance.

671. Water, from its constitution and the nature of its elements, and from
its frequent presence in cases of electrolytic action, has hitherto stood
foremost in this respect. Though a compound formed by very powerful
affinity, it yields up its elements under the influence of a very feeble
electric current; and it is doubtful whether a case of electrolyzation can
occur, where, being present, it is not resolved into its first principles.

672. The various oxides, chlorides, iodides, and salts, which I have shown
are decomposable by the electric current when in the liquid state, under
the same general law with water (402.), illustrate in an equally striking
manner the activity, in such decompositions, of elements directly and
powerfully opposed to each other by their chemical relations.

673. On the other hand, bodies dependent on weak affinities very rarely
give way. Take, for instance, glasses: many of those formed of silica,
lime, alkali, and oxide of lead, may be considered as little more than
solutions of substances one in another[A]. If bottle-glass be fused, and
subjected to the voltaic pile, it does not appear to be at all decomposed
(408.). If flint glass, which contains substances more directly opposed, be
operated upon, it suffers some decomposition; and if borate of lead glass,
which is a definite chemical compound, be experimented with, it readily
yields up its elements (408.).

  [A] Philosophical Transactions, 1830, p. 49.

674. But the result which is found to be so striking in the instances
quoted is not at all borne out by reference to other cases where a similar
consequence might have been expected. It may be said, that my own theory of
electro-chemical decomposition would lead to the expectation that all
compound bodies should give way under the influence of the electric current
with a facility proportionate to the strength of the affinity by which
their elements, either proximate or ultimate, are combined. I am not sure
that that follows as a consequence of the theory; but if the objection is
supposed to be one presented by the facts, I have no doubt it will be
removed when we obtain a more intimate acquaintance with, and precise idea
of, the nature of chemical affinity and the mode of action of an electric
current over it (518. 524.): besides which, it is just as directly opposed
to any other theory of electro-chemical decomposition as the one I have
propounded; for if it be admitted, as is generally the case, that the more
directly bodies are opposed to each other in their attractive forces, the
more powerfully do they combine, then the objection applies with equal
force to any of the theories of electrolyzation which have been considered,
and is an addition to those which I have taken against them.

675. Amongst powerful compounds which are not decomposed, boracic acids
stand prominent (408.). Then again, the iodide of sulphur, and the
chlorides of sulphur, phosphorus, and carbon, are not decomposable under
common circumstances, though their elements are of a nature which would
lead to a contrary expectation. Chloride of antimony (402. 690.), the
hydro-carbons, acetic acid, ammonia, and many other bodies undecomposable
by the voltaic pile, would seem to be formed by an affinity sufficiently
strong to indicate that the elements were so far contrasted in their nature
as to sanction the expectation that, the pile would separate them,
especially as in some cases of mere solution (530. 544.), where the
affinity must by comparison be very weak, separation takes place[A].

  [A] With regard to solution, I have met with some reasons for
  supposing that it will probably disappear as a cause of transference,
  and intend resuming the consideration at a convenient opportunity.

676. It must not be forgotten, however, that much of this difficulty, and
perhaps the whole, may depend upon the absence of conducting power, which,
preventing the transmission of the current, prevents of course the effects
due to it. All known compounds being non-conductors when solid, but
conductors when liquid, are decomposed, with _perhaps_ the single exception
at present known of periodide of mercury (679. 691.)[A]; and even water
itself, which so easily yields up its elements when the current passes, if
rendered quite pure, scarcely suffers change, because it then becomes a
very bad conductor.

  [A] See now, 1340, 1341.--_Dec. 1838._

677. If it should hereafter be proved that the want of decomposition in
those cases where, from chemical considerations, it might be so strongly
expected (669, 672. 674.), is due to the absence or deficiency of
conducting power, it would also at the same time be proved that
decomposition _depends_ upon conduction, and not the latter upon the former
(413.); and in water this seems to be very nearly decided. On the other
hand, the conclusion is almost irresistible, that in electrolytes the power
of transmitting the electricity across the substance is _dependent_ upon
their capability of suffering decomposition; taking place only whilst they
are decomposing, and being proportionate to the quantity of elements
separated (821.). I may not, however, stop to discuss this point
experimentally at present.

678. When a compound contains such elements as are known to pass towards
the opposite extremities of the voltaic pile, still the proportions in
which they are present appear to be intimately connected with capability in
the compound of suffering or resisting decomposition. Thus, the
protochloride of tin readily conducts, and is decomposed (402.), but the
perchloride neither conducts nor is decomposed (406.). The protiodide of
tin is decomposed when fluid (402.); the periodide is not (405.). The
periodide of mercury when fused is not decomposed (691.), even though it
does conduct. I was unable to contrast it with the protiodide, the latter
being converted into mercury and periodide by heat.

679. These important differences induced me to look more closely to certain
binary compounds, with a view of ascertaining whether a _law_ regulating
the _decomposability_ according to some _relation of the proportionals or
equivalents_ of the elements, could be discovered. The proto compounds
only, amongst those just referred to, were decomposable; and on referring
to the substances quoted to illustrate the force and generality of the law
of conduction and decomposition which I discovered (402.), it will be found
that all the oxides, chlorides, and iodides subject to it, except the
chloride of antimony and the periodide of mercury, (to which may now
perhaps be added corrosive sublimate,) are also decomposable, whilst many
per compounds of the same elements, not subject to the law, were not so
(405. 406.).

680. The substances which appeared to form the strongest exceptions to this
general result were such bodies as the sulphuric, phosphoric, nitric,
arsenic, and other acids.

681. On experimenting with sulphuric acid, I found no reason to believe
that it was by itself a conductor of, or decomposable by, electricity,
although I had previously been of that opinion (552.). When very strong it
is a much worse conductor than if diluted[A]. If then subjected to the
action of a powerful battery, oxygen appears at the _anode_, or positive
electrode, although much is absorbed (728.), and hydrogen and sulphur
appear at the _cathode_, or negative electrode. Now the hydrogen has with
me always been pure, not sulphuretted, and has been deficient in proportion
to the sulphur present, so that it is evident that when decomposition
occurred water must have been decomposed. I endeavoured to make the
experiment with anhydrous sulphuric acid; and it appeared to me that, when
fused, such acid was not a conductor, nor decomposed; but I had not enough
of the dry acid in my possession to allow me to decide the point
satisfactorily. My belief is, that when sulphur appears during the action
of the pile on sulphuric acid, it is the result of a secondary action, and
that the acid itself is not electrolyzable (757.).

  [A] De la Rive.

682. Phosphoric acid is, I believe, also in the same condition; but I have
found it impossible to decide the point, because of the difficulty of
operating on fused anhydrous phosphoric acid. Phosphoric acid which has
once obtained water cannot be deprived of it by heat alone. When heated,
the hydrated acid volatilizes. Upon subjecting phosphoric acid, fused upon
the ring end of a wire (401.), to the action of the voltaic apparatus, it
conducted, and was decomposed; but gas, which I believe to be hydrogen, was
always evolved at the negative electrode, and the wire was not affected as
would have happened had phosphorus been separated. Gas was also evolved at
the positive electrode. From all the facts, I conclude it was the water and
not the acid which was decomposed.

683. _Arsenic acid_. This substance conducted, and was decomposed; but it
contained water, and I was unable at the time to press the investigation so
as to ascertain whether a fusible anhydrous arsenic acid could be obtained.
It forms, therefore, at present no exception to the general result.

684. Nitrous acid, obtained by distilling nitrate of lead, and keeping it
in contact with strong sulphuric acid, was found to conduct and decompose
slowly. But on examination there were strong reasons for believing that
water was present, and that the decomposition and conduction depended upon
it. I endeavoured to prepare a perfectly anhydrous portion, but could not
spare the time required to procure an unexceptionable result.

685. Nitric acid is a substance which I believe is not decomposed directly
by the electric current. As I want the facts in illustration of the
distinction existing between primary and secondary decomposition, I will
merely refer to them in this place (752.).

686. That these mineral acids should confer facility of conduction and
decomposition on water, is no proof that they are competent to favour and
suffer these actions in themselves. Boracic acid does the same thing,
though not decomposable. M. de la Rive has pointed out that chlorine has
this power also; but being to us an elementary substance, it cannot be due
to its capability of suffering decomposition.

687. _Chloride of sulphur_ does not conduct, nor is it decomposed. It
consists of single proportionals of its elements, but is not on that
account an exception to the rule (679.), which does not affirm that _all_
compounds of single proportionals of elements are decomposable, but that
such as are decomposable are so constituted.

688. _Protochloride of phosphorus_ does not conduct nor become decomposed.

689. _Protochloride of carbon_ does not conduct nor suffer decomposition.
In association with this substance, I submitted the _hydro-chloride of
carbon_ from olefiant gas and chlorine to the action of the electric
current; but it also refused to conduct or yield up its elements.

600. With regard to the exceptions (679.), upon closer examination some of
them disappear. Chloride of antimony (a compound of one proportional of
antimony and one and a half of chlorine) of recent preparation was put into
a tube (fig. 68.) (789.), and submitted when fused to the action of the
current, the positive electrode being of plumbago. No electricity passed,
and no appearance of decomposition was visible at first; but when the
positive and negative electrodes were brought very near each other in the
chloride, then a feeble action occurred and a feeble current passed. The
effect altogether was so small (although quite amenable to the law before
given (394.)), and so unlike the decomposition and conduction occurring in
all the other cases, that I attribute it to the presence of a minute
quantity of water, (for which this and many other chlorides have strong
attractions, producing hydrated chlorides,) or perhaps of a true
protochloride consisting of single proportionals (695, 796.).

691. _Periodide of mercury_ being examined in the same manner, was found
most distinctly to insulate whilst solid, but conduct when fluid, according
to the law of _liquido-conduction_ (402.); but there was no appearance of
decomposition. No iodine appeared at the _anode_, nor mercury or other
substance at the _cathode_. The case is, therefore, no exception to the
rule, that only compounds of single proportionals are decomposable; but it
is an exception, and I think the only one, to the statement, that all
bodies subject to the law of liquido-conduction are decomposable. I
incline, however, to believe, that a portion of protiodide of mercury is
retained dissolved in the periodide, and that to its slow decomposition the
feeble conducting power is due. Periodide would be formed, as a secondary
result, at the _anode_; and the mercury at the _cathode_ would also form,
as a secondary result, protiodide. Both these bodies would mingle with the
fluid mass, and thus no final separation appear, notwithstanding the
continued decomposition.

692. When _perchloride of mercury_ was subjected to the voltaic current, it
did not conduct in the solid state, but it did conduct when fluid. I think,
also, that in the latter case it was decomposed; but there are many
interfering circumstances which require examination before a positive
conclusion can be drawn[A].

  [A] With regard to perchloride and periodide of mercury, see now 1340,
  1341.--_Dec. 1838._

693. When the ordinary protoxide of antimony is subjected to the voltaic
current in a fused state, it also is decomposed, although the effect from
other causes soon ceases (402, 801.). This oxide consists of one
proportional of antimony and one and a half of oxygen, and is therefore an
exception to the general law assumed. But in working with this oxide and
the chloride, I observed facts which lead me to doubt whether the compounds
usually called the protoxide and the protochloride do not often contain
other compounds, consisting of single proportions, which are the true proto
compounds, and which, in the case of the oxide, might give rise to the
decomposition above described.

694. The ordinary sulphuret of antimony its considered as being the
compound with the smallest quantity of sulphur, and analogous in its
proportions to the ordinary protoxide. But I find that if it be fused with
metallic antimony, a new sulphuret is formed, containing much more of the
metal than the former, and separating distinctly, when fused, both from the
pure metal on the one hand, and the ordinary gray sulphuret on the other.
In some rough experiments, the metal thus taken up by the ordinary
sulphuret of antimony was equal to half the proportion of that previously
in the sulphuret, in which case the new sulphuret would consist of _single_
proportionals.

695. When this new sulphuret was dissolved in muriatic acid, although a
little antimony separated, yet it appeared to me that a true protochloride,
consisting of _single_ proportionals, was formed, and from that by
alkalies, &c., a true protoxide, consisting also of _single_ proportionals,
was obtainable. But I could not stop to ascertain this matter strictly by
analysis.

696. I believe, however, that there is such an oxide; that it is often
present in variable proportions in what is commonly called protoxide,
throwing uncertainty upon the results of its analysis, and causing the
electrolytic decomposition above described[A].

  [A] In relation to this and the three preceding paragraphs, and also
  801, see Berzelius's correction of the nature of the supposed now
  sulphuret and oxide, Phil. Mag. 1836, vol. viii. 476: and for the
  probable explanation of the effects obtained with the protoxide, refer
  to 1340, 1341.--_Dec. 1838._

697. Upon the whole, it appears probable that all those binary compounds of
elementary bodies which are capable of being electrolyzed when fluid, but
not whilst solid, according to the law of liquido-conduction (394.),
consist of single proportionals of their elementary principles; and it may
be because of their departure from this simplicity of composition, that
boracic acid, ammonia, perchlorides, periodides, and many other direct
compounds of elements, are indecomposable.

698. With regard to salts and combinations of compound bodies, the same
simple relation does not appear to hold good. I could not decide this by
bisulphates of the alkalies, for as long as the second proportion of acid
remained, water was retained with it. The fused salts conducted, and were
decomposed; but hydrogen always appeared at the negative electrode.

699. A biphosphate of soda was prepared by heating, and ultimately fusing,
the ammonia-phosphate of soda. In this case the fused bisalt conducted, and
was decomposed; but a little gas appeared at the negative electrode; and
though I believe the salt itself was electrolyzed, I am not quite satisfied
that water was entirely absent.

700. Then a biborate of soda was prepared; and this, I think, is an
unobjectionable case. The salt, when fused, conducted, and was decomposed,
and gas appeared at both electrodes: even when the boracic acid was
increased to three proportionals, the same effect took place.

701. Hence this class of compound combinations does not seem to be subject
to the same simple law as the former class of binary combinations. Whether
we may find reason to consider them as mere solutions of the compound of
single proportionals in the excess of acid, is a matter which, with some
apparent exceptions occurring amongst the sulphurets, must be left for
decision by future examination.

702. In any investigation of these points, great care must be taken to
exclude water; for if present, secondary effects are so frequently produced
as often seemingly to indicate an electro-decomposition of substances, when
no true result of the kind has occurred (742, &c.).

703. It is evident that all the cases in which decomposition _does not
occur, may_ depend upon the want of conduction (677. 413.); but that does
not at all lessen the interest excited by seeing the great difference of
effect due to a change, not in the nature of the elements, but merely in
their proportions; especially in any attempt which may be made to elucidate
and expound the beautiful theory put forth by Sir Humphry Davy[A], and
illustrated by Berzelius and other eminent philosophers, that ordinary
chemical affinity is a mere result of the electrical attractions of the
particles of matter.

  [A] Philosophical Transactions, 1807, pp. 32, 39; also 1826, pp. 387,
  389.


¶ v. _On a new measure of Volta-electricity._

704. I have already said, when engaged in reducing common and voltaic
electricity to one standard of measurement (377.), and again when
introducing my theory of electro-chemical decomposition (504. 505. 510.),
that the chemical decomposing action of a current _is constant for a
constant quantity of electricity_, notwithstanding the greatest variations
in its sources, in its intensity, in the size of the _electrodes_ used, in
the nature of the conductors (or non-conductors (307.)) through which it is
passed, or in other circumstances. The conclusive proofs of the truth of
these statements shall be given almost immediately (783, &c.).

705. I endeavoured upon this law to construct an instrument which should
measure out the electricity passing through it, and which, being interposed
in the course of the current used in any particular experiment, should
serve at pleasure, either as a _comparative standard_ of effect, or as a
_positive measurer_ of this subtile agent.

706. There is no substance better fitted, under ordinary circumstances, to
be the indicating body in such an instrument than water; for it is
decomposed with facility when rendered a better conductor by the addition
of acids or salts; its elements may in numerous cases be obtained and
collected without any embarrassment from secondary action, and, being
gaseous, they are in the best physical condition for separation and
measurement. Water, therefore, acidulated by sulphuric acid, is the
substance I shall generally refer to, although it may become expedient in
peculiar cases or forms of experiment to use other bodies (843.).

707. The first precaution needful in the construction of the instrument was
to avoid the recombination of the evolved gases, an effect which the
positive electrode has been found so capable of producing (571.). For this
purpose various forms of decomposing apparatus were used. The first
consisted of straight tubes, each containing a plate and wire of platina
soldered together by gold, and fixed hermetically in the glass at the
closed extremity of the tube (Plate V. fig. 60.). The tubes were about
eight inches long, 0.7 of an inch in diameter, and graduated. The platina
plates were about an inch long, as wide as the tubes would permit, and
adjusted as near to the mouths of the tubes as was consistent with the safe
collection of the gases evolved. In certain cases, where it was required to
evolve the elements upon as small a surface as possible, the metallic
extremity, instead of being a plate, consisted of the wire bent into the
form of a ring (fig. 61.). When these tubes were used as measurers, they
were filled with the dilute sulphuric acid, inverted in a basin of the same
liquid (fig. 62.), and placed in an inclined position, with their mouths
near to each other, that as little decomposing matter should intervene as
possible; and also, in such a direction that the platina plates should be
in vertical planes (720).

708. Another form of apparatus is that delineated (fig. 63.). The tube is
bent in the middle; one end is closed; in that end is fixed a wire and
plate, _a_, proceeding so far downwards, that, when in the position
figured, it shall be as near to the angle as possible, consistently with
the collection, at the closed extremity of the tube, of all the gas evolved
against it. The plane of this plate is also perpendicular (720.). The other
metallic termination, _b_, is introduced at the time decomposition is to be
effected, being brought as near the angle as possible, without causing any
gas to pass from it towards the closed end of the instrument. The gas
evolved against it is allowed to escape.

709. The third form of apparatus contains both electrodes in the same tube;
the transmission, therefore, of the electricity, and the consequent
decomposition, is far more rapid than in the separate tubes. The resulting
gas is the sum of the portions evolved at the two electrodes, and the
instrument is better adapted than either of the former as a measurer of the
quantity of voltaic electricity transmitted in ordinary cases. It consists
of a straight tube (fig. 64.) closed at the upper extremity, and graduated,
through the sides of which pass platina wires (being fused into the glass),
which are connected with two plates within. The tube is fitted by grinding
into one mouth of a double-necked bottle. If the latter be one-half or
two-thirds full of the dilute sulphuric acid (706.), it will, upon
inclination of the whole, flow into the tube and fill it. When an electric
current is passed through the instrument, the gases evolved against the
plates collect in the upper portion of the tube, and are not subject to the
recombining power of the platina.

710. Another form of the instrument is given at fig. 65.

711. A fifth form is delineated (fig. 66.). This I have found exceedingly
useful in experiments continued in succession for days together, and where
large quantities of indicating gas were to be collected. It is fixed on a
weighted foot, and has the form of a small retort containing the two
electrodes: the neck is narrow, and sufficiently long to deliver gas
issuing from it into a jar placed in a small pneumatic trough. The
electrode chamber, sealed hermetically at the part held in the stand, is
five inches in length, and 0.6 of an inch in diameter; the neck about nine
inches in length, and 0.4 of an inch in diameter internally. The figure
will fully indicate the construction.

712. It can hardly be requisite to remark, that in the arrangement of any
of these forms of apparatus, they, and the wires connecting them with the
substance, which is collaterally subjected to the action of the same
electric current, should be so far insulated as to ensure a certainty that
all the electricity which passes through the one shall also be transmitted
through the other.

       *       *       *       *       *

713. Next to the precaution of collecting the gases, if mingled, out of
contact with the platinum, was the necessity of testing the law of a
_definite electrolytic_ action, upon water at least, under all varieties of
condition; that, with a conviction of its certainty, might also be obtained
a knowledge of those interfering circumstances which would require to be
practically guarded against.

714. The first point investigated was the influence or indifference of
extensive variations in the size of the electrodes, for which purpose
instruments like those last described (709. 710. 711.) were used. One of
these had plates 0.7 of an inch wide, and nearly four inches long; another
had plates only 0.5 of an inch wide, and 0.8 of an inch long; a third had
wires 0.02 of an inch in diameter, and three inches long; and a fourth,
similar wires only half an inch in length. Yet when these were filled with
dilute sulphuric acid, and, being placed in succession, had one common
current of electricity passed through them, very nearly the same quantity
of gas was evolved in all. The difference was sometimes in favour of one
and sometimes on the side of another; but the general result was that the
largest quantity of gases was evolved at the smallest electrodes, namely,
those consisting merely of platina wires.

715. Experiments of a similar kind were made with the single-plate,
straight tubes (707.), and also with the curved tubes (708.), with similar
consequences; and when these, with the former tubes, were arranged together
in various ways, the result, as to the equality of action of large and
small metallic surfaces when delivering and receiving the same current of
electricity, was constantly the same. As an illustration, the following
numbers are given. An instrument with two wires evolved 74.3 volumes of
mixed gases; another with plates 73.25 volumes; whilst the sum of the
oxygen and hydrogen in two separate tubes amounted to 73.65 volumes. In
another experiment the volumes were 55.3, 55.3, and 54.4.

716. But it was observed in these experiments, that in single-plate tubes
(707.) more hydrogen was evolved at the negative electrode than was
proportionate to the oxygen at the positive electrode; and generally, also,
more than was proportionate to the oxygen and hydrogen in a double-plate
tube. Upon more minutely examining these effects, I was led to refer them,
and also the differences between wires and plates (714.), to the solubility
of the gases evolved, especially at the positive electrode.

717. When the positive and negative electrodes are equal in surface, the
bubbles which rise from them in dilute sulphuric acid are always different
in character. Those from the positive plate are exceedingly small, and
separate instantly from every part of the surface of the metal, in
consequence of its perfect cleanliness (633.); whilst in the liquid they
give it a hazy appearance, from their number and minuteness; are easily
carried down by currents, and therefore not only present far greater
surface of contact with the liquid than larger bubbles would do, but are
retained a much longer time in mixture with it. But the bubbles at the
negative surface, though they constitute twice the volume of the gas at the
positive electrode, are nevertheless very inferior in number. They do not
rise so universally from every part of the surface, but seem to be evolved
at different parts; and though so much larger, they appear to cling to the
metal, separating with difficulty from it, and when separated, instantly
rising to the top of the liquid. If, therefore, oxygen and hydrogen had
equal solubility in, or powers of combining with, water under similar
circumstances, still under the present conditions the oxygen would be far
the most liable to solution; but when to these is added its well-known
power of forming a compound with water, it is no longer surprising that
such a compound should be produced in small quantities at the positive
electrode; and indeed the blenching power which some philosophers have
observed in a solution at this electrode, when chlorine and similar bodies
have been carefully excluded, is probably due to the formation there, in
this manner, of oxywater.

718. That more gas was collected from the wires than from the plates, I
attribute to the circumstance, that as equal quantities were evolved in
equal times, the bubbles at the wires having been more rapidly produced, in
relation to any part of the surface, must have been much larger; have been
therefore in contact with the fluid by a much smaller surface, and for a
much shorter time than those at the plates; hence less solution and a
greater amount collected.

719. There was also another effect produced, especially by the use of large
electrodes, which was both a consequence and a proof of the solution of
part of the gas evolved there. The collected gas, when examined, was found
to contain small portions of nitrogen. This I attribute to the presence of
air dissolved in the acid used for decomposition. It is a well-known fact,
that when bubbles of a gas but slightly soluble in water or solutions pass
through them, the portion of this gas which is dissolved displaces a
portion of that previously in union with the liquid: and so, in the
decompositions under consideration, as the oxygen dissolves, it displaces a
part of the air, or at least of the nitrogen, previously united to the
acid; and this effect takes place _most extensively_ with large plates,
because the gas evolved at them is in the most favourable condition for
solution,

720. With the intention of avoiding this solubility of the gases as much as
possible, I arranged the decomposing plates in a vertical position (707.
708.), that the bubbles might quickly escape upwards, and that the downward
currents in the fluid should not meet ascending currents of gas. This
precaution I found to assist greatly in producing constant results, and
especially in experiments to be hereafter referred to, in which other
liquids than dilute sulphuric acid, as for instance solution of potash,
were used.

721. The irregularities in the indications of the measurer proposed,
arising from the solubility just referred to, are but small, and may be
very nearly corrected by comparing the results of two or three experiments.
They may also be almost entirely avoided by selecting that solution which
is found to favour them in the least degree (728.); and still further by
collecting the hydrogen only, and using that as the indicating gas; for
being much less soluble than oxygen, being evolved with twice the rapidity
and in larger bubbles (717.), it can be collected more perfectly and in
greater purity.

722. From the foregoing and many other experiments, it results that
_variation in the size of the electrodes causes no variation in the
chemical action of a given quantity of electricity upon water_.

723. The next point in regard to which the principle of constant
electro-chemical action was tested, was _variation of intensity_. In the
first place, the preceding experiments were repeated, using batteries of an
_equal_ number of plates, _strongly_ and _weakly_ charged; but the results
were alike. They were then repeated, using batteries sometimes containing
forty, and at other times only five pairs of plates; but the results were
still the same. _Variations therefore in the intensity_, caused by
difference in the strength of charge, or in the number of alternations
used, _produced no difference as to the equal action of large and small
electrodes_.

724. Still these results did not prove that variation in the intensity of
the current was not accompanied by a corresponding variation in the
electro-chemical effects, since the actions at _all_ the surfaces might
have increased or diminished together. The deficiency in the evidence is,
however, completely supplied by the former experiments on different-sized
electrodes; for with variation in the size of these, a variation in the
intensity must have occurred. The intensity of an electric current
traversing conductors alike in their nature, quality, and length, is
probably as the quantity of electricity passing through a given sectional
area perpendicular to the current, divided by the time (360. _note_); and
therefore when large plates were contrasted with wires separated by an
equal length of the same decomposing conductor (714.), whilst one current
of electricity passed through both arrangements, that electricity must have
been in a very different state, as to _tension_, between the plates and
between the wires; yet the chemical results were the same.

725. The difference in intensity, under the circumstances described, may be
easily shown practically, by arranging two decomposing apparatus as in fig.
67, where the same fluid is subjected to the decomposing power of the same
current of electricity, passing in the vessel A. between large platina
plates, and in the vessel B. between small wires. If a third decomposing
apparatus, such as that delineated fig. 66. (711.), be connected with the
wires at _ab_, fig. 67, it will serve sufficiently well, by the degree of
decomposition occurring in it, to indicate the relative state of the two
plates as to intensity; and if it then be applied in the same way, as a
test of the state of the wires at _a'b'_, it will, by the increase of
decomposition within, show how much greater the intensity is there than at
the former points. The connexions of P and N with the voltaic battery are
of course to be continued during the whole time.

726. A third form of experiment, in which difference of intensity was
obtained, for the purpose of testing the principle of equal chemical
action, was to arrange three volta-electrometers, so that after the
electric current had passed through one, it should divide into two parts,
each of which should traverse one of the remaining instruments, and should
then reunite. The sum of the decomposition in the two latter vessels was
always equal to the decomposition in the former vessel. But the _intensity_
of the divided current could not be the same as that it had in its original
state; and therefore _variation of intensity has no influence on the
results if the quantity of electricity remain the same_. The experiment, in
fact, resolves itself simply into an increase in the size of the electrodes
(725.).

727. The _third point_, in respect to which the principle of equal
electro-chemical action on water was tested, was _variation of the strength
of the solution used_. In order to render the water a conductor, sulphuric
acid had been added to it (707.); and it did not seem unlikely that this
substance, with many others, might render the water more subject to
decomposition, the electricity remaining the same in quantity. But such did
not prove to be the case. Diluted sulphuric acid, of different strengths,
was introduced into different decomposing apparatus, and submitted
simultaneously to the action of the same electric current (714.). Slight
differences occurred, as before, sometimes in one direction, sometimes in
another; but the final result was, that _exactly the same quantity of water
was decomposed in all the solutions by the same quantity of electricity_,
though the sulphuric acid in some was seventy-fold what it was in others.
The strengths used were of specific gravity 1.495, and downwards.

728. When an acid having a specific gravity of about 1.336 was employed,
the results were most uniform, and the oxygen and hydrogen (716.) most
constantly in the right proportion to each other. Such an acid gave more
gas than one much weaker acted upon by the same current, apparently because
it had less solvent power. If the acid were very strong, then a remarkable
disappearance of oxygen took place; thus, one made by mixing two measures
of strong oil of vitriol with one of water, gave forty-two volumes of
hydrogen, but only twelve of oxygen. The hydrogen was very nearly the same
with that evolved from acid of the specific gravity 1.232. I have not yet
had time to examine minutely the circumstances attending the disappearance
of the oxygen in this case, but imagine it is due to the formation of
oxywater, which Thenard has shown is favoured by the presence of acid.

729. Although not necessary for the practical use of the instrument I am
describing, yet as connected with the important point of constant
chemical action upon water, I now investigated the effects produced by an
electro-electric current passing through aqueous solutions of acids, salts,
and compounds, exceedingly different from each other in their nature, and
found them to yield astonishingly uniform results. But many of them which
are connected with a secondary action will be more usefully described
hereafter (778.).

730. When solutions of caustic potassa or soda, or sulphate of magnesia, or
sulphate of soda, were acted upon by the electric current, just as much
oxygen and hydrogen was evolved from them as from the diluted sulphuric
acid, with which they were compared. When a solution of ammonia, rendered a
better conductor by sulphate of ammonia (554.), or a solution of
subcarbonate of potassa was experimented with, the _hydrogen_ evolved was
in the same quantity as that set free from the diluted sulphuric acid with
which they were compared. Hence _changes in the nature of the solution do
not alter the constancy of electrolytic action upon water_.

731. I have already said, respecting large and small electrodes, that
change of order caused no change in the general effect (715.). The same was
the case with different solutions, or with different intensities; and
however the circumstances of an experiment might be varied, the results
came forth exceedingly consistent, and proved that the electro-chemical
action was still the same.

732. I consider the foregoing investigation as sufficient to prove the very
extraordinary and important principle with respect to WATER, _that when
subjected to the influence of the electric current, a quantity of it is
decomposed exactly proportionate to the quantity of electricity which has
passed_, notwithstanding the thousand variations in the conditions and
circumstances under which it may at the time be placed; and further, that
when the interference of certain secondary effects (742. &c.), together
with the solution or recombination of the gas and the evolution of air, are
guarded against, _the products of the decomposition may be collected with
such accuracy, as to afford a very excellent and valuable measurer of the
electricity concerned in their evolution_.

733. The forms of instrument which I have given, figg. 64, 65, 66. (709.
710. 711.), are probably those which will be found most useful, as they
indicate the quantity of electricity by the largest volume of gases, and
cause the least obstruction to the passage of the current. The fluid which
my present experience leads me to prefer, is a solution of sulphuric acid
of specific gravity about 1.336, or from that to 1.25; but it is very
essential that there should be no organic substance, nor any vegetable
acid, nor other body, which, by being liable to the action of the oxygen or
hydrogen evolved at the electrodes (773. &c.), shall diminish their
quantity, or add other gases to them.

734. In many cases when the instrument is used as a _comparative standard_,
or even as _a measurer_, it may be desirable to collect the hydrogen only,
as being less liable to absorption or disappearance in other ways than the
oxygen; whilst at the same time its volume is so large, as to render it a
good and sensible indicator. In such cases the first and second form of
apparatus have been used, figg. 62, 63. (707. 708.). The indications
obtained were very constant, the variations being much smaller than in
those forms of apparatus collecting both gases; and they can also be
procured when solutions are used in comparative experiments, which,
yielding no oxygen or only secondary results of its action, can give no
indications if the educts at both electrodes be collected. Such is the case
when solutions of ammonia, muriatic acid, chlorides, iodides, acetates or
other vegetable salts, &c., are employed.

735. In a few cases, as where solutions of metallic salts liable to
reduction at the negative electrode are acted upon, the oxygen may be
advantageously used as the measuring substance. This is the case, for
instance, with sulphate of copper.

736. There are therefore two general forms of the instrument which I submit
as a measurer of electricity; one, in which both the gases of the water
decomposed are collected (709. 710. 711.); and the other, in which a single
gas, as the hydrogen only, is used (707. 708.). When referred to as a
_comparative instrument_, (a use I shall now make of it very extensively,)
it will not often require particular precaution in the observation; but
when used as an _absolute measurer_, it will be needful that the barometric
pressure and the temperature be taken into account, and that the graduation
of the instruments should be to one scale; the hundredths and smaller
divisions of a cubical inch are quite fit for this purpose, and the
hundredth may be very conveniently taken as indicating a DEGREE of
electricity.

737. It can scarcely be needful to point out further than has been done how
this instrument is to be used. It is to be introduced into the course of
the electric current, the action of which is to be exerted anywhere else,
and if 60° or 70° of electricity are to be measured out, either in one or
several portions, the current, whether strong or weak, is to be continued
until the gas in the tube occupies that number of divisions or hundredths
of a cubical inch. Or if a quantity competent to produce a certain effect
is to be measured, the effect is to be obtained, and then the indication
read off. In exact experiments it is necessary to correct the volume of gas
for changes in temperature and pressure, and especially for moisture[A].
For the latter object the volta-electrometer (fig. 66.) is most accurate,
as its gas can be measured over water, whilst the others retain it over
acid or saline solutions.

  [A] For a simple table of correction for moisture, I may take the
  liberty of referring to my Chemical Manipulation, edition of 1830,
  p. 376.

738. I have not hesitated to apply the term _degree_ (736.), in analogy
with the use made of it with respect to another most important imponderable
agent, namely, heat; and as the definite expansion of air, water, mercury,
&c., is there made use of to measure heat, so the equally definite
evolution of gases is here turned to a similar use for electricity.

739. The instrument offers the only _actual measurer_ of voltaic
electricity which we at present possess. For without being at all affected
by variations in time or intensity, or alterations in the current itself,
of any kind, or from any cause, or even of intermissions of action, it
takes note with accuracy of the quantity of electricity which has passed
through it, and reveals that quantity by inspection; I have therefore named
it a VOLTA-ELECTROMETER.

740. Another mode of measuring volta-electricity may be adopted with
advantage in many cases, dependent on the quantities of metals or other
substances evolved either as primary or as secondary results; but I refrain
from enlarging on this use of the products, until the principles on which
their constancy depends have been fully established (791. 848.);

741. By the aid of this instrument I have been able to establish the
definite character of electro-chemical action in its most general sense;
and I am persuaded it will become of the utmost use in the extensions of
the science which these views afford. I do not pretend to have made its
detail perfect, but to have demonstrated the truth of the principle, and
the utility of the application[A].

  [A] As early as the year 1811, Messrs. Gay-Lussac and Thénard employed
  chemical decomposition as a measure of the electricity of the voltaic
  pile. See _Recherches Physico-chymiques_, p. 12. The principles and
  precautions by which it becomes an exact measure were of course not
  then known.--_Dec. 1838._


¶ vi. _On the primary or secondary character of the bodies evolved at the
Electrodes._

742. Before the _volta-electrometer_ could be employed in determining, as a
_general law_, the constancy of electro-decomposition, it became necessary
to examine a distinction, already recognised among scientific men, relative
to the products of that action, namely, their primary or secondary
character; and, if possible, by some general rule or principle, to decide
when they were of the one or the other kind. It will appear hereafter that
great mistakes inspecting electro-chemical action and its consequences have
arisen from confounding these two classes of results together.

743. When a substance under decomposition yields at the electrodes those
bodies uncombined and unaltered which the electric current has separated,
then they may be considered as primary results, even though themselves
compounds. Thus the oxygen and hydrogen from water are primary results; and
so also are the acid and alkali (themselves compound bodies) evolved from
sulphate of soda. But when the substances separated by the current are
changed at the electrodes before their appearance, then they give rise to
secondary results, although in many cases the bodies evolved are
elementary.

744. These secondary results occur in two ways, being sometimes due to the
mutual action of the evolved substance and the matter of the electrode, and
sometimes to its action upon the substances contained in the body itself
under decomposition. Thus, when carbon is made the positive electrode in
dilute sulphuric acid, carbonic oxide and carbonic acid occasionally appear
there instead of oxygen; for the latter, acting upon the matter of the
electrode, produces these secondary results. Or if the positive electrode,
in a solution of nitrate or acetate of lead, be platina, then peroxide of
lead appears there, equally a secondary result with the former, but now
depending upon an action of the oxygen on a substance in the solution.
Again, when ammonia is decomposed by platina electrodes, nitrogen appears
at the _anode_[A]; but though an _elementary_ body, it is a _secondary_
result in this case, being derived from the chemical action of the oxygen
electrically evolved there, upon the ammonia in the surrounding solution
(554.). In the same manner when aqueous solutions of metallic salts are
decomposed by the current, the metals evolved at the _cathode_, though
elements, are _always_ secondary results, and not immediate consequences of
the decomposing power of the electric current.

  [A] Annales de Chimie, 1801, tom. li. p. 167.

745. Many of these secondary results are extremely valuable; for instance,
all the interesting compounds which M. Becquerel has obtained by feeble
electric currents are of this nature; but they are essentially chemical,
and must, in the theory of electrolytic action, be carefully distinguished
from those which are directly due to the action of the electric current.

746. The nature of the substances evolved will often lead to a correct
judgement of their primary or secondary character, but is not sufficient
alone to establish that point. Thus, nitrogen is said to be attracted
sometimes by the positive and sometimes by the negative electrode,
according to the bodies with which it may be combined (554. 555.), and it
is on such occasions evidently viewed as a primary result[A]; but I think I
shall show, that, when it appears at the positive electrode, or rather at
the _anode_, it is a secondary result (748.). Thus, also, Sir Humphry
Davy[B], and with him the great body of chemical philosophers, (including
myself,) have given the appearance of copper, lead, tin, silver, gold, &c.,
at the negative electrode, when their aqueous solutions were acted upon by
the voltaic current, as proofs that the metals, as a class, were attracted
to that surface; thus assuming the metal, in each case, to be a primary
result. These, however, I expect to prove, are all secondary results; the
mere consequence of chemical action, and no proofs either of the attraction
or of the law announced respecting their places[C].

  [A] Annales de Chimie, 1804, tom. li. p. 172.

  [B] Elements of Chemical Philosophy, pp. 144. 161.

  [C] It is remarkable that up to 1804 it was the received opinion that
  the metals were reduced by the nascent hydrogen. At that date the
  general opinion was reversed by Hisinger and Berzelius (Annales de
  Chimie, 1804, tom. li. p. 174,), who stated that the metals were
  evolved directly by the electricity: in which opinion it appears, from
  that time, Davy coincided (Philosophical Transactions, 1826, p. 388).

747. But when we take to our assistance the law of _constant
electro-chemical action_ already proved with regard to water (732.), and
which I hope to extend satisfactorily to all bodies (821.), and consider
the _quantities_ as well as the _nature_ of the substances set free, a
generally accurate judgement of the primary or secondary character of the
results may be formed: and this important point, so essential to the theory
of electrolyzation, since it decides what are the particles directly under
the influence of the current, (distinguishing them from such as are not
affected,) and what are the results to be expected, may be established with
such degree of certainty as to remove innumerable ambiguities and doubtful
considerations from this branch of the science.

748. Let us apply these principles to the case of ammonia, and the supposed
determination of nitrogen to one or the other _electrode_ (554. 555,). A
pure strong solution of ammonia is as bad a conductor, and therefore as
little liable to electrolyzation, as pure water; but when sulphate of
ammonia is dissolved in it, the whole becomes a conductor; nitrogen
_almost_ and occasionally _quite_ pure is evolved at the _anode_, and
hydrogen at the _cathode_; the ratio of the volume of the former to that of
the latter varying, but being as 1 to about 3 or 4. This result would seem
at first to imply that the electric current had decomposed ammonia, and
that the nitrogen had been determined towards the positive electrode. But
when the electricity used was measured out by the volta-electrometer (707.
736.), it was found that the hydrogen obtained was exactly in the
proportion which would have been supplied by decomposed water, whilst the
nitrogen had no certain or constant relation whatever. When, upon
multiplying experiments, it was found that, by using a stronger or weaker
solution, or a more or less powerful battery, the gas evolved at the
_anode_ was a mixture of oxygen and nitrogen, varying both in proportion
and absolute quantity, whilst the hydrogen at the _cathode_ remained
constant, no doubt could be entertained that the nitrogen at the _anode_
was a secondary result, depending upon the chemical action of the nascent
oxygen, determined to that surface by the electric current, upon the
ammonia in solution. It was the water, therefore, which was electrolyzed,
not the ammonia. Further, the experiment gives no real indication of the
tendency of the element nitrogen to either one electrode or the other; nor
do I know of any experiment with nitric acid, or other compounds of
nitrogen, which shows the tendency of this element, under the influence of
the electric current, to pass in either direction along its course.

749. As another illustration of secondary results, the effects on a
solution of acetate of potassa, may be quoted. When a very strong solution
was used, more gas was evolved at the _anode_ than at the _cathode_, in the
proportion of 4 to 3 nearly: that from the _anode_ was a mixture of
carbonic oxide and carbonic acid; that from the _cathode_ pure hydrogen.
When a much weaker solution was used, less gas was evolved at the _anode_
than at the _cathode_; and it now contained carburetted hydrogen, as well
as carbonic oxide and carbonic acid. This result of carburetted hydrogen at
the positive electrode has a very anomalous appearance, if considered as an
immediate consequence of the decomposing power of the current. It, however,
as well as the carbonic oxide and acid, is only a _secondary result_; for
it is the water alone which suffers electro-decomposition, and it is the
oxygen eliminated at the _anode_ which, reacting on the acetic acid, in the
midst of which it is evolved, produces those substances that finally appear
there. This is fully proved by experiments with the volta-electrometer
(707.); for then the hydrogen evolved from the acetate at the _cathode_ is
always found to be definite, being exactly proportionate to the electricity
which has passed through the solution, and, in quantity, the same as the
hydrogen evolved in the volta-electrometer itself. The appearance of the
carbon in combination with the hydrogen at the positive electrode, and its
non-appearance at the negative electrode, are in curious contrast with the
results which might have been expected from the law usually accepted
respecting the final places of the elements.

750. If the salt in solution be an acetate of lead, then the results at
both electrodes are secondary, and cannot be used to estimate or express
the amount of electro-chemical action, except by a circuitous process
(843.). In place of oxygen or even the gases already described (749.),
peroxide of lead now appears at the positive, and lead itself at the
negative electrode. When other metallic solutions are used, containing, for
instance, peroxides, as that of copper, combined with this or any other
decomposable acid, still more complicated results will be obtained; which,
viewed as direct results of the electro-chemical action, will, in their
proportions, present nothing but confusion, but will appear perfectly
harmonious and simple if they be considered as secondary results, and will
accord in their proportions with the oxygen and hydrogen evolved from water
by the action of a definite quantity of electricity.

751. I have experimented upon many bodies, with a view to determine whether
the results were primary or secondary. I have been surprised to find how
many of them, in ordinary cases, are of the latter class, and how
frequently water is the only body electrolyzed in instances where other
substances have been supposed to give way. Some of these results I will
give in as few words as possible.

752. _Nitric acid._--When very strong, it conducted well, and yielded
oxygen at the positive electrode. No gas appeared at the negative
electrode; but nitrous acid, and apparently nitric oxide, were formed
there, which, dissolving, rendered the acid yellow or red, and at last even
effervescent, from the spontaneous separation of nitric oxide. Upon
diluting the acid with its bulk or more of water, gas appeared at the
negative electrode. Its quantity could be varied by variations, either in
the strength of the acid or of the voltaic current: for that acid from
which no gas separated at the _cathode_, with a weak voltaic battery, did
evolve gas there with a stronger; and that battery which evolved no gas
there with a strong acid, did cause its evolution with an acid more dilute.
The gas at the _anode_ was always oxygen; that at the _cathode_ hydrogen.
When the quantity of products was examined by the volta-electrometer
(707.), the oxygen, whether from strong or weak acid, proved to be in the
same proportion as from water. When the acid was diluted to specific
gravity 1.24, or less, the hydrogen also proved to be the same in quantity
as from water. Hence I conclude that the nitric acid does not undergo
electrolyzation, but the water only; that the oxygen at the _anode_ is
always a primary result, but that the products at the _cathode_ are often
secondary, and due to the reaction of the hydrogen upon the nitric acid.

753. _Nitre._--A solution of this salt yields very variable results,
according as one or other form of tube is used, or as the electrodes are
large or small. Sometimes the whole of the hydrogen of the water decomposed
may be obtained at the negative electrode; at other times, only a part of
it, because of the ready formation of secondary results. The solution is a
very excellent conductor of electricity.

754. _Nitrate of ammonia_, in aqueous solution, gives rise to secondary
results very varied and uncertain in their proportions.

755. _Sulphurous acid._--Pure liquid sulphurous acid does not conduct nor
suffer decomposition by the voltaic current[A], but, when dissolved in
water, the solution acquires conducting power, and is decomposed, yielding
oxygen at the _anode_, and hydrogen and sulphur at the _cathode_.

  [A] See also De la Rive, Bibliothèque Universelle, tom. xl. p. 205; or
  Quarterly Journal of Science, vol. xxvii. p, 407.

756. A solution containing sulphuric acid in addition to the sulphurous
acid, was a better conductor. It gave very little gas at either electrode:
that at the _anode_ was oxygen, that at the _cathode_ pure hydrogen. From
the _cathode_ also rose a white turbid stream, consisting of diffused
sulphur, which soon rendered the whole solution milky. The volumes of gases
were in no regular proportion to the quantities evolved from water in the
voltameter. I conclude that the sulphurous acid was not at all affected by
the electric current in any of these cases, and that the water present was
the only body electro-chemically decomposed; that, at the _anode_, the
oxygen from the water converted the sulphurous acid into sulphuric acid,
and, at the _cathode_, the hydrogen electrically evolved decomposed the
sulphurous acid, combining with its oxygen, and setting its sulphur free. I
conclude that the sulphur at the negative electrode was only a secondary
result; and, in fact, no part of it was found combined with the small
portion of hydrogen which escaped when weak solutions of sulphurous acid
were used.

757. _Sulphuric acid._--I have already given my reasons for concluding that
sulphuric acid is not electrolyzable, i.e. not decomposable directly by the
electric current, but occasionally suffering by a secondary action at the
_cathode_ from the hydrogen evolved there (681.). In the year 1800, Davy
considered the sulphur from sulphuric acid as the result of the action of
the nascent hydrogen[A]. In 1804, Hisinger and Berzelius stated that it was
the direct result of the action of the voltaic pile[B], an opinion which
from that time Davy seems to have adopted, and which has since been
commonly received by all. The change of my own opinion requires that I
should correct what I have already said of the decomposition of sulphuric
acid in a former series of these Researches (552.): I do not now think that
the appearance of the sulphur at the negative electrode is an immediate
consequence of electrolytic action.

  [A] Nicholson's Quarterly Journal, vol. iv. pp. 280, 281.

  [B] Annales de Chimie, 1804, tom. li. p. 173.

758. _Muriatic acid._--A strong solution gave hydrogen at the negative
electrode, and chlorine only at the positive electrode; of the latter, a
part acted on the platina and a part was dissolved. A minute bubble of gas
remained; it was not oxygen, but probably air previously held in solution.

759. It was an important matter to determine whether the chlorine was a
primary result, or only a secondary product, due to the action of the
oxygen evolved from water at the _anode_ upon the muriatic acid; i.e.
whether the muriatic acid was electrolyzable, and if so, whether the
decomposition was _definite_.

760. The muriatic acid was gradually diluted. One part with six of water
gave only chlorine at the _anode_. One part with eight of water gave only
chlorine; with nine of water, a little oxygen appeared with the chlorine;
but the occurrence or non-occurrence of oxygen at these strengths depended,
in part, on the strength of the voltaic battery used. With fifteen parts of
water, a little oxygen, with much chlorine, was evolved at the _anode_. As
the solution was now becoming a bad conductor of electricity, sulphuric
acid was added to it: this caused more ready decomposition, but did not
sensibly alter the proportion of chlorine and oxygen.

761. The muriatic acid was now diluted with 100 times its volume of dilute
sulphuric acid. It still gave a large proportion of chlorine at the
_anode_, mingled with oxygen; and the result was the same, whether a
voltaic battery of 40 pairs of plates or one containing only 5 pairs were
used. With acid of this strength, the oxygen evolved at the _anode_ was to
the hydrogen at the _cathode_, in volume, as 17 is to 64; and therefore the
chlorine would have been 30 volumes, had it not been dissolved by the
fluid.

762. Next with respect to the quantity of elements evolved. On using the
volta-electrometer, it was found that, whether the strongest or the weakest
muriatic acid were used, whether chlorine alone or chlorine mingled with
oxygen appeared at the _anode_, still the hydrogen evolved at the _cathode_
was a constant quantity, i.e. exactly the same as the hydrogen which the
_same quantity of electricity_ could evolve from water.

763. This constancy does not decide whether the muriatic acid is
electrolyzed or not, although it proves that if so, it must be in definite
proportions to the quantity of electricity used. Other considerations may,
however, be allowed to decide the point. The analogy between chlorine and
oxygen, in their relations to hydrogen, is so strong, as to lead almost to
the certainty, that, when combined with that element, they would perform
similar parts in the process of electro-decomposition. They both unite with
it in single proportional or equivalent quantities; and the number of
proportionals appearing to have an intimate and important relation to the
decomposability of a body (697.), those in muriatic acid, as well as in
water, are the most favourable, or those perhaps even necessary, to
decomposition. In other binary compounds of chlorine also, where nothing
equivocal depending on the simultaneous presence of it and oxygen is
involved, the chlorine is directly eliminated at the _anode_ by the
electric current. Such is the case with the chloride of lead (395.), which
may be justly compared with protoxide of lead (402.), and stands in the
same relation to it as muriatic acid to water. The chlorides of potassium,
sodium, barium, &c., are in the same relation to the protoxides of the same
metals and present the same results under the influence of the electric
current (402.).

764. From all the experiments, combined with these considerations, I
conclude that muriatic acid is decomposed by the direct influence of the
electric current, and that the quantities evolved are, and therefore the
chemical action is, _definite for a definite quantity of electricity_. For
though I have not collected and measured the chlorine, in its separate
state, at the _anode_, there can exist no doubt as to its being
proportional to the hydrogen at the _cathode_; and the results are
therefore sufficient to establish the general law of _constant
electro-chemical action_ in the case of muriatic acid.

765. In the dilute acid (761.), I conclude that a part of the water is
electro-chemically decomposed, giving origin to the oxygen, which appears
mingled with the chlorine at the _anode_. The oxygen _may_ be viewed as a
secondary result; but I incline to believe that it is not so; for, if it
were, it might be expected in largest proportion from the stronger acid,
whereas the reverse is the fact. This consideration, with others, also
leads me to conclude that muriatic acid is more easily decomposed by the
electric current than water; since, even when diluted with eight or nine
times its quantity of the latter fluid, it alone gives way, the water
remaining unaffected.

766. _Chlorides._--On using solutions of chlorides in water,--for instance,
the chlorides of sodium or calcium,--there was evolution of chlorine only
at the positive electrode, and of hydrogen, with the oxide of the base, as
soda or lime, at the negative electrode. The process of decomposition may
be viewed as proceeding in two or three ways, all terminating in the same
results. Perhaps the simplest is to consider the chloride as the substance
electrolyzed, its chlorine being determined to and evolved at the _anode_,
and its metal passing to the _cathode_, where, finding no more chlorine, it
acts upon the water, producing hydrogen and an oxide as secondary results.
As the discussion would detain me from more important matter, and is not of
immediate consequence, I shall defer it for the present. It is, however, of
_great consequence_ to state, that, on using the volta-electrometer, the
hydrogen in both cases was definite; and if the results do not prove the
definite decomposition of chlorides, (which shall be proved
elsewhere,--789. 794. 814.,) they are not in the slightest degree opposed
to such a conclusion, and do support the _general law_.

767. _Hydriodic acid._--A solution of hydriodic acid was affected exactly
in the same manner as muriatic acid. When strong, hydrogen was evolved at
the negative electrode, in definite proportion to the quantity of
electricity which had passed, i.e. in the same proportion as was evolved by
the same current from water; and iodine without any oxygen was evolved at
the positive electrode. But when diluted, small quantities of oxygen
appeared with the iodine at the _anode_, the proportion of hydrogen at the
_cathode_ remaining undisturbed.

768. I believe the decomposition of the hydriodic acid in this case to be
direct, for the reasons already given respecting muriatic acid (763. 764.).

769. _Iodides._--A solution of iodide of potassium being subjected to the
voltaic current, iodine appeared at the positive electrode (without any
oxygen), and hydrogen with free alkali at the negative electrode. The same
observations as to the mode of decomposition are applicable here as were
made in relation to the chlorides when in solution (766.).

770. _Hydro-fluoric acid and fluorides._--Solution of hydrofluoric acid did
not appear to be decomposed under the influence of the electric current: it
was the water which gave way apparently. The fused fluorides were
electrolysed (417.); but having during these actions obtained _fluorine_ in
the separate state, I think it better to refer to a future series of these
Researches, in which I purpose giving a fuller account of the results than
would be consistent with propriety here[A].

  [A] I have not obtained fluorine: my expectations, amounting to
  conviction, passed away one by one when subjected to rigorous
  examination; some very singular results were obtained; and to one of
  these I refer at 1340.--_Dec. 1838._

771. _Hydro-cyanic acid_ in solution conducts very badly. The definite
proportion of hydrogen (equal to that from water) was set free at the
_cathode_, whilst at the _anode_ a small quantity of oxygen was evolved and
apparently a solution of cyanogen formed. The action altogether
corresponded with that on a dilute muriatic or hydriodic acid. When the
hydrocyanic acid was made a better conductor by sulphuric acid, the same
results occurred.

_Cyanides._--With a solution of the cyanide of potassium, the result was
precisely the same as with a chloride or iodide. No oxygen was evolved at
the positive electrode, but a brown solution formed there. For the reasons
given when speaking of the chlorides (766.), and because a fused cyanide of
potassium evolves cyanogen at the positive electrode[A], I incline to
believe that the cyanide in solution is _directly_ decomposed.

  [A] It is a very remarkable thing to see carbon and nitrogen in this
  case determined powerfully towards the positive surface of the voltaic
  battery; but it is perfectly in harmony with the theory of
  electro-chemical decomposition which I have advanced.

772. _Ferro-cyanic acid_ and the _ferro-cyanides_, as also _sulpho-cyanic
acid_ and the _sulpho-cyanides_, presented results corresponding with those
just described (771.).

773. _Acetic acid._--Glacial acetic acid, when fused (405.), is not
decomposed by, nor does it conduct, electricity. On adding a little water
to it, still there were no signs of action; on adding more water, it acted
slowly and about as pure water would do. Dilute sulphuric acid was added to
it in order to make it a better conductor; then the definite proportion of
hydrogen was evolved at the _cathode_, and a mixture of oxygen in very
deficient quantity, with carbonic acid, and a little carbonic oxide, at the
_anode_. Hence it appears that acetic acid is not electrolyzable, but that
a portion of it is decomposed by the oxygen evolved at the _anode_,
producing secondary results, varying with the strength of the acid, the
intensity of the current, and other circumstances.

774. _Acetates._--One of these has been referred to already, as affording
only secondary results relative to the acetic acid (749.). With many of the
metallic acetates the results at both electrodes are secondary (746. 750.).

Acetate of soda fused and anhydrous is directly decomposed, being, as I
believe, a true electrolyte, and evolving soda and acetic acid at the
_cathode_ and _anode_. These however have no sensible duration, but are
immediately resolved into other substances; charcoal, sodiuretted hydrogen,
&c., being set free at the former, and, as far as I could judge under the
circumstances, acetic acid mingled with carbonic oxide, carbonic acid, &c.
at the latter.

775. _Tartaric acid._--Pure solution of tartaric acid is almost as bad a
conductor as pure water. On adding sulphuric acid, it conducted well, the
results at the positive electrode being primary or secondary in different
proportions, according to variations in the strength of the acid and the
power of the electric current (752.). Alkaline tartrates gave a large
proportion of secondary results at the positive electrode. The hydrogen at
the negative electrode remained constant unless certain triple metallic
salts were used.

776. Solutions, of salts containing other vegetable acids, as the
benzoates; of sugar, gum, &c., dissolved in dilute sulphuric acid; of
resin, albumen, &c., dissolved in alkalies, were in turn submitted to the
electrolytic power of the voltaic current. In all these cases, secondary
results to a greater or smaller extent were produced at the positive
electrode.

777. In concluding this division of these Researches, it cannot but occur
to the mind that the final result of the action of the electric current
upon substances, placed between the electrodes, instead of being simple may
be very complicated. There are two modes by which these substances may be
decomposed, either by the direct force of the electric current, or by the
action of bodies which that current may evolve. There are also two modes by
which new compounds may be formed, i.e. by combination of the evolving
substances whilst in their nascent state (658.), directly with the matter
of the electrode; or else their combination with those bodies, which being
contained in, or associated with, the body suffering decomposition, are
necessarily present at the _anode_ and _cathode_. The complexity is
rendered still greater by the circumstance that two or more of these
actions may occur simultaneously, and also in variable proportions to each
other. But it may in a great measure be resolved by attention to the
principles already laid down (747.).

778. When _aqueous_ solutions of bodies are used, secondary results are
exceedingly frequent. Even when the water is not present in large quantity,
but is merely that of combination, still secondary results often ensue: for
instance, it is very possible that in Sir Humphry Davy's decomposition of
the hydrates of potassa and soda, a part of the potassium produced was the
result of a secondary action. Hence, also, a frequent cause for the
disappearance of the oxygen and hydrogen which would otherwise be evolved:
and when hydrogen does _not_ appear at the _cathode_ in an _aqueous
solution_, it perhaps always indicates that a secondary action has taken
place there. No exception to this rule has as yet occurred to my
observation.

779. Secondary actions are _not confined to aqueous solutions_, or cases
where water is present. For instance, various chlorides acted upon, when
fused (402.), by platina electrodes, have the chlorine determined
electrically to the _anode_. In many cases, as with the chlorides of lead,
potassium, barium, &c., the chlorine acts on the platina and forms a
compound with it, which dissolves; but when protochloride of tin is used,
the chlorine at the _anode_ does not act upon the platina, but upon the
chloride already there, forming a perchloride which rises in vapour (790.
804.). These are, therefore, instances of secondary actions of both kinds,
produced in bodies containing no water.

780. The production of boron from fused borax (402. 417.) is also a case of
secondary action; for boracic acid is not decomposable by electricity
(408.), and it was the sodium evolved at the _cathode_ which, re-acting on
the boracic acid around it, took oxygen from it and set boron free in the
experiments formerly described.

781. Secondary actions have already, in the hands of M. Becquerel, produced
many interesting results in the formation of compounds; some of them new,
others imitations of those occurring naturally[A]. It is probable they may
prove equally interesting in an opposite direction, i.e. as affording cases
of analytic decomposition. Much information regarding the composition, and
perhaps even the arrangement, of the particles of such bodies as the
vegetable acids and alkalies, and organic compounds generally, will
probably be obtained by submitting them to the action of nascent oxygen,
hydrogen, chlorine, &c. at the electrodes; and the action seems the more
promising, because of the thorough command which we possess over attendant
circumstances, such as the strength of the current, the size of the
electrodes, the nature of the decomposing conductor, its strength, &c., all
of which may be expected to have their corresponding influence upon the
final result.

782. It is to me a great satisfaction that the extreme variety of secondary
results has presented nothing opposed to the doctrine of a constant and
definite electro-chemical action, to the particular consideration of which
I shall now proceed.


¶ vii. _On the definite nature and extent of Electro-chemical
Decomposition._

783. In the third series of these Researches, after proving the identity of
electricities derived from different sources, and showing, by actual
measurement, the extraordinary quantity of electricity evolved by a very
feeble voltaic arrangement (371. 376.), I announced a law, derived from
experiment, which seemed to me of the utmost importance to the science of
electricity in general, and that branch of it denominated electro-chemistry
in particular. The law was expressed thus: _The chemical power of a current
of electricity is in direct proportion to the absolute quantity of
electricity which passes_ (377.).

  [A] Annales de Chimie, tom, xxxv. p. 113.

784. In the further progress of the successive investigations, I have had
frequent occasion to refer to the same law, sometimes in circumstances
offering powerful corroboration of its truth (456. 504. 505.); and the
present series already supplies numerous new cases in which it holds good
(704. 722. 726. 732.). It is now my object to consider this great principle
more closely, and to develope some of the consequences to which it leads.
That the evidence for it may be the more distinct and applicable, I shall
quote cases of decomposition subject to as few interferences from secondary
results as possible, effected upon bodies very simple, yet very definite in
their nature.

785. In the first place, I consider the law as so fully established with
respect to the decomposition of _water_, and under so many circumstances
which might be supposed, if anything could, to exert an influence over it,
that I may be excused entering into further detail respecting that
substance, or even summing up the results here (732.). I refer, therefore,
to the whole of the subdivision of this series of Researches which contains
the account of the _volta-electrometer_ (704. &c.).

786. In the next place, I also consider the law as established with respect
to _muriatic acid_ by the experiments and reasoning already advanced, when
speaking of that substance, in the subdivision respecting primary and
secondary results (758. &c.).

787. I consider the law as established also with regard to _hydriodic acid_
by the experiments and considerations already advanced in the preceding
division of this series of Researches (767. 768.).

788. Without speaking with the same confidence, yet from the experiments
described, and many others not described, relating to hydro-fluoric,
hydro-cyanic, ferro-cyanic, and sulpho-cyanic acids (770. 771. 772.), and
from the close analogy which holds between these bodies and the hydracids
of chlorine, iodine, bromine, &c., I consider these also as coming under
subjection to the law, and assisting to prove its truth.

789. In the preceding cases, except the first, the water is believed to be
inactive; but to avoid any ambiguity arising from its presence, I sought
for substances from which it should be absent altogether; and, taking
advantage of the law of conduction already developed (380. &c.), I soon
found abundance, amongst which _protochloride of tin_ was first subjected
to decomposition in the following manner. A piece of platina wire had one
extremity coiled up into a small knob, and, having been carefully weighed,
was sealed hermetically into a piece of bottle-glass tube, so that the knob
should be at the bottom of the tube within (fig. 68.). The tube was
suspended by a piece of platina wire, so that the heat of a spirit-lamp
could be applied to it. Recently fused protochloride of tin was introduced
in sufficient quantity to occupy, when melted, about one-half of the tube;
the wire of the tube was connected with a volta-electrometer (711.), which
was itself connected with the negative end of a voltaic battery; and a
platina wire connected with the positive end of the same battery was dipped
into the fused chloride in the tube; being however so bent, that it could
not by any shake of the hand or apparatus touch the negative electrode at
the bottom of the vessel. The whole arrangement is delineated in fig. 69.

790. Under these circumstances the chloride of tin was decomposed: the
chlorine evolved at the positive electrode formed bichloride of tin (779.),
which passed away in fumes, and the tin evolved at the negative electrode
combined with the platina, forming an alloy, fusible at the temperature to
which the tube was subjected, and therefore never occasioning metallic
communication through the decomposing chloride. When the experiment had
been continued so long as to yield a reasonable quantity of gas in the
volta-electrometer, the battery connexion was broken, the positive
electrode removed, and the tube and remaining chloride allowed to cool.
When cold, the tube was broken open, the rest of the chloride and the glass
being easily separable from the platina wire and its button of alloy. The
latter when washed was then reweighed, and the increase gave the weight of
the tin reduced.

791. I will give the particular results of one experiment, in illustration
of the mode adopted in this and others, the results of which I shall have
occasion to quote. The negative electrode weighed at first 20 grains; after
the experiment, it, with its button of alloy, weighed 23.2 grains. The tin
evolved by the electric current at the _cathode_: weighed therefore 3.2
grains. The quantity of oxygen and hydrogen collected in the
volta-electrometer = 3.85 cubic inches. As 100 cubic inches of oxygen and
hydrogen, in the proportions to form water, may be considered as weighing
12.92 grains, the 3.85 cubic inches would weigh 0.49742 of a grain; that
being, therefore, the weight of water decomposed by the same electric
current as was able to decompose such weight of protochloride of tin as
could yield 3.2 grains of metal. Now 0.49742 : 3.2 :: 9 the equivalent of
water is to 57.9, which should therefore be the equivalent of tin, if the
experiment had been made without error, and if the electro-chemical
decomposition _is in this case also definite_. In some chemical works 58 is
given as the chemical equivalent of tin, in others 57.9. Both are so near
to the result of the experiment, and the experiment itself is so subject to
slight causes of variation (as from the absorption of gas in the
volta-electrometer (716.), &c.), that the numbers leave little doubt of the
applicability of the _law of definite action_ in this and all similar cases
of electro-decomposition.

792. It is not often I have obtained an accordance in numbers so near as
that I have just quoted. Four experiments were made on the protochloride of
tin, the quantities of gas evolved in the volta-electrometer being from
2.05 to 10.29 cubic inches. The average of the four experiments gave 58.53
as the electro-chemical equivalent for tin.

793. The chloride remaining after the experiment was pure protochloride of
tin; and no one can doubt for a moment that the equivalent of chlorine had
been evolved at the _anode_, and, having formed bichloride of tin as a
secondary result, had passed away.

794. _Chloride of lead_ was experimented upon in a manner exactly similar,
except that a change was made in the nature of the positive electrode; for
as the chlorine evolved at the _anode_ forms no perchloride of lead, but
acts directly upon the platina, it produces, if that metal be used, a
solution of chloride of platina in the chloride of lead; in consequence of
which a portion of platina can pass to the _cathode_, and would then
produce a vitiated result. I therefore sought for, and found in plumbago,
another substance, which could be used safely as the positive electrode in
such bodies as chlorides, iodides, &c.

The chlorine or iodine does not act upon it, but is evolved in the free
state; and the plumbago has no re-action, under the circumstances, upon the
fused chloride or iodide in which it is plunged. Even if a few particles of
plumbago should separate by the heat or the mechanical action of the
evolved gas, they can do no harm in the chloride.

795. The mean of three experiments gave the number of 100.85 as the
equivalent for lead. The chemical equivalent is 103.5. The deficiency in my
experiments I attribute to the solution of part of the gas (716.) in the
volta-electrometer; but the results leave no doubt on my mind that both the
lead and the chlorine are, in this case, evolved in _definite quantities_
by the action of a given quantity of electricity (814. &c.).

796. _Chloride of antimony._--It was in endeavouring to obtain the
electro-chemical equivalent of antimony from the chloride, that I found
reasons for the statement I have made respecting the presence of water in
it in an earlier part of these Researches (690. 693. &c.).

797. I endeavoured to experiment upon the _oxide of lead_ obtained by
fusion and ignition of the nitrate in a platina crucible, but found great
difficulty, from the high temperature required for perfect fusion, and the
powerful fluxing qualities of the substance. Green-glass tubes repeatedly
failed. I at last fused the oxide in a small porcelain crucible, heated
fully in a charcoal fire; and, as it is was essential that the evolution of
the lead at the _cathode_ should take place beneath the surface, the
negative electrode was guarded by a green-glass tube, fused around it in
such a _manner as to expose only the knob of platina_ at the lower end
(fig. 70.), so that it could be plunged beneath the surface, and thus
exclude contact of air or oxygen with the lead reduced there. A platina
wire was employed for the positive electrode, that metal not being subject
to any action from the oxygen evolved against it. The arrangement is given
in fig. 71.

798. In an experiment of this kind the equivalent for the lead came out
93.17, which is very much too small. This, I believe, was because of the
small interval between the positive and negative electrodes in the oxide of
lead; so that it was not unlikely that some of the froth and bubbles formed
by the oxygen at the _anode_ should occasionally even touch the lead
reduced at the _cathode_, and re-oxidize it. When I endeavoured to correct
this by having more litharge, the greater heat required to keep it all
fluid caused a quicker action on the crucible, which was soon eaten
through, and the experiment stopped.

799. In one experiment of this kind I used borate of lead (408. 673.). It
evolves lead, under the influence of the electric current, at the _anode_,
and oxygen at the _cathode_; and as the boracic acid is not either directly
(408.) or incidentally decomposed during the operation, I expected a result
dependent on the oxide of lead. The borate is not so violent a flux as the
oxide, but it requires a higher temperature to make it quite liquid; and if
not very hot, the bubbles of oxygen cling to the positive electrode, and
retard the transfer of electricity. The number for lead came out 101.29,
which is so near to 103.5 as to show that the action of the current had
been definite.

800. _Oxide of bismuth._--I found this substance required too high a
temperature, and acted too powerfully as a flux, to allow of any experiment
being made on it, without the application of more time and care than I
could give at present.

801. The ordinary _protoxide of antimony_, which consists of one
proportional of metal and one and a half of oxygen, was subjected to the
action of the electric current in a green-glass tube (789.), surrounded by
a jacket of platina foil, and heated in a charcoal fire. The decomposition
began and proceeded very well at first, apparently indicating, according to
the general law (679. 697.), that this substance was one containing such
elements and in such proportions as made it amenable to the power of the
electric current. This effect I have already given reasons for supposing
may be due to the presence of a true protoxide, consisting of single
proportionals (696. 693.). The action soon diminished, and finally ceased,
because of the formation of a higher oxide of the metal at the positive
electrode. This compound, which was probably the peroxide, being infusible
and insoluble in the protoxide, formed a crystalline crust around the
positive electrode; and thus insulating it, prevented the transmission of
the electricity. Whether, if it had been fusible and still immiscible, it
would have decomposed, is doubtful, because of its departure from the
required composition (697.). It was a very natural secondary product at the
positive electrode (779.). On opening the tube it was found that a little
antimony had been separated at the negative electrode; but the quantity was
too small to allow of any quantitative result being obtained[A].

  [A] This paragraph is subject to the corrective note now appended to
  paragraph 696.--_Dec. 1838._

802. _Iodide of lead._--This substance can be experimented with in tubes
heated by a spirit-lamp (789.); but I obtained no good results from it,
whether I used positive electrodes of platina or plumbago. In two
experiments the numbers for the lead came out only 75.46 and 73.45, instead
of 103.5. This I attribute to the formation of a periodide at the positive
electrode, which, dissolving in the mass of liquid iodide, came in contact
with the lead evolved at the negative electrode, and dissolved part of it,
becoming itself again protiodide. Such a periodide does exist; and it is
very rarely that the iodide of lead formed by precipitation, and
well-washed, can be fused without evolving much iodine, from the presence
of this percompound; nor does crystallization from its hot aqueous solution
free it from this substance. Even when a little of the protiodide and
iodine are merely rubbed together in a mortar, a portion of the periodide
is formed. And though it is decomposed by being fused and heated to dull
redness for a few minutes, and the whole reduced to protiodide, yet that is
not at all opposed to the possibility, that a little of that which is
formed in great excess of iodine at the _anode_, should be carried by the
rapid currents in the liquid into contact with the _cathode_.

803. This view of the result was strengthened by a third experiment, where
the space between the electrodes was increased to one third of an inch; for
now the interfering effects were much diminished, and the number of the
lead came out 89.04; and it was fully confirmed by the results obtained in
the cases of _transfer_ to be immediately described (818.).

The experiments on iodide of lead therefore offer no exception to the
_general law_ under consideration, but on the contrary may, from general
considerations, be admitted as included in it.

804. _Protiodide of tin._--This substance, when fused (402.), conducts and
is decomposed by the electric current, tin is evolved at the _anode_, and
periodide of tin as a secondary result (779. 790.) at the _cathode_. The
temperature required for its fusion is too high to allow of the production
of any results fit for weighing.

805. _Iodide of potassium_ was subjected to electrolytic action in a tube,
like that in fig. 68. (789.). The negative electrode was a globule of lead,
and I hoped in this way to retain the potassium, and obtain results that
could be weighed and compared with the volta-electrometer indication; but
the difficulties dependent upon the high temperature required, the action
upon the glass, the fusibility of the platina induced by the presence of
the lead, and other circumstances, prevented me from procuring such
results. The iodide was decomposed with the evolution of iodine at the
_anode_, and of potassium at the _cathode_, as in former cases.

806. In some of these experiments several substances were placed in
succession, and decomposed simultaneously by the same electric current:
thus, protochloride of tin, chloride of lead, and water, were thus acted on
at once. It is needless to say that the results were comparable, the tin,
lead, chlorine, oxygen, and hydrogen evolved being _definite in quantity_
and electro-chemical equivalents to each other.

       *       *       *       *       *

807. Let us turn to another kind of proof of the _definite chemical action
of electricity_. If any circumstances could be supposed to exert an
influence over the quantity of the matters evolved during electrolytic
action, one would expect them to be present when electrodes of different
substances, and possessing very different chemical affinities for such
matters, were used. Platina has no power in dilute sulphuric acid of
combining with the oxygen at the _anode_, though the latter be evolved in
the nascent state against it. Copper, on the other hand, immediately unites
with the oxygen, as the electric current sets it free from the hydrogen;
and zinc is not only able to combine with it, but can, without any help
from the electricity, abstract it directly from the water, at the same time
setting torrents of hydrogen free. Yet in cases where these three
substances were used as the positive electrodes in three similar portions
of the same dilute sulphuric acid, specific gravity 1.336, precisely the
same quantity of water was decomposed by the electric current, and
precisely the same quantity of hydrogen set free at the _cathodes_ of the
three solutions.

808. The experiment was made thus. Portions of the dilute sulphuric acid
were put into three basins. Three volta-electrometer tubes, of the form
figg. 60. 62. were filled with the same acid, and one inverted in each
basin (707.). A zinc plate, connected with the positive end of a voltaic
battery, was dipped into the first basin, forming the positive electrode
there, the hydrogen, which was abundantly evolved from it by the direct
action of the acid, being allowed to escape. A copper plate, which dipped
into the acid of the second basin, was connected with the negative
electrode of the _first_ basin; and a platina plate, which dipped into the
acid of the third basin, was connected with the negative electrode of the
_second_ basin. The negative electrode of the third basin was connected
with a volta-electrometer (711.), and that with the negative end of the
voltaic battery.

809. Immediately that the circuit was complete, the _electro-chemical
action_ commenced in all the vessels. The hydrogen still rose in,
apparently, undiminished quantities from the positive zinc electrode in the
first basin. No oxygen was evolved at the positive copper electrode in the
second basin, but a sulphate of copper was formed there; whilst in the
third basin the positive platina electrode evolved pure oxygen gas, and was
itself unaffected. But in _all_ the basins the hydrogen liberated at the
_negative_ platina electrodes was the _same in quantity_, and the same with
the volume of hydrogen evolved in the volta-electrometer, showing that in
all the vessels the current had decomposed an equal quantity of water. In
this trying case, therefore, the _chemical action of electricity_ proved to
be _perfectly definite_.

810. A similar experiment was made with muriatic acid diluted with its bulk
of water. The three positive electrodes were zinc, silver, and platina; the
first being able to separate and combine with the chlorine _without_ the
aid of the current; the second combining with the chlorine only after the
current had set it free; and the third rejecting almost the whole of it.
The three negative electrodes were, as before, platina plates fixed within
glass tubes. In this experiment, as in the former, the quantity of hydrogen
evolved at the _cathodes_ was the same for all, and the same as the
hydrogen evolved in the volta-electrometer. I have already given my reasons
for believing that in these experiments it is the muriatic acid which is
directly decomposed by the electricity (764.); and the results prove that
the quantities so decomposed are _perfectly definite_ and proportionate to
the quantity of electricity which has passed.

811. In this experiment the chloride of silver formed in the second basin
retarded the passage of the current of electricity, by virtue of the law of
conduction before described (394.), so that it had to be cleaned off four
or five times during the course of the experiment; but this caused no
difference between the results of that vessel and the others.

812. Charcoal was used as the positive electrode in both sulphuric and
muriatic acids (808. 810.); but this change produced no variation of the
results. A zinc positive electrode, in sulphate of soda or solution of
common salt, gave the same constancy of operation.

813. Experiments of a similar kind were then made with bodies altogether in
a different state, i.e. with _fused_ chlorides, iodides, &c. I have already
described an experiment with fused chloride of silver, in which the
electrodes were of metallic silver, the one rendered negative becoming
increased and lengthened by the addition of metal, whilst the other was
dissolved and eaten away by its abstraction. This experiment was repeated,
two weighed pieces of silver wire being used as the electrodes, and a
volta-electrometer included in the circuit. Great care was taken to
withdraw the negative electrodes so regularly and steadily that the
crystals of reduced silver should not form a _metallic_ communication
beneath the surface of the fused chloride. On concluding the experiment the
positive electrode was re-weighed, and its loss ascertained. The mixture of
chloride of silver, and metal, withdrawn in successive portions at the
negative electrode, was digested in solution of ammonia, to remove the
chloride, and the metallic silver remaining also weighed: it was the
reduction at the _cathode_, and exactly equalled the solution at the
_anode_; and each portion was as nearly as possible the equivalent to the
water decomposed in the volta-electrometer.

814. The infusible condition of the silver at the temperature used, and the
length and ramifying character of its crystals, render the above experiment
difficult to perform, and uncertain in its results. I therefore wrought
with chloride of lead, using a green-glass tube, formed as in fig. 72. A
weighed platina wire was fused into the bottom of a small tube, as before
described (789.). The tube was then bent to an angle, at about half an inch
distance from the closed end; and the part between the angle and the
extremity being softened, was forced upward, as in the figure, so as to
form a bridge, or rather separation, producing two little depressions or
basins _a, b_, within the tube. This arrangement was suspended by a platina
wire, as before, so that the heat of a spirit-lamp could be applied to it,
such inclination being given to it as would allow all air to escape during
the fusion of the chloride of lead. A positive electrode was then provided,
by bending up the end of a platina wire into a knot, and fusing about
twenty grains of metallic lead on to it, in a small closed tube of glass,
which was afterwards broken away. Being so furnished, the wire with its
lead was weighed, and the weight recorded.

815. Chloride of lead was now introduced into the tube, and carefully
fused. The leaded electrode was also introduced; after which the metal, at
its extremity, soon melted. In this state of things the tube was filled up
to _c_ with melted chloride of lead; the end of the electrode to be
rendered negative was in the basin _b_, and the electrode of melted lead
was retained in the basin _a_, and, by connexion with the proper conducting
wire of a voltaic battery, was rendered positive. A volta-electrometer was
included in the circuit.

816. Immediately upon the completion of the communication with the voltaic
battery, the current passed, and decomposition proceeded. No chlorine was
evolved at the positive electrode; but as the fused chloride was
transparent, a button of alloy could be observed gradually forming and
increasing in size at _b_, whilst the lead at _a_ could also be seen
gradually to diminish. After a time, the experiment was stopped; the tube
allowed to cool, and broken open; the wires, with their buttons, cleaned
and weighed; and their change in weight compared with the indication of the
volta-electrometer.

817. In this experiment the positive electrode had lost just as much lead
as the negative one had gained (795.), and the loss and gain were very
nearly the equivalents of the water decomposed in the volta-electrometer,
giving for lead the number 101.5. It is therefore evident, in this
instance, that causing a _strong affinity_, or _no affinity_, for the
substance evolved at the _anode_, to be active during the experiment
(807.), produces no variation in the definite action of the electric
current.

818. A similar experiment was then made with iodide of lead, and in this
manner all confusion from the formation of a periodide avoided (803.). No
iodine was evolved during the whole action, and finally the loss of lead at
the _anode_ was the same as the gain at the _cathode_, the equivalent
number, by comparison with the result in the volta-electrometer, being
103.5.

819. Then protochloride of tin was subjected to the electric current in the
same manner, using of course, a tin positive electrode. No bichloride of
tin was now formed (779. 790.). On examining the two electrodes, the
positive had lost precisely as much as the negative had gained; and by
comparison with the volta-electrometer, the number for tin came out 59.

820. It is quite necessary in these and similar experiments to examine the
interior of the bulbs of alloy at the ends of the conducting wires; for
occasionally, and especially with those which have been positive, they are
cavernous, and contain portions of the chloride or iodide used, which must
be removed before the final weight is ascertained. This is more usually the
case with lead than tin.

821. All these facts combine into, I think, an irresistible mass of
evidence, proving the truth of the important proposition which I at first
laid down, namely, _that the chemical power of a current of electricity is
in direct proportion to the absolute quantity of electricity which passes_
(377. 783.). They prove, too, that this is not merely true with one
substance, as water, but generally with all electrolytic bodies; and,
further, that the results obtained with any _one substance_ do not merely
agree amongst themselves, but also with those obtained from _other
substances_, the whole combining together into _one series of definite
electro-chemical actions_ (505.). I do not mean to say that no exceptions
will appear: perhaps some may arise, especially amongst substances existing
only by weak affinity; but I do not expect that any will seriously disturb
the result announced. If, in the well-considered, well-examined, and, I may
surely say, well-ascertained doctrines of the definite nature of ordinary
chemical affinity, such exceptions occur, as they do in abundance, yet,
without being allowed to disturb our minds as to the general conclusion,
they ought also to be allowed if they should present themselves at this,
the opening of a new view of electro-chemical action; not being held up as
obstructions to those who may be engaged in rendering that view more and
more perfect, but laid aside for a while, in hopes that their perfect and
consistent explanation will ultimately appear.

       *       *       *       *       *

822. The doctrine of _definite electro-chemical action_ just laid down,
and, I believe, established, leads to some new views of the relations and
classifications of bodies associated with or subject to this action. Some
of these I shall proceed to consider.

823. In the first place, compound bodies may be separated into two great
classes, namely, those which are decomposable by the electric current, and
those which are not: of the latter, some are conductors, others
non-conductors, of voltaic electricity[A]. The former do not depend for
their decomposability upon the nature of their elements only; for, of the
same two elements, bodies may be formed, of which one shall belong to one
class and another to the other class; but probably on the proportions also
(697.). It is further remarkable, that with very few, if any, exceptions
(414. 691.), these decomposable bodies are exactly those governed by the
remarkable law of conduction I have before described (694.); for that law
does not extend to the many compound fusible substances that are excluded
from this class. I propose to call bodies of this, the decomposable class,
_Electrolytes_ (664.).

  [A] I mean here by voltaic electricity, merely electricity from a most
  abundant source, but having very small intensity.

824. Then, again, the substances into which these divide, under the
influence of the electric current, form an exceedingly important general
class. They are combining bodies; are directly associated with the
fundamental parts of the doctrine of chemical affinity; and have each a
definite proportion, in which they are always evolved during electrolytic
action. I have proposed to call these bodies generally _ions_, or
particularly _anions_ and _cations_, according as they appear at the
_anode_ or _cathode_ (665.); and the numbers representing the proportions
in which they are evolved _electro-chemical equivalents_. Thus hydrogen,
oxygen, chlorine, iodine, lead, tin are _ions_; the three former are
_anions_, the two metals are _cations_, and 1, 8, 3, 125, 104, 58, are
their _electro-chemical equivalents_ nearly.

825. A summary of certain points already ascertained respecting
_electrolytes, ions_, and _electro-chemical equivalents_, may be given in
the following general form of propositions, without, I hope, including any
serious error.

826. i. A single _ion_, i.e. one not in combination with another, will have
no tendency to pass to either of the electrodes, and will be perfectly
indifferent to the passing current, unless it be itself a compound of more
elementary _ions_, and so subject to actual decomposition. Upon this fact
is founded much of the proof adduced in favour of the new theory of
electro-chemical decomposition, which I put forth in a former series of
these Researches (518. &c.).

827. ii. If one _ion_ be combined in right proportions (697.) with another
strongly opposed to it in its ordinary chemical relations, i.e. if an
_anion_ be combined with a _cation_, then both will travel, the one to the
_anode_, the other to the _cathode_, of the decomposing body (530, 542.
547.).

828. iii. If, therefore, an _ion_ pass towards one of the electrodes,
another _ion_ must also be passing simultaneously to the other electrode,
although, from secondary action, it may not make its appearance (743.).

829. iv. A body decomposable directly by the electric current, i.e. an
_electrolyte_, must consist of two _ions_, and must also render them up
during the act of decomposition.

830. v. There is but one _electrolyte_ composed of the same two elementary
_ions_; at least such appears to be the fact (697.), dependent upon a law,
that _only single electro-chemical equivalents of elementary ions can go to
the electrodes, and not multiples_.

831. vi. A body not decomposable when alone, as boracic acid, is not
directly decomposable by the electric current when in combination (780.).
It may act as an _ion_ going wholly to the _anode_ or _cathode_, but does
not yield up its elements, except occasionally by a secondary action.
Perhaps it is superfluous for me to point out that this proposition has _no
relation_ to such cases as that of water, which, by the presence of other
bodies, is rendered a better conductor of electricity, and _therefore_ is
more freely decomposed.

832. vii. The nature of the substance of which the electrode is formed,
provided it be a conductor, causes no difference in the
electro-decomposition, either in kind or degree (807. 813.): but it
seriously influences, by secondary action (714.), the state in which the
finally appear. Advantage may be taken of this principle in combining and
_ions_ collecting such _ions_ as, if evolved in their _free_ state, would
be unmanageable[A].

  [A] It will often happen that the electrodes used may be of such a
  nature as, with the fluid in which they are immersed, to produce an
  electric current, either according with or opposing that of the
  voltaic arrangement used, and in this way, or by direct chemical
  action, may sadly disturb the results. Still, in the midst of all
  these confusing effects, the electric current, which actually passes
  in any direction through the body suffering decomposition, will
  produce its own definite electrolytic action.

833. viii. A substance which, being used as the electrode, can combine with
the _ion_ evolved against it, is also, I believe, an _ion_, and combines,
in such cases, in the quantity represented by its _electro-chemical
equivalent_. All the experiments I have made agree with this view; and it
seems to me, at present, to result as a necessary consequence. Whether, in
the secondary actions that take place, where the _ion_ acts, not upon the
matter of the electrode, but on that which is around it in the liquid
(744.), the same consequence follows, will require more extended
investigation to determine.

834. ix. Compound _ions_ are not necessarily composed of electro-chemical
equivalents of simple _ions_. For instance, sulphuric acid, boracic acid,
phosphoric acid, are _ions_, but not _electrolytes_, i.e. not composed of
electro-chemical equivalents of simple _ions_.

835. x. Electro-chemical equivalents are always consistent; i.e. the same
number which represents the equivalent of a substance A when it is
separating from a substance B, will also represent A when separating from a
third substance C. Thus, 8 is the electro-chemical equivalent of oxygen,
whether separating from hydrogen, or tin, or lead; and 103.5 is the
electrochemical equivalent of lead, whether separating from oxygen, or
chlorine, or iodine.

836. xi. Electro-chemical equivalents coincide, and are the same, with
ordinary chemical equivalents.

837. By means of experiment and the preceding propositions, a knowledge of
_ions_ and their electro-chemical equivalents may be obtained in various
ways.

838. In the first place, they may be determined directly, as has been done
with hydrogen, oxygen, lead, and tin, in the numerous experiments already
quoted.

839. In the next place, from propositions ii. and iii., may be deduced the
knowledge of many other _ions_, and also their equivalents. When chloride
of lead was decomposed, platina being used for both electrodes (395.),
there could remain no more doubt that chlorine was passing to the _anode_,
although it combined with the platina there, than when the positive
electrode, being of plumbago (794.), allowed its evolution in the free
state; neither could there, in either case, remain any doubt that for every
103.5 parts of lead evolved at the _cathode_, 36 parts of chlorine were
evolved at the _anode_, for the remaining chloride of lead was unchanged.
So also, when in a metallic solution one volume of oxygen, or a secondary
compound containing that proportion, appeared at the _anode_, no doubt
could arise that hydrogen, equivalent to two volumes, had been determined
to the _cathode_, although, by a secondary action, it had been employed in
reducing oxides of lead, copper, or other metals, to the metallic state. In
this manner, then, we learn from the experiments already described in these
Researches, that chlorine, iodine, bromine, fluorine, calcium, potassium,
strontium, magnesium, manganese, &c., are _ions_ and that their
_electro-chemical equivalents_ are the same as their _ordinary chemical
equivalents_.

840. Propositions iv. and v. extend our means of gaining information. For
if a body of known chemical composition is found to be decomposable, and
the nature of the substance evolved as a primary or even a secondary result
(743. 777.) at one of the electrodes, be ascertained, the electro-chemical
equivalent of that body may be deduced from the known constant composition
of the substance evolved. Thus, when fused protiodide of tin is decomposed
by the voltaic current (804.), the conclusion may be drawn, that both the
iodine and tin are _ions_, and that the proportions in which they combine
in the fused compound express their electro-chemical equivalents. Again,
with respect to the fused iodide of potassium (805.), it is an electrolyte;
and the chemical equivalents will also be the electro-chemical equivalents.

841. If proposition viii. sustain extensive experimental investigation,
then it will not only help to confirm the results obtained by the use of
the other propositions, but will give abundant original information of its
own.

842. In many instances, the _secondary results_ obtained by the action of
the evolved _ion_ on the substances present in the surrounding liquid or
solution, will give the electro-chemical equivalent. Thus, in the solution
of acetate of lead, and, as far as I have gone, in other proto-salts
subjected to the reducing action of the nascent hydrogen at the _cathode_,
the metal precipitated has been in the same quantity as if it had been a
primary product, (provided no free hydrogen escaped there,) and therefore
gave accurately the number representing its electro-chemical equivalent.

843. Upon this principle it is that secondary results may occasionally be
used as measurers of the volta-electric current (706. 740.); but there are
not many metallic solutions that answer this purpose well: for unless the
metal is easily precipitated, hydrogen will be evolved at the _cathode_ and
vitiate the result. If a soluble peroxide is formed at the _anode_, or if
the precipitated metal crystallize across the solution and touch the
positive electrode, similar vitiated results are obtained. I expect to find
in some salts, as the acetates of mercury and zinc, solutions favourable
for this use.

844. After the first experimental investigations to establish the definite
chemical action of electricity, I have not hesitated to apply the more
strict results of chemical analysis to correct the numbers obtained as
electrolytic results. This, it is evident, may be done in a great number of
cases, without using too much liberty towards the due severity of
scientific research. The series of numbers representing electro-chemical
equivalents must, like those expressing the ordinary equivalents of
chemically acting bodies, remain subject to the continual correction of
experiment and sound reasoning.

845. I give the following brief Table of _ions_ and their electro-chemical
equivalents, rather as a specimen of a first attempt than as anything that
can supply the want which must very quickly be felt, of a full and complete
tabular account of this class of bodies. Looking forward to such a table as
of extreme utility (if well-constructed) in developing the intimate
relation of ordinary chemical affinity to electrical actions, and
identifying the two, not to the imagination merely, but to the conviction
of the senses and a sound judgement, I may be allowed to express a hope,
that the endeavour will always be to make it a table of _real_, and not
_hypothetical_, electro-chemical equivalents; for we shall else overrun the
facts, and lose all sight and consciousness of the knowledge lying directly
in our path.

846. The equivalent numbers do not profess to be exact, and are taken
almost entirely from the chemical results of other philosophers in whom I
could repose more confidence, as to these points, than in myself.

847. TABLE OF IONS.

_Anions_.

Oxygen           8
Chlorine        35.5
Iodine         126
Bromine         78.3
Fluorine        18.7
Cyanogen        26
Sulphuric acid  40
Selenic acid    64
Nitric acid     54
Chloric acid    75.5
Phosphoric acid 35.7
Carbonic acid   22
Boracic acid    24
Acetic acid     51
Tartaric acid   66
Citric acid     58
Oxalic acid     36
Sulphur (?)     16
Selenium (?)
Salpho-cyanogen

_Cations_.

Hydrogen         1
Potassium       39.2
Sodium          23.3
Lithium         10
Barium          68.7
Strontium       43.8
Calcium         20.5
Magnesium       12.7
Manganese       27.7
Zinc            32.5
Tin             57.9
Lead           103.5
Iron            28
Copper          31.6
Cadmium         55.8
Cerium          46
Cobalt          29.5
Nickel          29.5
Antimony        61.67
Bismuth         71
Mercury        200
Silver         108
Platina         98.6?
Gold           (?)

Ammonia         17
Potassa         47.2
Soda            31.3
Lithia          18
Baryta          76.7
Strontia        51.8
Lime            28.5
Magnesia        20.7
Alumina.       (?)
Protoxides generally.
Quinia         171.6
Cinchona       160
Morphia        290
Vegeto-alkalies generally.

848. This Table might be further arrange into groups of such substances as
either act with, or replace, each other. Thus, for instance, acids and
bases act in relation to each other; but they do not act in association
with oxygen, hydrogen, or elementary substances. There is indeed little or
no doubt that, when the electrical relations of the particles of matter
come to be closely examined, this division must be made. The simple
substances, with cyanogen, sulpho-cyanogen, and one or two other compound
bodies, will probably form the first group; and the acids and bases, with
such analogous compounds as may prove to be _ions_, the second group.
Whether these will include all _ions_, or whether a third class of more
complicated results will be required, must be decided by future
experiments.

849. It is _probable_ that all our present elementary bodies are _ions_,
but that is not as yet certain. There are some, such as carbon, phosphorus,
nitrogen, silicon, boron, alumium, the right of which to the title of _ion_
it is desirable to decide as soon as possible. There are also many compound
bodies, and amongst them alumina and silica, which it is desirable to class
immediately by unexceptionable experiments. It is also _possible_, that all
combinable bodies, compound as well as simple, may enter into the class of
_ions_; but at present it does not seem to me probable. Still the
experimental evidence I have is so small in proportion to what must
gradually accumulate around, and bear upon, this point, that I am afraid to
give a strong opinion upon it.

850. I think I cannot deceive myself in considering the doctrine of
definite electro-chemical action as of the utmost importance. It touches by
its facts more directly and closely than any former fact, or set of facts,
have done, upon the beautiful idea, that ordinary chemical affinity is a
mere consequence of the electrical attractions of the particles of
different kinds of matter; and it will probably lead us to the means by
which we may enlighten that which is at present so obscure, and either
fully demonstrate the truth of the idea, or develope that which ought to
replace it.

851. A very valuable use of electro-chemical equivalents will be to decide,
in cases of doubt, what is the true chemical equivalent, or definite
proportional, or atomic number of a body; for I have such conviction that
the power which governs electro-decomposition and ordinary chemical
attractions is the same; and such confidence in the overruling influence of
those natural laws which render the former definite, as to feel no
hesitation in believing that the latter must submit to them also. Such
being the case, I can have, no doubt that, assuming hydrogen as 1, and
dismissing small fractions for the simplicity of expression, the equivalent
number or atomic weight of oxygen is 8, of chlorine 36, of bromine 78.4, of
lead 103.5, of tin 59, &c., notwithstanding that a very high authority
doubles several of these numbers.


§ 13. _On the absolute quantity of Electricity associated with the
particles or atoms of Matter._


852. The theory of definite electrolytical or electro-chemical action
appears to me to touch immediately upon the _absolute quantity_ of
electricity or electric power belonging to different bodies. It is
impossible, perhaps, to speak on this point without committing oneself
beyond what present facts will sustain; and yet it is equally impossible,
and perhaps would be impolitic, not to reason upon the subject. Although we
know nothing of what an atom is, yet we cannot resist forming some idea of
a small particle, which represents it to the mind; and though we are in
equal, if not greater, ignorance of electricity, so as to be unable to say
whether it is a particular matter or matters, or mere motion of ordinary
matter, or some third kind of power or agent, yet there is an immensity of
facts which justify us in believing that the atoms of matter are in some
way endowed or associated with electrical powers, to which they owe their
most striking qualities, and amongst them their mutual chemical affinity.
As soon as we perceive, through the teaching of Dalton, that chemical
powers are, however varied the circumstances in which they are exerted,
definite for each body, we learn to estimate the relative degree of force
which resides in such bodies: and when upon that knowledge comes the fact,
that the electricity, which we appear to be capable of loosening from its
habitation for a while, and conveying from place to place, _whilst it
retains its chemical force_, can be measured out, and being so measured is
found to be _as definite in its action_ as any of _those portions_ which,
remaining associated with the particles of matter, give them their
_chemical relation_; we seem to have found the link which connects the
proportion of that we have evolved to the proportion of that belonging to
the particles in their natural state.

853. Now it is wonderful to observe how small a quantity of a compound body
is decomposed by a certain portion of electricity. Let us, for instance,
consider this and a few other points in relation to water. _One grain_ of
water, acidulated to facilitate conduction, will require an electric
current to be continued for three minutes and three quarters of time to
effect its decomposition, which current must be powerful enough to retain a
platina wire 1/104 of an inch in thickness[A], red-hot, in the air during
the whole time; and if interrupted anywhere by charcoal points, will
produce a very brilliant and constant star of light. If attention be paid
to the instantaneous discharge of electricity of tension, as illustrated in
the beautiful experiments of Mr. Wheatstone[B], and to what I have said
elsewhere on the relation of common and voltaic electricity (371. 375.), it
will not be too much to say that this necessary quantity of electricity is
equal to a very powerful flash of lightning. Yet we have it under perfect
command; can evolve, direct, and employ it at pleasure; and when it has
performed its full work of electrolyzation, it has only separated the
elements of _a single grain of water_.

  [A] I have not stated the length of wire used, because I find by
  experiment, as would be expected in theory, that it is indifferent.
  The same quantity of electricity which, passed in a given time, can
  heat an inch of platina wire of a certain diameter red-hot, can also
  heat a hundred, a thousand, or any length of the same wire to the same
  degree, provided the cooling circumstances are the same for every part
  in all cases. This I have proved by the volta-electrometer. I found
  that whether half an inch or eight inches were retained at one
  constant temperature of dull redness, equal quantities of water were
  decomposed in equal times. When the half-inch was used, only the
  centre portion of wire was ignited. A fine wire may even be used as a
  rough but ready regulator of a voltaic current; for if it be made part
  of the circuit, and the larger wires communicating with it be shifted
  nearer to or further apart, so as to keep the portion of wire in the
  circuit sensibly at the same temperature, the current passing through
  it will be nearly uniform.

  [B] Literary Gazette, 1833, March 1 and 8. Philosophical Magazine,
  1833, p. 201. L'Institut, 1833, p.261.

854. On the other hand, the relation between the conduction of the
electricity and the decomposition of the water is so close, that one cannot
take place without the other. If the water is altered only in that small
degree which consists in its having the solid instead of the fluid state,
the conduction is stopped, and the decomposition is stopped with it.
Whether the conduction be considered as depending upon the decomposition,
or not (443. 703.), still the relation of the two functions is equally
intimate and inseparable.

855. Considering this close and twofold relation, namely, that without
decomposition transmission of electricity does not occur; and, that for a
given definite quantity of electricity passed, an equally definite and
constant quantity of water or other matter is decomposed; considering also
that the agent, which is electricity, is simply employed in overcoming
electrical powers in the body subjected to its action; it seems a probable,
and almost a natural consequence, that the quantity which passes is the
_equivalent_ of, and therefore equal to, that of the particles separated;
i.e. that if the electrical power which holds the elements of a grain of
water in combination, or which makes a grain of oxygen and hydrogen in the
right proportions unite into water when they are made to combine, could be
thrown into the condition of _a current_, it would exactly equal the
current required for the separation of that grain of water into its
elements again.

856. This view of the subject gives an almost overwhelming idea of the
extraordinary quantity or degree of electric power which naturally belongs
to the particles of matter; but it is not inconsistent in the slightest
degree with the facts which can be brought to bear on this point. To
illustrate this I must say a few words on the voltaic pile[A].

  [A] By the term voltaic pile, I mean such apparatus or arrangement of
  metals as up to this time have been called so, and which contain
  water, brine, acids, or other aqueous solutions or decomposable
  substances (476.), between their plates. Other kinds of electric
  apparatus may be hereafter invented, and I hope to construct some not
  belonging to the class of instruments discovered by Volta.

857. Intending hereafter to apply the results given in this and the
preceding series of Researches to a close investigation of the source of
electricity in the voltaic instrument, I have refrained from forming any
decided opinion on the subject; and without at all meaning to dismiss
metallic contact, or the contact of dissimilar substances, being
conductors, but not metallic, as if they had nothing to do with the origin
of the current,

I still am fully of opinion with Davy, that it is at least continued by
chemical action, and that the supply constituting the current is almost
entirely from that source.

858. Those bodies which, being interposed between the metals of the voltaic
pile, render it active, _are all of them electrolytes_ (476.); and it
cannot but press upon the attention of every one engaged in considering
this subject, that in those bodies (so essential to the pile) decomposition
and the transmission of a current are so intimately connected, that one
cannot happen without the other. This I have shown abundantly in water, and
numerous other cases (402. 476.). If, then, a voltaic trough have its
extremities connected by a body capable of being decomposed, as water, we
shall have a continuous current through the apparatus; and whilst it
remains in this state we may look at the part where the acid is acting upon
the plates, and that where the current is acting upon the water, as the
reciprocals of each other. In both parts we have the two conditions
_inseparable in such bodies as these_, namely, the passing of a current,
and decomposition; and this is as true of the cells in the battery as of
the water cell; for no voltaic battery has as yet been constructed in which
the chemical action is only that of combination: _decomposition is always
included_, and is, I believe, an essential chemical part.

859. But the difference in the two parts of the connected battery, that is,
the decomposition or experimental cell, and the acting cells, is simply
this. In the former we urge the current through, but it, apparently of
necessity, is accompanied by decomposition: in the latter we cause
decompositions by ordinary chemical actions, (which are, however,
themselves electrical,) and, as a consequence, have the electrical current;
and as the decomposition dependent upon the current is definite in the
former case, so is the current associated with the decomposition also
definite in the latter (862. &c.).

860. Let us apply this in support of what I have surmised respecting the
enormous electric power of each particle or atom of matter (856.). I showed
in a former series of these Researches on the relation by measure of common
and voltaic electricity, that two wires, one of platina and one of zinc,
each one-eighteenth of an inch in diameter, placed five-sixteenths of an
inch apart, and immersed to the depth of five-eighths of an inch in acid,
consisting of one drop of oil of vitriol and four ounces of distilled water
at a temperature of about 60° Fahr., and connected at the other extremities
by a copper wire eighteen feet long, and one-eighteenth of an inch in
thickness, yielded as much electricity in little more than three seconds of
time as a Leyden battery charged by thirty turns of a very large and
powerful plate electric machine in full action (371.). This quantity,
though sufficient if passed at once through the head of a rat or cat to
have killed it, as by a flash of lightning, was evolved by the mutual
action of so small a portion of the zinc wire and water in contact with it,
that the loss of weight sustained by either would be inappreciable by our
most delicate instruments; and as to the water which could be decomposed by
that current, it must have been insensible in quantity, for no trace of
hydrogen appeared upon the surface of the platina during those three
seconds.

861. What an enormous quantity of electricity, therefore, is required for
the decomposition of a single grain of water! We have already seen that it
must be in quantity sufficient to sustain a platina wire 1/104 of an inch
in thickness, red-hot, in contact with the air, for three minutes and three
quarters (853.), a quantity which is almost infinitely greater than that
which could be evolved by the little standard voltaic arrangement to which
I have just referred (860. 871.). I have endeavoured to make a comparison
by the loss of weight of such a wire in a given time in such an acid,
according to a principle and experiment to be almost immediately described
(862.); but the proportion is so high that I am almost afraid to mention
it. It would appear that 800,000 such charges of the Leyden battery as I
have referred to above, would be necessary to supply electricity sufficient
to decompose a single grain of water; or, if I am right, to equal the
quantity of electricity which is naturally associated with the elements of
that grain of water, endowing them with their mutual chemical affinity.

862. In further proof of this high electric condition of the particles of
matter, and the _identity as to quantity of that belonging to them with
that necessary for their separation_, I will describe an experiment of
great simplicity but extreme beauty, when viewed in relation to the
evolution of an electric current and its decomposing powers.

863. A dilute sulphuric acid, made by adding about one part by measure of
oil of vitriol to thirty parts of water, will act energetically upon a
piece of zinc plate in its ordinary and simple state: but, as Mr. Sturgeon
has shown[A], not at all, or scarcely so, if the surface of the metal has
in the first instance been amalgamated; yet the amalgamated zinc will act
powerfully with platina as an electromotor, hydrogen being evolved on the
surface of the latter metal, as the zinc is oxidized and dissolved. The
amalgamation is best effected by sprinkling a few drops of mercury upon the
surface of the zinc, the latter being moistened with the dilute acid, and
rubbing with the fingers or two so as to extend the liquid metal over the
whole of the surface. Any mercury in excess, forming liquid drops upon the
zinc, should be wiped off[B].

  [A] Recent Experimental Researches, &c., 1830, p.74, &c.

  [B] The experiment may be made with pure zinc, which, as chemists well
  know, is but slightly acted upon by dilute sulphuric acid in
  comparison with ordinary zinc, which during the action is subject to
  an infinity of voltaic actions. See De la Rive on this subject,
  Bibliothèque Universelle, 1830, p.391.

864. Two plates of zinc thus amalgamated were dried and accurately weighed;
one, which we will call A, weighed 163.1 grains; the other, to be called B,
weighed 148.3 grains. They were about five inches long, and 0.4 of an inch
wide. An earthenware pneumatic trough was filled with dilute sulphuric
acid, of the strength just described (863.), and a gas jar, also filled
with the acid, inverted in it[A]. A plate of platina of nearly the same
length, but about three times as wide as the zinc plates, was put up into
this jar. The zinc plate A was also introduced into the jar, and brought in
contact with the platina, and at the same moment the plate B was put into
the acid of the trough, but out of contact with other metallic matter.

  [A] The acid was left during a night with a small piece of
  unamalgamated zinc in it, for the purpose of evolving such air as
  might be inclined to separate, and bringing the whole into a constant
  state.

865. Strong action immediately occurred in the jar upon the contact of the
zinc and platina plates. Hydrogen gas rose from the platina, and was
collected in the jar, but no hydrogen or other gas rose from _either_ zinc
plate. In about ten or twelve minutes, sufficient hydrogen having been
collected, the experiment was stopped; during its progress a few small
bubbles had appeared upon plate B, but none upon plate A. The plates were
washed in distilled water, dried, and reweighed. Plate B weighed 148.3
grains, as before, having lost nothing by the direct chemical action of the
acid. Plate A weighed 154.65 grains, 8.45 grains of it having been oxidized
and dissolved during the experiment.

866. The hydrogen gas was next transferred to a water-trough and measured;
it amounted to 12.5 cubic inches, the temperature being 52°, and the
barometer 29.2 inches. This quantity, corrected for temperature, pressure,
and moisture, becomes 12.15453 cubic inches of dry hydrogen at mean
temperature and pressure; which, increased by one half for the oxygen that
must have gone to the _anode_, i.e. to the zinc, gives 18.232 cubic inches
as the quantity of oxygen and hydrogen evolved from the water decomposed by
the electric current. According to the estimate of the weight of the mixed
gas before adopted (791.), this volume is equal to 2.3535544 grains, which
therefore is the weight of water decomposed; and this quantity is to 8.45,
the quantity of zinc oxidized, as 9 is to 32.31. Now taking 9 as the
equivalent number of water, the number 32.5 is given as the equivalent
number of zinc; a coincidence sufficiently near to show, what indeed could
not but happen, that for an equivalent of zinc oxidized an equivalent of
water must be decomposed[A].

  [A] The experiment was repeated several times with the same results.

867. But let us observe _how_ the water is decomposed. It is electrolyzed,
i.e. is decomposed voltaically, and not in the ordinary manner (as to
appearance) of chemical decompositions; for the oxygen appears at the
_anode_ and the hydrogen at the _cathode_ of the body under decomposition,
and these were in many parts of the experiment above an inch asunder.
Again, the ordinary chemical affinity was not enough under the
circumstances to effect the decomposition of the water, as was abundantly
proved by the inaction on plate B; the voltaic current was essential. And
to prevent any idea that the chemical affinity was almost sufficient to
decompose the water, and that a smaller current of electricity might, under
the circumstances, cause the hydrogen to pass to the _cathode_, I need only
refer to the results which I have given (807. 813.) to shew that the
chemical action at the electrodes has not the slightest influence over the
_quantities_ of water or other substances decomposed between them, but that
they are entirely dependent upon the quantity of electricity which passes.

868. What, then, follows as a necessary consequence of the whole
experiment? Why, this: that the chemical action upon 32.31 parts, or one
equivalent of zinc, in this simple voltaic circle, was able to evolve such
quantity of electricity in the form of a current, as, passing through
water, should decompose 9 parts, or one equivalent of that substance: and
considering the definite relations of electricity as developed in the
preceding parts of the present paper, the results prove that the quantity
of electricity which, being naturally associated with the particles of
matter, gives them their combining power, is able, when thrown into a
current, to separate those particles from their state of combination; or,
in other words, that _the electricity which decomposes, and that which is
evolved by the decomposition of a certain quantity of matter, are alike._

869. The harmony which this theory of the definite evolution and the
equivalent definite action of electricity introduces into the associated
theories of definite proportions and electrochemical affinity, is very
great. According to it, the equivalent weights of bodies are simply those
quantities of them which contain equal quantities of electricity, or have
naturally equal electric powers; it being the ELECTRICITY which
_determines_ the equivalent number, _because_ it determines the combining
force. Or, if we adopt the atomic theory or phraseology, then the atoms of
bodies which are equivalents to each other in their ordinary chemical
action, have equal quantities of electricity naturally associated with
them. But I must confess I am jealous of the term _atom_; for though it is
very easy to talk of atoms, it is very difficult to form a clear idea of
their nature, especially when compound bodies are under consideration.

870. I cannot refrain from recalling here the beautiful idea put forth, I
believe, by Berzelius (703.) in his development of his views of the
electro-chemical theory of affinity, that the heat and light evolved during
cases of powerful combination are the consequence of the electric discharge
which is at the moment taking place. The idea is in perfect accordance with
the view I have taken of the _quantity_ of electricity associated with the
particles of matter.

871. In this exposition of the law of the definite action of electricity,
and its corresponding definite proportion in the particles of bodies, I do
not pretend to have brought, as yet, every case of chemical or
electro-chemical action under its dominion. There are numerous
considerations of a theoretical nature, especially respecting the compound
particles of matter and the resulting electrical forces which they ought to
possess, which I hope will gradually receive their development; and there
are numerous experimental cases, as, for instance, those of compounds
formed by weak affinities, the simultaneous decomposition of water and
salts, &c., which still require investigation. But whatever the results on
these and numerous other points may be, I do not believe that the facts
which I have advanced, or even the general laws deduced from them, will
suffer any serious change; and they are of sufficient importance to justify
their publication, though much may yet remain imperfect or undone. Indeed,
it is the great beauty of our science, CHEMISTRY, that advancement in it,
whether in a degree great or small, instead of exhausting the subjects of
research, opens the doors to further and more abundant knowledge,
overflowing with beauty and utility, to those who will be at the easy
personal pains of undertaking its experimental investigation.

872. The definite production of electricity (868.) in association with its
definite action proves, I think, that the current of electricity in the
voltaic pile: is sustained by chemical decomposition, or rather by chemical
action, and not by contact only. But here, as elsewhere (857.), I beg to
reserve my opinion as to the real action of contact, not having yet been
able to make up my mind as to whether it is an exciting cause of the
current, or merely necessary to allow of the conduction of electricity,
otherwise generated, from one metal to the other.

873. But admitting that chemical action is the source of electricity, what
an infinitely small fraction of that which is active do we obtain and
employ in our voltaic batteries! Zinc and platina wires, one-eighteenth of
an inch in diameter and about half an inch long, dipped into dilute
sulphuric acid, so weak that it is not sensibly sour to the tongue, or
scarcely to our most delicate test-papers, will evolve more electricity in
one-twentieth of a minute (860.) than any man would willingly allow to pass
through his body at once. The chemical action of a grain of water upon four
grains of zinc can evolve electricity equal in quantity to that of a
powerful thunder-storm (868. 861.). Nor is it merely true that the quantity
is active; it can be directed and made to perform its full equivalent duty
(867. &c.). Is there not, then, great reason to hope and believe that, by a
closer _experimental_ investigation of the principles which govern the
development and action of this subtile agent, we shall be able to increase
the power of our batteries, or invent new instruments which shall a
thousandfold surpass in energy those which we at present possess?

874. Here for a while I must leave the consideration of the _definite
chemical action of electricity_. But before I dismiss this series of
experimental Researches, I would call to mind that, in a former series, I
showed the current of electricity was also _definite in its magnetic
action_ (216. 366. 367. 376. 377.); and, though this result was not pursued
to any extent, I have no doubt that the success which has attended the
development of the chemical effects is not more than would accompany an
investigation of the magnetic phenomena.


_Royal Institution,
December 31st, 1833._




EIGHTH SERIES.


§14. _On the Electricity of the Voltaic Pile; its source, quantity,
intensity, and general characters._ ¶ i. _On simple Voltaic Circles._ ¶ ii.
_On the intensity necessary for Electrolyzation._ ¶ iii. _On associated
Voltaic Circles, or the Voltaic Battery._ ¶ iv. _On the resistance of an
Electrolyte to Electrolytic action._ ¶ v. _General remarks on the active
Voltaic Battery._

Received April 7,--Read June 5, 1831.


¶ i. _On simple Voltaic Circles._


875. The great question of the source of electricity, in the voltaic pile
has engaged the attention of so many eminent philosophers, that a man of
liberal mind and able to appreciate their powers would probably conclude,
although he might not have studied the question, that the truth was
somewhere revealed. But if in pursuance of this impression he were induced
to enter upon the work of collating results and conclusions, he would find
such contradictory evidence, such equilibrium of opinion, such variation
and combination of theory, as would leave him in complete doubt respecting
what he should accept as the true interpretation of nature: he would be
forced to take upon himself the labour of repeating and examining the
facts, and then use his own judgement on them in preference to that of
others.

876. This state of the subject must, to those who have made up their minds
on the matter, be my apology for entering upon its investigation. The views
I have taken of the definite action of electricity in decomposing bodies
(783.), and the identity of the power so used with the power to be overcome
(855.), founded not on a mere opinion or general notion, but on facts
which, being altogether new, were to my mind precise and conclusive, gave
me, as I conceived, the power of examining the question with advantages not
before possessed by any, and which might compensate, on my part, for the
superior clearness and extent of intellect on theirs. Such are the
considerations which have induced me to suppose I might help in deciding
the question, and be able to render assistance in that great service of
removing _doubtful knowledge_. Such knowledge is the early morning light of
every advancing science, and is essential to its development; but the man
who is engaged in dispelling that which is deceptive in it, and revealing
more clearly that which is true, is as useful in his place, and as
necessary to the general progress of the science, as he who first broke
through the intellectual darkness, and opened a path into knowledge before
unknown to man.

877. The identity of the force constituting the voltaic current or
electrolytic agent, with that which holds the elements of electrolytes
together (855.), or in other words with chemical affinity, seemed to
indicate that the electricity of the pile itself was merely a mode of
exertion, or exhibition, or existence of _true chemical action_, or rather
of its cause; and I have consequently already said that I agree with those
who believe that the _supply_ of electricity is due to chemical powers
(857.).

878. But the great question of whether it is originally due to metallic
contact or to chemical action, i.e. whether it is the first or the second
which _originates_ and determines the current, was to me still doubtful;
and the beautiful and simple experiment with amalgamated zinc and platina,
which I have described minutely as to its results (863, &c.), did not
decide the point; for in that experiment the chemical action does not take
place without the contact of the metals, and the metallic contact is
inefficient without the chemical action. Hence either might be looked upon
as the _determining_ cause of the current.

879. I thought it essential to decide this question by the simplest
possible forms of apparatus and experiment, that no fallacy might be
inadvertently admitted. The well-known difficulty of effecting
decomposition by a single pair of plates, except in the fluid exciting them
into action (863.), seemed to throw insurmountable obstruction in the way
of such experiments; but I remembered the easy decomposability of the
solution of iodide of potassium (316.), and seeing no theoretical reason,
if metallic contact was not _essential_, why true electro-decomposition
should not be obtained without it, even in a single circuit, I persevered
and succeeded.

880. A plate of zinc, about eight inches long and half an inch wide, was
cleaned and bent in the middle to a right angle, fig. 73 _a_, Plate VI. A
plate of platina, about three inches long and half an inch wide, was
fastened to a platina wire, and the latter bent as in the figure, _b_.
These two pieces of metal were arranged together as delineated, but as yet
without the vessel _c_, and its contents, which consisted of dilute
sulphuric acid mingled with a little nitric acid. At _x_ a piece of folded
bibulous paper, moistened in a solution of iodide of potassium, was placed
on the zinc, and was pressed upon by the end of the platina wire. When
under these circumstances the plates were dipped into the acid of the
vessel _c_, there was an immediate effect at _x_, the iodide being
decomposed, and iodine appearing at the _anode_ (663.), i.e. against the
end of the platina wire.

881. As long as the lower ends of the plates remained in the acid the
electric current continued, and the decomposition proceeded at _x_. On
removing the end of the wire from place to place on the paper, the effect
was evidently very powerful; and on placing a piece of turmeric paper
between the white paper and zinc, both papers being moistened with the
solution of iodide of potassium, alkali was evolved at the _cathode_ (663.)
against the zinc, in proportion to the evolution of iodine at the _anode_.
Hence the decomposition was perfectly polar, and decidedly dependent upon a
current of electricity passing from the zinc through the acid to the
platina in the vessel _c_, and back from the platina through the solution
to the zinc at the paper _x_.

882. That the decomposition at _x_ was a true electrolytic action, due to a
current determined by the state of things in the vessel _c_, and not
dependent upon any mere direct chemical action of the zinc and platina on
the iodide, or even upon any _current_ which the solution of iodide might
by its action on those metals tend to form at _x_, was shown, in the first
place, by removing the vessel _c_ and its acid from the plates, when all
decomposition at _x_ ceased, and in the next by connecting the metals,
either in or out of the acid, together, when decomposition of the iodide at
_x_ occurred, but in a _reverse order_; for now alkali appeared against the
end of the platina wire, and the iodine passed to the zinc, the current
being the contrary of what it was in the former instance, and produced
directly by the difference of action of the solution in the paper on the
two metals. The iodine of course _combined_ with the zinc.

883. When this experiment was made with pieces of zinc amalgamated over the
whole surface (863.), the results were obtained with equal facility and in
the same direction, even when only dilute sulphuric acid was contained in
the vessel _c_ (fig. 73.). Whichsoever end of the zinc was immersed in the
acid, still the effects were the same: so that if, for a moment, the
mercury might be supposed to supply the metallic contact, the inversion of
the amalgamated piece destroys that objection. The use of _unamalgamated
zinc_ (880.) removes all possibility of doubt[A].

  [A] The following is a more striking mode of making the above
  elementary experiment. Prepare a plate of zinc, ten or twelve inches
  long and two inches wide, and clean it thoroughly: provide also two
  discs of clean platina, about one inch and a half in diameter:--dip
  three or four folds of bibulous paper into a strong solution of iodide
  of potassium, place them on the clean zinc at one end of the plate,
  and put on them one of the platina discs: finally dip similar folds of
  paper or a piece of linen cloth into a mixture of equal parts nitric
  acid and water, and place it at the other end of the zinc plate with
  the second platina disc upon it. In this state of things no change at
  the solution of the iodide will be perceptible; but if the two discs
  be connected by a platina (or any other) wire for a second or two, and
  then that over the iodide be raised, it will be found that the _whole_
  of the surface beneath is deeply stained with _evolved iodine_.--_Dec.
  1838._

884 When, in pursuance of other views (930.), the vessel _c_ was made to
contain a solution of caustic potash in place of acid, still the same
results occurred. Decomposition of the iodide was effected freely, though
there was no metallic contact of dissimilar metals, and the current of
electricity was in the _same direction_ as when acid was used at the place
of excitement.

885. Even a solution of common salt in the glass _c_ could produce all
these effects.

886. Having made a galvanometer with platina wires, and introduced it into
the course of the current between the platina plate and the place of
decomposition _x_, it was affected, giving indications of currents in the
same direction as those shown to exist by the chemical action.

887. If we consider these results generally, they lead to very important
conclusions. In the first place, they prove, in the most decisive manner,
that _metallic contact is not necessary for the production of the voltaic
current._ In the next place, they show a most extraordinary mutual relation
of the chemical affinities of the fluid which _excites_ the current, and
the fluid which is _decomposed_ by it.

888. For the purpose of simplifying the consideration, let us take the
experiment with amalgamated zinc. The metal so prepared exhibits no effect
until the current can pass: it at the same time introduces no new action,
but merely removes an influence which is extraneous to those belonging
either to the production or the effect of the electric current under
investigation (1000.); an influence also which, when present, tends only to
confuse the results.

889. Let two plates, one of amalgamated zinc and the other of platina, be
placed parallel to each other (fig. 74.), and introduce a drop of dilute
sulphuric acid, _y_, between them at one end: there will be no sensible
chemical action at that spot unless the two plates are connected somewhere
else, as at PZ, by a body capable of conducting electricity. If that body
be a metal or certain forms of carbon, then the current passes, and, as it
circulates through the fluid at _y_, decomposition ensues.

890. Then remove the acid from _y_, and introduce a drop of the solution of
iodide of potassium at _x_ (fig. 75.). Exactly the same set of effects
occur, except that when the metallic communication is made at PZ, the
electric current is in the opposite direction to what it was before, as is
indicated by the arrows, which show the courses of the currents (667.).

891. Now _both_ the solutions used are conductors, but the conduction in
them is essentially connected with decomposition (858.) in a certain
constant order, and therefore the appearance of the elements in certain
places _shows_ in what direction a current has passed when the solutions
are thus employed. Moreover, we find that when they are used at opposite
ends of the plates, as in the last two experiments (889. 890.), metallic
contact being allowed at the other extremities, the currents are in
opposite directions. We have evidently, therefore, the power of opposing
the actions of the two fluids simultaneously to each other at the opposite
ends of the plates, using each one as a conductor for the discharge of the
current of electricity, which the other tends to generate; in fact,
substituting them for metallic contact, and combining both experiments into
one (fig. 76.). Under these circumstances, there is an opposition of
forces: the fluid, which brings into play the stronger set of chemical
affinities for the zinc, (being the dilute acid,) overcomes the force of
the other, and determines the formation and direction of the electric
current; not merely making that current pass through the weaker liquid, but
actually reversing the tendency which the elements of the latter have in
relation to the zinc and platina if not thus counteracted, and forcing them
in the contrary direction to that they are inclined to follow, that its own
current may have free course. If the dominant action at _y_ be removed by
making metallic contact there, then the liquid at _x_ resumes its power; or
if the metals be not brought into contact at _y_ but the affinities of the
solution there weakened, whilst those active _x_ are strengthened, then the
latter gains the ascendency, and the decompositions are produced in a
contrary order.

892. Before drawing a _final_ conclusion from this mutual dependence and
state of the chemical affinities of two distant portions of acting fluids
(916.), I will proceed to examine more minutely the various circumstances
under which the re-action of the body suffering decomposition is rendered
evident upon the action of the body, also undergoing decomposition, which
produces the voltaic current.

893. The use of _metallic contact_ in a single pair of plates, and the
cause of its great superiority above contact made by other kinds of matter,
become now very evident. When an amalgamated zinc plate is dipped into
dilute sulphuric acid, the force of chemical affinity exerted between the
metal and the fluid is not sufficiently powerful to cause sensible action
at the surfaces of contact, and occasion the decomposition of water by the
oxidation of the metal, although it _is_ sufficient to produce such a
condition of the electricity (or the power upon which chemical affinity
depends) as would produce a current if there were a path open for it (916.
956.); and that current would complete the conditions necessary, under the
circumstances, for the decomposition of the water.

894. Now the presence of a piece of platina touching both the zinc and the
fluid to be decomposed, opens the path required for the electricity. Its
_direct communication_ with the zinc is effectual, far beyond any
communication made between it and that metal, (i.e. between the platina
and zinc,) by means of decomposable conducting bodies, or, in other words,
_electrolytes_, as in the experiment already described (891.); because,
when _they_ are used, the chemical affinities between them and the zinc
produce a contrary and opposing action to that which is influential in the
dilute sulphuric acid; or if that action be but small, still the affinity
of their component parts for each other has to be overcome, for they cannot
conduct without suffering decomposition; and this decomposition is found
_experimentally_ to re-act back upon the forces which in the acid tend to
produce the current (904. 910. &c.), and in numerous cases entirely to
neutralize them. Where direct contact of the zinc and platina occurs, these
obstructing forces are not brought into action, and therefore the
production and the circulation of the electric current and the concomitant
action of decomposition are then highly favoured.

895. It is evident, however, that one of these opposing actions may be
dismissed, and yet an electrolyte be used for the purpose of completing the
circuit between the zinc and platina immersed separately into the dilute
acid; for if, in fig. 73, the platina wire be retained in metallic contact
with the zinc plate _a_, at _x_, and a division of the platina be made
elsewhere, as at _s_, then the solution of iodide placed there, being in
contact with platina at both surfaces, exerts no chemical affinities for
that metal; or if it does, they are equal on both sides. Its power,
therefore, of forming a current in opposition to that dependent upon the
action of the acid in the vessel _c_, is removed, and only its resistance
to decomposition remains as the obstacle to be overcome by the affinities
exerted in the dilute sulphuric acid.

896. This becomes the condition of a single pair of active plates where
_metallic contact_ is allowed. In such cases, only one set of opposing
affinities are to be overcome by those which are dominant in the vessel
_c_; whereas, when metallic contact is not allowed, two sets of opposing
affinities must be conquered (894.).

897. It has been considered a difficult, and by some an impossible thing,
to decompose bodies by the current from a single pair of plates, even when
it was so powerful as to heat bars of metal red-hot, as in the case of
Hare's calorimeter, arranged as a single voltaic circuit, or of Wollaston's
powerful single pair of metals. This difficulty has arisen altogether from
the antagonism of the chemical affinity engaged in producing the current
with the chemical affinity to be overcome, and depends entirely upon their
relative intensity; for when the sum of forces in one has a certain degree
of superiority over the sum of forces in the other, the former gain the
ascendency, determine the current, and overcome the latter so as to make
the substance exerting them yield up its elements in perfect accordance,
both as to direction and quantity, with the course of those which are
exerting the most intense and dominant action.

898. Water has generally been the substance, the decomposition of which has
been sought for as a chemical test of the passage of an electric current.
But I now began to perceive a reason for its failure, and for a fact which
I had observed long before (315. 316.) with regard to the iodide of
potassium, namely, that bodies would differ in facility of decomposition by
a given electric current, according to the condition and intensity of their
ordinary chemical affinities. This reason appeared in their _re-action upon
the affinities_ tending to cause the current; and it appeared probable,
that many substances might be found which could be decomposed by the
current of a single pair of zinc and platina plates immersed in dilute
sulphuric acid, although water resisted its action. I soon found this to be
the case, and as the experiments offer new and beautiful proofs of the
direct relation and opposition of the chemical affinities concerned in
producing and in resisting the stream of electricity, I shall briefly
describe them.

899. The arrangement of the apparatus was as in fig. 77. The vessel _v_
contained dilute sulphuric acid; Z and P are the zinc and platina plates;
_a_, _b_, and _c_ are platina wires; the decompositions were effected at
_x_, and occasionally, indeed generally, a galvanometer was introduced into
the circuit at _g_: its place only is here given, the circle at _g_ having
no reference to the size of the instrument. Various arrangements were made
at _x_, according to the kind of decomposition to be effected. If a drop of
liquid was to be acted upon, the two ends were merely dipped into it; if a
solution contained in the pores of paper was to be decomposed, one of the
extremities was connected with a platina plate supporting the paper, whilst
the other extremity rested on the paper, _e_, fig. 81: or sometimes, as
with sulphate of soda, a plate of platina sustained two portions of paper,
one of the ends of the wires resting upon each piece, _c_, fig. 86. The
darts represent the direction of the electric current (667.).

900. Solution of _iodide of potassium_, in moistened paper, being placed at
the interruption of the circuit at _x_, was readily decomposed. Iodine was
evolved at the _anode_, and alkali at the _cathode_, of the decomposing
body.

901. _Protochloride of tin_, when fused and placed at _x_, was also readily
decomposed, yielding perchloride of tin at the _anode_ (779.), and tin at
the _cathode_.

902. Fused chloride of silver, placed at _x_, was also easily decomposed;
chlorine was evolved at the _anode_, and brilliant metallic silver, either
in films upon the surface of the liquid, or in crystals beneath, evolved at
the _cathode_.

903. Water acidulated with sulphuric acid, solution of muriatic acid,
solution of sulphate of soda, fused nitre, and the fused chloride and
iodide of lead were not decomposed by this single pair of plates, excited
only by dilute sulphuric acid.

904. These experiments give abundant proofs that a single pair of plates
can electrolyze bodies and separate their elements. They also show in a
beautiful manner the direct relation and opposition of the chemical
affinities concerned at the two points of action. In those cases where the
sum of the opposing affinities at _x_ was sufficiently beneath the sum of
the acting affinities in _v_, decomposition took place; but in those cases
where they rose higher, decomposition was effectually resisted and the
current ceased to pass (891.).

905. It is however, evident, that the sum of acting affinities in _v_ may
be increased by using other fluids than dilute sulphuric acid, in which
latter case, as I believe, it is merely the affinity of the zinc for the
oxygen already combined with hydrogen in the water that is exerted in
producing the electric current (919.): and when the affinities are so
increased, the view I am supporting leads to the conclusion, that bodies
which resisted in the preceding experiments would then be decomposed,
because of the increased difference between their affinities and the acting
affinities thus exalted. This expectation was fully confirmed in the
following manner.

906. A little nitric acid was added to the liquid in the vessel _r_, so as
to make a mixture which I shall call diluted nitro-sulphuric acid. On
repeating the experiments with this mixture, all the substances before
decomposed again gave way, and much more readily. But, besides that, many
which before resisted electrolyzation, now yielded up their elements. Thus,
solution of sulphate of soda, acted upon in the interstices of litmus and
turmeric paper, yielded acid at the _anode_ and alkali at the _cathode_;
solution of muriatic acid tinged by indigo yielded chlorine at the _anode_
and hydrogen at the _cathode_; solution of nitrate of silver yielded silver
at the _cathode_. Again, fused nitre and the fused iodide and chloride of
lead were decomposable by the current of this single pair of plates, though
they were not by the former (903.).

907. A solution of acetate of lead was apparently not decomposed by this
pair, nor did water acidulated by sulphuric acid seem at first to give way
(973.).

908. The increase of intensity or power of the current produced by a simple
voltaic circle, with the increase of the force of the chemical action at
the exciting place, is here sufficiently evident. But in order to place it
in a clearer point of view, and to show that the decomposing effect was not
at all dependent, in the latter cases, upon the mere capability of evolving
_more_ electricity, experiments were made in which the quantity evolved
could be increased without variation in the intensity of the exciting
cause. Thus the experiments in which dilute sulphuric acid was used (899.),
were repeated, using large plates of zinc and platina in the acid; but
still those bodies which resisted decomposition before, resisted it also
under these new circumstances. Then again, where nitro-sulphuric acid was
used (906.), mere wires of platina and zinc were immersed in the exciting
acid; yet, notwithstanding this change, those bodies were now decomposed
which resisted any current tending to be formed by the dilute sulphuric
acid. For instance, muriatic acid could not be decomposed by a single pair
of plates when immersed in dilute sulphuric acid; nor did making the
solution of sulphuric acid strong, nor enlarging the size of the zinc and
platina plates immersed in it, increase the power; but if to a weak
sulphuric acid a very little nitric acid was added, then the electricity
evolved had power to decompose the muriatic acid, evolving chlorine at the
_anode_ and hydrogen at the _cathode_, even when mere wires of metals were
used. This mode of increasing the intensity of the electric current, as it
excludes the effect dependent upon many pairs of plates, or even the effect
of making any one acid stronger or weaker, is at once referable to the
condition and force of the chemical affinities which are brought into
action, and may, both in principle and practice, be considered as perfectly
distinct from any other mode.

909. The direct reference which is thus experimentally made in the simple
voltaic circle of the _intensity_ of the electric current to the
_intensity_ of the chemical action going on at the place where the
existence and direction of the current is determined, leads to the
conclusion that by using selected bodies, as fused chlorides, salts,
solutions of acids, &c., which may act upon the metals employed with
different degrees of chemical force; and using also metals in association
with platina, or with each other, which shall differ in the degree of
chemical action exerted between them and the exciting fluid or electrolyte,
we shall be able to obtain a series of comparatively constant effects due
to electric currents of different intensities, which will serve to assist
in the construction of a scale competent to supply the means of determining
relative degrees of intensity with accuracy in future researches[A].

  [A] In relation to this difference and its probable cause, see
  considerations on inductive polarization, 1354, &c.--_Dec. 1838._

910. I have already expressed the view which I take of the decomposition in
the experimental place, as being the direct consequence of the superior
exertion at some other spot of the same kind of power as that to be
overcome, and therefore as the result of an antagonism of forces of the
_same_ nature (891. 904.). Those at the place of decomposition have a
re-action upon, and a power over, the exerting or determining set
proportionate to what is needful to overcome their own power; and hence a
curious result of _resistance_ offered by decompositions to the original
determining force, and consequently to the current. This is well shown in
the cases where such bodies as chloride of lead, iodide of lead, and water
would not decompose with the current produced by a single pair of zinc and
platina plates in sulphuric acid (903.), although they would with a current
of higher intensity produced by stronger chemical powers. In such cases no
sensible portion of the current passes (967.); the action is stopped; and I
am now of opinion that in the case of the law of conduction which I
described in the Fourth Series of these Researches (413.), the bodies which
are electrolytes in the fluid state cease to be such in the solid form,
because the attractions of the particles by which they are retained in
combination and in their relative position, are then too powerful for the
electric current[A]. The particles retain their places; and as
decomposition is prevented, the transmission of the electricity is
prevented also; and although a battery of many plates may be used, yet if
it be of that perfect kind which allows of no extraneous or indirect action
(1000.), the whole of the affinities concerned in the activity of that
battery are at the same time also suspended and counteracted.

  [A] Refer onwards to 1705.--_Dec. 1838._

911. But referring to the _resistance_ of each single case of
decomposition, it would appear that as these differ in force according to
the affinities by which the elements in the substance tend to retain their
places, they also would supply cases constituting a series of degrees by
which to measure the initial intensities of simple voltaic or other
currents of electricity, and which, combined with the scale of intensities
determined by different degrees of _acting force_ (909.), would probably
include a sufficient set of differences to meet almost every important case
where a reference to intensity would be required.

912. According to the experiments I have already had occasion to make, I
find that the following bodies are electrolytic in the order in which I
have placed them, those which are first being decomposed by the current of
lowest intensity. These currents were always from a single pair of plates,
and may be considered as elementary _voltaic forces_.

Iodide of potassium (solution).
Chloride of silver (fused).
Protochloride of tin (fused).
Chloride of lead (fused).
Iodide of lead (fused).
Muriatic acid (solution).
Water, acidulated with sulphuric acid.

913. It is essential that, in all endeavours to obtain the relative
electrolytic intensity necessary for the decomposition of different bodies,
attention should be paid to the nature of the electrodes and the other
bodies present which may favour secondary actions (986.). If in
electro-decomposition one of the elements separated has an affinity for the
electrode, or for bodies present in the surrounding fluid, then the
affinity resisting decomposition is in part balanced by such power, and the
true place of the electrolyte in a table of the above kind is not obtained:
thus, chlorine combines with a positive platina electrode freely, but
iodine scarcely at all, and therefore I believe it is that the fused
chlorides stand first in the preceding Table. Again, if in the
decomposition of water not merely sulphuric but also a little nitric acid
be present, then the water is more freely decomposed, for the hydrogen at
the _cathode_ is not ultimately expelled, but finds oxygen in the nitric
acid, with which it can combine to produce a secondary result; the
affinities opposing decomposition are in this way diminished, and the
elements of the water can then be separated by a current of lower
intensity.

914. Advantage may be taken of this principle to interpolate more minute
degrees into the scale of initial intensities already referred to (909.
911.) than is there spoken of; for by combining the force of a current
_constant_ in its intensity, with the use of electrodes consisting of
matter, having more or less affinity for the elements evolved from the
decomposing electrolyte, various intermediate degrees may be obtained.

        *        *        *        *        *

915. Returning to the consideration of the source of electricity (878.
&c.), there is another proof of the most perfect kind that metallic contact
has nothing to do with the _production_ of electricity in the voltaic
circuit, and further, that electricity is only another mode of the exertion
of chemical forces. It is, the production of the _electric spark_ before
any contact of metals is made, and by the exertion of _pure and unmixed
chemical forces_. The experiment, which will be described further on
(956.), consists in obtaining the spark upon making contact between a plate
of zinc and a plate of copper plunged into dilute sulphuric acid. In order
to make the arrangement as elementary as possible, mercurial surfaces were
dismissed, and the contact made by a copper wire connected with the copper
plate, and then brought to touch a clean part of the zinc plate. The
electric spark appeared, and it must of necessity have existed and passed
_before the zinc and the copper were in contact_.

916. In order to render more distinct the principles which I have been
endeavouring to establish, I will restate them in their simplest form,
according to my present belief. The electricity of the voltaic pile (856.
note) is not dependent either in its origin or its continuance upon the
contact of the metals with each other (880. 915.). It is entirely due to
chemical action (882.), and is proportionate in its intensity to the
intensity of the affinities concerned in its production (908.); and in its
quantity to the quantity of matter which has been chemically active during
its evolution (869.). This definite production is again one of the
strongest proofs that the electricity is of chemical origin.

917. As _volta-electro-generation_ is a case of mere chemical action, so
_volta-electro-decomposition_ is simply a case of the preponderance of one
set of chemical affinities more powerful in their nature, over another set
which are less powerful: and if the instance of two opposing sets of such
forces (891.) be considered, and their mutual relation and dependence borne
in mind, there appears no necessity for using, in respect to such cases,
any other term than chemical affinity, (though that of electricity may be
very convenient,) or supposing any new agent to be concerned in producing
the results; for we may consider that the powers at the two places of
action are in direct communion and balanced against each other through the
medium of the metals (891.), fig. 76, in a manner analogous to that in
which mechanical forces are balanced against each other by the intervention
of the lever (1031.).

918. All the facts show us that that power commonly called chemical
affinity, can be communicated to a distance through the metals and certain
forms of carbon; that the electric current is only another form of the
forces of chemical affinity; that its power is in proportion to the
chemical affinities producing it; that when it is deficient in force it may
be helped by calling in chemical aid, the want in the former being made up
by an equivalent of the latter; that, in other words, _the forces termed
chemical affinity and electricity are one and the same._

919. When the circumstances connected with the production of electricity in
the ordinary voltaic circuit are examined and compared, it appears that the
source of that agent, always meaning the electricity which circulates and
completes the current in the voltaic apparatus, and gives that apparatus
power and character (947. 996.), exists in the chemical action which takes
place directly between the metal and the body with which it combines, and
not at all in the subsequent action of the substance so produced with the
acid present[A]. Thus, when zinc, platina, and dilute sulphuric acid are
used, it is the union of the zinc with the oxygen of the water which
determines the current; and though the acid is essential to the removal of
the oxide so formed, in order that another portion of zinc may act on
another portion of water, it does not, by combination with that oxide,
produce any sensible portion of the current of electricity which
circulates; for the quantity of electricity is dependent upon the quantity
of zinc oxidized, and in definite proportion to it: its intensity is in
proportion to the intensity of the chemical affinity of the zinc for the
oxygen under the circumstances, and is scarcely, if at all, affected by the
use of either strong or weak acid (908.).

  [A] Wollaston, Philosophical Transactions, 1801, p. 427.

920. Again, if zinc, platina, and muriatic acid are used, the electricity
appears to be dependent upon the affinity of the zinc for the chlorine, and
to be circulated in exact proportion to the number of particles of zinc and
chlorine which unite, being in fact an equivalent to them.

921. But in considering this oxidation, or other direct action upon the
METAL itself, as the cause and source of the electric current, it is of the
utmost importance to observe that the oxygen or other body must be in a
peculiar condition, namely, in the state of _combination_; and not only so,
but limited still further to such a state of combination and in such
proportions as will constitute an _electrolyte_ (823.). A pair of zinc and
platina plates cannot be so arranged in oxygen gas as to produce a current
of electricity, or act as a voltaic circle, even though the temperature may
be raised so high as to cause oxidation of the zinc far more rapidly than
if the pair of plates were plunged into dilute sulphuric acid; for the
oxygen is not part of an electrolyte, and cannot therefore conduct the
forces onwards by decomposition, or even as metals do by itself. Or if its
gaseous state embarrass the minds of some, then liquid chlorine may be
taken. It does not excite a current of electricity through the two plates
by combining with the zinc, for its particles cannot transfer the
electricity active at the point of combination across to the platina. It is
not a conductor of itself, like the metals; nor is it an electrolyte, so as
to be capable of conduction during decomposition, and hence there is simple
chemical action at the spot, and no electric current[A].

  [A] I do not mean to affirm that no traces of electricity ever appear
  in such cases. What I mean is, that no electricity is evolved in any
  way, due or related to the causes which excite voltaic electricity, or
  proportionate to them. That which does appear occasionally is the
  smallest possible fraction of that which the acting matter could
  produce if arranged so as to act voltaically, probably not the one
  hundred thousandth, or even the millionth part, and is very probably
  altogether different in its source.

922. It might at first be supposed that a conducting body not electrolytic,
might answer as the third substance between the zinc and the platina; and
it is true that we have some such capable of exerting chemical action upon
the metals. They must, however, be chosen from the metals themselves, for
there are no bodies of this kind except those substances and charcoal. To
decide the matter by experiment, I made the following arrangement. Melted
tin was put into a glass tube bent into the form of the letter V, fig. 78,
so as to fill the half of each limb, and two pieces of thick platina wire,
_p_, _w_, inserted, so as to have their ends immersed some depth in the
tin: the whole was then allowed to cool, and the ends _p_ and _w_ connected
with a delicate galvanometer. The part of the tube at _x_ was now reheated,
whilst the portion _y_ was retained cool. The galvanometer was immediately
influenced by the thermo-electric current produced. The heat was steadily
increased at _x_, until at last the tin and platina combined there; an
effect which is known to take place with strong chemical action and high
ignition; but not the slightest additional effect occurred at the
galvanometer. No other deflection than that due to the thermo-electric
current was observable the whole time. Hence, though a conductor, and one
capable of exerting chemical action on the tin, was used, yet, not being an
_electrolyte_, not the slightest effect of an electrical current could be
observed (947.).

923. From this it seems apparent that the peculiar character and condition
of an electrolyte is _essential_ in one part of the voltaic circuit; and
its nature being considered, good reasons appear why it and it alone should
be effectual. An electrolyte is always a compound body: it can conduct, but
only whilst decomposing. Its conduction depends upon its decomposition and
the _transmission of its particles_ in directions parallel to the current;
and so intimate is this connexion, that if their transition be stopped, the
current is stopped also; if their course be changed, its course and
direction change with them; if they proceed in one direction, it has no
power to proceed in any other than a direction invariably dependent on
them. The particles of an electrolytic body are all so mutually connected,
are in such relation with each other through their whole extent in the
direction of the current, that if the last is not disposed of, the first is
not at liberty to take up its place in the new combination which the
powerful affinity of the most active metal tends to produce; and then the
current itself is stopped; for the dependencies of the current and the
decomposition are so mutual, that whichsoever be originally determined,
i.e. the motion of the particles or the motion of the current, the other is
invariable in its concomitant production and its relation to it.

924. Consider, then, water as an electrolyte and also as an oxidizing body.
The attraction of the zinc for the oxygen is greater, under the
circumstances, than that of the oxygen for the hydrogen; but in combining
with it, it tends to throw into circulation a current of electricity in a
certain direction. This direction is consistent (as is found by innumerable
experiments) with the transfer of the hydrogen from the zinc towards the
platina, and the transfer in the opposite direction of fresh oxygen from
the platina towards the zinc; so that the current _can pass_ in that one
line, and, whilst it passes, can consist with and favour the renewal of the
conditions upon the surface of the zinc, which at first determined both the
combination and circulation. Hence the continuance of the action there, and
the continuation of the current. It therefore appears quite as essential
that there should be an electrolyte in the circuit, in order that the
action may be transferred forward, in a _certain constant direction,_ as
that there should be an oxidizing or other body capable of acting directly
on the metal; and it also appears to be essential that these two should
merge into one, or that the principle directly active on the metal by
chemical action should be one of the _ions_ of the electrolyte used.
Whether the voltaic arrangement be excited by solution of acids, or
alkalies, or sulphurets, or by fused substances (476.), this principle has
always hitherto, as far as I am aware, been an _anion_ (943.); and I
anticipate, from a consideration of the principles of electric action, that
it must of necessity be one of that class of bodies.

925. If the action of the sulphuric acid used in the voltaic circuit be
considered, it will be found incompetent to produce any sensible portion of
the electricity of the current by its combination with the oxide formed,
for this simple reason, it is deficient in a most essential condition: it
forms no part of an electrolyte, nor is it in relation with any other body
present in the solution which will permit of the mutual transfer of the
particles and the consequent transfer of the electricity. It is true, that
as the plane at which the acid is dissolving the oxide of zinc formed by
the action of the water, is in contact with the metal zinc, there seems no
difficulty in considering how the oxide there could communicate an
electrical state, proportionate to its own chemical action on the acid, to
the metal, which is a conductor without decomposition. But on the side of
the acid there is no substance to complete the circuit: the water, as
water, cannot conduct it, or at least only so small a proportion that it is
merely an incidental and almost inappreciable effect (970.); and it cannot
conduct it as an electrolyte, because an electrolyte conducts in
consequence of the _mutual_ relation and action of its particles; and
neither of the elements of the water, nor even the water itself, as far as
we can perceive, are _ions_ with respect to the sulphuric acid (848.)[A].

  [A] It will be seen that I here agree with Sir Humphry Davy, who has
  experimentally supported the opinion that acids and alkalies in
  combining do not produce any current of electricity. Philosophical
  Transactions, 1826, p. 398.

926. This view of the secondary character of the sulphuric acid as an agent
in the production of the voltaic current, is further confirmed by the fact,
that the current generated and transmitted is directly and exactly
proportional to the quantity of water decomposed and the quantity of zinc
oxidized (868. 991.), and is the same as that required to decompose the
same quantity of water. As, therefore, the decomposition of the water shows
that the electricity has passed by its means, there remains no other
electricity to be accounted for or to be referred to any action other than
that of the zinc and the water on each other.

927. The general case (for it includes the former one (924.),) of acids and
bases, may theoretically be stated in the following manner. Let _a_, fig.
79, be supposed to be a dry oxacid, and _b_ a dry base, in contact at _c_,
and in electric communication at their extremities by plates of platina
_pp_, and a platina wire _w_. If this acid and base were fluid, and
combination took place at _c_, with an affinity ever so vigorous, and
capable of originating an electric current, the current could not circulate
in any important degree; because, according to the experimental results,
neither _a_ nor _b_ could conduct without being decomposed, for they are
either electrolytes or else insulators, under all circumstances, except to
very feeble and unimportant currents (970. 986.). Now the affinities at _c_
are not such as tend to cause the _elements_ either of _a_ or _b_ to
separate, but only such as would make the two bodies combine together as a
whole; the point of action is, therefore, insulated, the action itself
local (921. 947.), and no current can be formed.

928. If the acid and base be dissolved in water, then it is possible that a
small portion of the electricity due to chemical action may be conducted by
the water without decomposition (966. 984.); but the quantity will be so
small as to be utterly disproportionate to that due to the equivalents of
chemical force; will be merely incidental; and, as it does not involve the
essential principles of the voltaic pile, it forms no part of the phenomena
at present under investigation[A].

  [A] It will I trust be fully understood, that in these investigations
  I am not professing to take an account of every small, incidental, or
  barely possible effect, dependent upon slight disturbances of the
  electric fluid during chemical action, but am seeking to distinguish
  and identify those actions on which the power of the voltaic battery
  essentially depends.

929. If for the oxacid a hydracid be substituted (927.),--as one analogous
to the muriatic, for instance,--then the state of things changes
altogether, and a current due to the chemical action of the acid on the
base is possible. But now both the bodies act as electrolytes, for it is
only one principle of each which combine mutually,--as, for instance, the
chlorine with the metal,--and the hydrogen of the acid and the oxygen of
the base are ready to traverse with the chlorine of the acid and the metal
of the base in conformity with the current and according to the general
principles already so fully laid down.

930. This view of the oxidation of the metal, or other _direct_ chemical
action upon it, being the sole cause of the production of the electric
current in the ordinary voltaic pile, is supported by the effects which
take place when alkaline or sulphuretted solutions (931. 943.) are used for
the electrolytic conductor instead of dilute sulphuric acid. It was in
elucidation of this point that the experiments without metallic contact,
and with solution of alkali as the exciting fluid, already referred to
(884.), were made.

931. Advantage was then taken of the more favourable condition offered,
when metallic contact is allowed (895.), and the experiments upon the
decomposition of bodies by a single pair of plates (899.) were repeated,
solution of caustic potassa being employed in the vessel _v_, fig. 77. in
place of dilute sulphuric acid. All the effects occurred as before: the
galvanometer was deflected; the decompositions of the solutions of iodide
of potassium, nitrate of silver, muriatic acid, and sulphate of soda ensued
at _x_; and the places where the evolved principles appeared, as well as
the deflection of the galvanometer, indicated a current in the _same
direction_ as when acid was in the vessel _v_; i.e. from the zinc through
the solution to the platina, and back by the galvanometer and substance
suffering decomposition to the zinc.

932. The similarity in the action of either dilute sulphuric acid or
potassa goes indeed far beyond this, even to the proof of identity in
_quantity_ as well as in _direction_ of the electricity produced. If a
plate of amalgamated zinc be put into a solution of potassa, it is not
sensibly acted upon; but if touched in the solution by a plate of platina,
hydrogen is evolved on the surface of the latter metal, and the zinc is
oxidized exactly as when immersed in dilute sulphuric acid (863.). I
accordingly repeated the experiment before described with weighed plates of
zinc (864. &c.), using however solution of potassa instead of dilute
sulphuric acid. Although the time required was much longer than when acid
was used, amounting to three hours for the oxidizement of 7.55 grains of
zinc, still I found that the hydrogen evolved at the platina plate was the
equivalent of the metal oxidized at the surface of the zinc. Hence the
whole of the reasoning which was applicable in the former instance applies
also here, the current being in the same direction, and its decomposing
effect in the same degree, as if acid instead of alkali had been used
(868.).

933. The proof, therefore, appears to me complete, that the combination of
the acid with the oxide, in the former experiment, had nothing to do with
the production of the electric current; for the same current is here
produced when the action of the acid is absent, and the reverse action of
an alkali is present. I think it cannot be supposed for a moment, that the
alkali acted chemically as an acid to the oxide formed; on the contrary,
our general chemical knowledge leads to the conclusion, that the ordinary
metallic oxides act rather as acids to the alkalies; yet that kind of
action would tend to give a reverse current in the present case, if any
were due to the union of the oxide of the exciting metal with the body
which combines with it. But instead of any variation of this sort, the
direction of the electricity was constant, and its quantity also directly
proportional to the water decomposed, or the zinc oxidized. There are
reasons for believing that acids and alkalies, when in contact with metals
upon which they cannot act directly, still have a power of influencing
their attractions for oxygen (941.); but all the effects in these
experiments prove, I think, that it is the oxidation of the metal
necessarily dependent upon, and associated as it is with, the
electrolyzation of the water (921. 923.) that produces the current; and
that the acid or alkali merely acts as solvents, and by removing the
oxidized zinc, allows other portions to decompose fresh water, and so
continues the evolution or determination of the current.

934. The experiments were then varied by using solution of ammonia instead
of solution of potassa; and as it, when pure, is like water, a bad
conductor (554.), it was occasionally improved in that power by adding
sulphate of ammonia to it. But in all the cases the results were the same
as before; decompositions of the same kind were effected, and the electric
current producing these was in the same direction as in the experiments
just described.

935. In order to put the equal and similar action of acid and alkali to
stronger proof, arrangements were made as in fig. 80.; the glass vessel A
contained dilute sulphuric acid, the corresponding glass vessel B solution
of potassa, PP was a plate of platina dipping into both solutions, and ZZ
two plates of amalgamated zinc connected with a delicate galvanometer. When
these were plunged at the same time into the two vessels, there was
generally a first feeble effect, and that in favour of the alkali, i.e. the
electric current tended to pass through the vessels in the direction of the
arrow, being the reverse direction of that which the acid in A would have
produced alone: but the effect instantly ceased, and the action of the
plates in the vessels was so equal, that, being contrary because of the
contrary position of the plates, no permanent current resulted.

936. Occasionally a zinc plate was substituted for the plate PP, and
platina plates for the plates ZZ; but this caused no difference in the
results: nor did a further change of the middle plate to copper produce any
alteration.

937. As the opposition of electro-motive pairs of plates produces results
other than those due to the mere difference of their independent actions
(1011. 1045.), I devised another form of apparatus, in which the action of
acid and alkali might be more directly compared. A cylindrical glass cup,
about two inches deep within, an inch in internal diameter, and at least a
quarter of an inch in thickness, was cut down the middle into halves, fig.
81. A broad brass ring, larger in diameter than the cup, was supplied with
a screw at one side; so that when the two halves of the cup were within the
ring, and the screw was made to press tightly against the glass, the cup
held any fluid put into it. Bibulous paper of different degrees of
permeability was then cut into pieces of such a size as to be easily
introduced between the loosened halves of the cup, and served when the
latter were tightened again to form a porous division down the middle of
the cup, sufficient to keep any two fluids on opposite sides of the paper
from mingling, except very slowly, and yet allowing them to act freely as
one _electrolyte_. The two spaces thus produced I will call the cells A and
B, fig. 82. This instrument I have found of most general application in the
investigation of the relation of fluids and metals amongst themselves and
to each other. By combining its use with that of the galvanometer, it is
easy to ascertain the relation of one metal with two fluids, or of two
metals with one fluid, or of two metals and two fluids upon each other.

938. Dilute sulphuric acid, sp. gr. 1.25, was put into the cell A, and a
strong solution of caustic potassa into the cell B; they mingled slowly
through the paper, and at last a thick crust of sulphate of potassa formed
on the side of the paper next to the alkali. A plate of clean platina was
put into each cell and connected with a delicate galvanometer, but no
electric current could be observed. Hence the _contact_ of acid with one
platina plate, and alkali with the other, was unable to produce a current;
nor was the combination of the acid with the alkali more effectual (925.).

939. When one of the platina plates was removed and a zinc plate
substituted, either amalgamated or not, a strong electric current was
produced. But, whether the zinc were in the acid whilst the platina was in
the alkali, or whether the reverse order were chosen, the electric current
was always from the zinc through the electrolyte to the platina, and back
through the galvanometer to the zinc, the current seeming to be strongest
when the zinc was in the alkali and the platina in the acid.

940. In these experiments, therefore, the acid seems to have no power over
the alkali, but to be rather inferior to it in force. Hence there is no
reason to suppose that the combination of the oxide formed with the acid
around it has any direct influence in producing the electricity evolved,
the whole of which appears to be due to the oxidation of the metal (919.).

941. The alkali, in fact, is superior to the acid in bringing a metal into
what is called the positive state; for if plates of the same metal, as
zinc, tin, lead, or copper, be used both in the acid or alkali, the
electric current is from the alkali across the cell to the acid, and back
through the galvanometer to the alkali, as Sir Humphry Davy formerly stated
[A]. This current is so powerful, that if amalgamated zinc, or tin, or lead
be used, the metal in the acid evolves hydrogen the moment it is placed in
communication with that in the alkali, not from any direct action of the
acid upon it, for if the contact be broken the action ceases, but because
it is powerfully negative with regard to the metal in the alkali.

  [A] Elements of Chemical Philosophy, p. 149; or Philosophical
  Transactions, 1826, p. 403.

942. The superiority of alkali is further proved by this, that if zinc and
tin be used, or tin and lead, whichsoever metal is put into the alkali
becomes positive, that in the acid being negative. Whichsoever is in the
alkali is oxidized, whilst that in the acid remains in the metallic state,
as far as the electric current is concerned.

943. When sulphuretted solutions are used (930.) in illustration of the
assertion, that it is the chemical action of the metal and one of the
_ions_ of the associated electrolyte that produces all the electricity of
the voltaic circuit, the proofs are still the same. Thus, as Sir Humphry
Davy[A] has shown, if iron and copper be plunged into dilute acid, the
current is from the iron through the liquid to the copper; in solution of
potassa it is in the same direction, but in solution of sulphuret of
potassa it is reversed. In the two first cases it is oxygen which combines
with the iron, in the latter sulphur which combines with the copper, that
produces the electric current; but both of these are _ions_, existing as
such in the electrolyte, which is at the same moment suffering
decomposition; and, what is more, both of these are _anions_, for they
leave the electrolytes at their _anodes_, and act just as chlorine, iodine,
or any other _anion_ would act which might have been previously chosen as
that which should be used to throw the voltaic circle into activity.

  [A] Elements of Chemical Philosophy, p. 148.

944. The following experiments complete the series of proofs of the origin
of the electricity in the voltaic pile. A fluid amalgam of potassium,
containing not more than a hundredth of that metal, was put into pure
water, and connected, through the galvanometer with a plate of platina in
the same water. There was immediately an electric current from the amalgam
through the electrolyte to the platina. This must have been due to the
oxidation only of the metal, for there was neither acid nor alkali to
combine with, or in any way act on, the body produced.

945. Again, a plate of clean lead and a plate of platina were put into
_pure_ water. There was immediately a powerful current produced from the
lead through the fluid to the platina: it was even intense enough to
decompose solution of the iodide of potassium when introduced into the
circuit in the form of apparatus already described (880.), fig. 73. Here no
action of acid or alkali on the oxide formed from the lead could supply the
electricity: it was due solely to the oxidation of the metal.

       *       *       *       *       *

946. There is no point in electrical science which seems to me of more
importance than the state of the metals and the electrolytic conductor in a
simple voltaic circuit _before and at_ the moment when metallic contact is
first completed. If clearly understood, I feel no doubt it would supply us
with a direct key to the laws under which the great variety of voltaic
excitements, direct and incidental, occur, and open out new fields of
research for our investigation[A].

  [A] In connexion with this part of the subject refer now to Series XI.
  1164, Series XII. 1343-1358, and Series XIII. 1621. &c.--_Dec. 1838._

947. We seem to have the power of deciding to a certain extent in numerous
cases of chemical affinity, (as of zinc with the oxygen of water, &c. &c.)
which of _two modes of action of the attractive power_ shall be exerted
(996.). In the one mode we can transfer the power onwards, and make it
produce elsewhere its equivalent of action (867. 917.); in the other, it is
not transferred, but exerted wholly at the spot. The first is the case of
volta-electric excitation, the other ordinary chemical affinity: but both
are chemical actions and due to one force or principle.

948. The general circumstances of the former mode occur in all instances of
voltaic currents, but may be considered as in their perfect condition, and
then free from those of the second mode, in some only of the cases; as in
those of plates of zinc and platina in solution of potassa, or of
amalgamated zinc and platina in dilute sulphuric acid.

949. Assuming it sufficiently proved, by the preceding experiments and
considerations, that the electro-motive action depends, when zinc, platina,
and dilute sulphuric acid are used, upon the mutual affinity of the metal
zinc and the oxygen of the water (921. 924.), it would appear that the
metal, when alone, has not power enough, under the circumstances, to take
the oxygen and expel the hydrogen from the water; for, in fact, no such
action takes place. But it would also appear that it has power so far to
act, by its attraction for the oxygen of the particles in contact with it,
as to place the similar forces already active between these and the other
particles of oxygen and the particles of hydrogen in the water, in a
peculiar state of tension or polarity, and probably also at the same time
to throw those of its own particles which are in contact with the water
into a similar but opposed state. Whilst this state is retained, no further
change occurs; but when it is relieved, by completion of the circuit, in
which case the forces determined in opposite directions, with respect to
the zinc and the electrolyte, are found exactly competent to neutralize
each other, then a series of decompositions and recompositions takes place
amongst the particles of oxygen and hydrogen constituting the water,
between the place of contact with the platina and the place where the zinc
is active; these intervening particles being evidently in close dependence
upon and relation to each other. The zinc forms a direct compound with
those particles of oxygen which were, previously, in divided relation to
both it and the hydrogen: the oxide is removed by the acid, and a fresh
surface of zinc is presented to the water, to renew and repeat the action.

950. Practically, the state of tension is best relieved by dipping a metal
which has less attraction for oxygen than the zinc, into the dilute acid,
and making it also touch the zinc. The force of chemical affinity, which
has been influenced or polarized in the particles of the water by the
dominant attraction of the zinc for the oxygen, is then transferred, in a
most extraordinary manner, through the two metals, so as to re-enter upon
the circuit in the electrolytic conductor, which, unlike the metals in that
respect, cannot convey or transfer it without suffering decomposition; or
rather, probably, it is exactly balanced and neutralized by the force which
at the same moment completes the combination of the zinc with the oxygen of
the water. The forces, in fact, of the two particles which are acting
towards each other, and which are therefore in opposite directions, are the
origin of the two opposite forces, or directions of force, in the current.
They are of necessity equivalent to each other. Being transferred forward
in contrary directions, they produce what is called the voltaic current:
and it seems to me impossible to resist the idea that it must be preceded
by a _state of tension_ in the fluid, and between the fluid and the zinc;
the _first consequence_ of the affinity of the zinc for the oxygen of the
water.

951. I have sought carefully for indications of a state of tension in the
electrolytic conductor; and conceiving that it might produce something like
structure, either before or during its discharge, I endeavoured to make
this evident by polarized light. A glass cell, seven inches long, one inch
and a half wide, and six inches deep, had two sets of platina electrodes
adapted to it, one set for the ends, and the other for the sides. Those for
the _sides_ were seven inches long by three inches high, and when in the
cell were separated by a little frame of wood covered with calico; so that
when made active by connexion with a battery upon any solution in the cell,
the bubbles of gas rising from them did not obscure the central parts of
the liquid.

952. A saturated solution of sulphate of soda was put into the cell, and
the electrodes connected with a battery of 150 pairs of 4-inch plates: the
current of electricity was conducted across the cell so freely, that the
discharge was as good as if a wire had been used. A ray of polarized light
was then transmitted through this solution, directly across the course of
the electric current, and examined by an analysing plate; but though it
penetrated seven inches of solution thus subject to the action of the
electricity, and though contact was sometimes made, sometimes broken, and
occasionally reversed during the observations, not the slightest trace of
action on the ray could be perceived.

953. The large electrodes were then removed, and others introduced which
fitted the _ends_ of the cell. In each a slit was cut, so as to allow the
light to pass. The course of the polarized ray was now parallel to the
current, or in the direction of its axis (517.); but still no effect, under
any circumstances of contact or disunion, could be perceived upon it.

954. A strong solution of nitrate of lead was employed instead of the
sulphate of soda, but no effects could be detected.

955. Thinking it possible that the discharge of the electric forces by the
successive decompositions and recompositions of the particles of the
electrolyte might neutralize and therefore destroy any effect which the
first state of tension could by possibility produce, I took a substance
which, being an excellent electrolyte when fluid, was a perfect insulator
when solid, namely, borate of lead, in the form of a glass plate, and
connecting the sides and the edges of this mass with the metallic plates,
sometimes in contact with the poles of a voltaic battery, and sometimes
even with the electric machine, for the advantage of the much higher
intensity then obtained, I passed a polarized ray across it in various
directions, as before, but could not obtain the slightest appearance of
action upon the light. Hence I conclude, that notwithstanding the new and
extraordinary state which must be assumed by an electrolyte, either during
decomposition (when a most enormous quantity of electricity must be
traversing it), or in the state of tension which is assumed as preceding
decomposition, and which might be supposed to be retained in the solid form
of the electrolyte, still it has no power of affecting a polarized ray of
light; for no kind of structure or tension can in this way be rendered
evident.

956. There is, however, one beautiful experimental proof of a state of
tension acquired by the metals and the electrolyte before the electric
current is produced, and _before contact_ of the different metals is made
(915.); in fact, at that moment when chemical forces only are efficient as
a cause of action. I took a voltaic apparatus, consisting of a single pair
of large plates, namely, a cylinder of amalgamated zinc, and a double
cylinder of copper. These were put into a jar containing dilute sulphuric
acid[A], and could at pleasure be placed in metallic communication by a
copper wire adjusted so as to dip at the extremities into two cups of
mercury connected with the two plates.

  [A] When nitro-sulphuric acid is used, the spark is more powerful, but
  local chemical action can then commence, and proceed without requiring
  metallic contact.

957. Being thus arranged, there was no chemical action whilst the plates
were not connected. On _making_ the connexion a spark was obtained[A], and
the solution was immediately decomposed. On breaking it, the usual spark
was obtained, and the decomposition ceased. In this case it is evident that
the first spark must have occurred before metallic contact was made, for it
passed through an interval of air; and also that it must have tended to
pass before the electrolytic action began; for the latter could not take
place until the current passed, and the current could not pass before the
spark appeared. Hence I think there is sufficient proof, that as it is the
zinc and water which by their mutual action produce the electricity of this
apparatus, so these, by their first contact with each other, were placed in
a state of powerful tension (951.), which, though it could not produce the
actual decomposition of the water, was able to make a spark of electricity
pass between the zinc and a fit discharger as soon as the interval was
rendered sufficiently small. The experiment demonstrates the direct
production of the electric spark from pure chemical forces.

  [A] It has been universally supposed that no spark is produced on
  making the contact between a single pair of plates. I was led to
  expect one from the considerations already advanced in this paper. The
  wire of communication should be short; for with a long wire,
  circumstances strongly affecting the spark are introduced.

958. There are a few circumstances connected with the production of this
spark by a single pair of plates, which should be known, to ensure success
to the experiment[B]. When the amalgamated surfaces of contact are quite
clean and dry, the spark, on making contact, is quite as brilliant as on
breaking it, if not even more so. When a film of oxide or dirt was present
at either mercurial surface, then the first spark was often feeble, and
often failed, the breaking spark, however, continuing very constant and
bright. When a little water was put over the mercury, the spark was greatly
diminished in brilliancy, but very regular both on making and breaking
contact. When the contact was made between clean platina, the spark was
also very small, but regular both ways. The true electric spark is, in
fact, very small, and when surfaces of mercury are used, it is the
combustion of the metal which produces the greater part of the light. The
circumstances connected with the burning of the mercury are most favourable
on breaking contact; for the act of separation exposes clean surfaces of
metal, whereas, on making contact, a thin film of oxide, or soiling matter,
often interferes. Hence the origin of the general opinion that it is only
when the contact is broken that the spark passes.

  [B] See in relation to precautions respecting a spark, 1074.--_Dec.
  1838._

959. With reference to the other set of cases, namely, those of local
action (947.) in which chemical affinity being exerted causes no
transference of the power to a distance where no electric current is
produced, it is evident that forces of the most intense kind must be
active, and in some way balanced in their activity, during such
combinations; these forces being directed so immediately and exclusively
towards each other, that no signs of the powerful electric current they can
produce become apparent, although the same final state of things is
obtained as if that current had passed. It was Berzelius, I believe, who
considered the heat and light evolved in cases of combustion as the
consequences of this mode of exertion of the electric powers of the
combining particles. But it will require a much more exact and extensive
knowledge of the nature of electricity, and the manner in which it is
associated with the atoms of matter, before we can understand accurately
the action of this power in thus causing their union, or comprehend the
nature of the great difference which it presents in the two modes of action
just distinguished. We may imagine, but such imaginations must for the time
be classed with the great mass of _doubtful knowledge_ (876.) which we
ought rather to strive to diminish than to increase; for the very extensive
contradictions of this knowledge by itself shows that but a small portion
of it can ultimately prove true[A].

  [A] Refer to 1738, &c. Series XIV.--_Dec. 1838._

960. Of the two modes of action in which chemical affinity is exerted, it
is important to remark, that that which produces the electric current is as
_definite_ as that which causes ordinary chemical combination; so that in
examining the _production_ or _evolution_ of electricity in cases of
combination or decomposition, it will be necessary, not merely to observe
certain effects dependent upon a current of electricity, but also their
_quantity_: and though it may often happen that the forces concerned in any
particular case of chemical action may be partly exerted in one mode and
partly in the other, it is only those which are efficient in producing the
current that have any relation to voltaic action. Thus, in the combination
of oxygen and hydrogen to produce water, electric powers to a most enormous
amount are for the time active (861. 873.); but any mode of examining the
flame which they form during energetic combination, which has as yet been
devised, has given but the feeblest traces. These therefore may not,
cannot, be taken as evidences of the nature of the action; but are merely
incidental results, incomparably small in relation to the forces concerned,
and supplying no information of the way in which the particles are active
on each other, or in which their forces are finally arranged.

961. That such cases of chemical action produce no _current of
electricity_, is perfectly consistent with what we know of the voltaic
apparatus, in which it is essential that one of the combining elements
shall form part of, or be in direct relation with, an electrolytic
conductor (921. 923.). That such cases produce _no free electricity of
tension_, and that when they are converted into cases of voltaic action
they produce a current in which the opposite forces are so equal as to
neutralize each other, prove the equality of the forces in the opposed
acting particles of matter, and therefore the equality of electric power in
those quantities of matter which are called _electro-chemical equivalents_
(824). Hence another proof of the definite nature of electro-chemical
action (783. &c.), and that chemical affinity and electricity are forms of
the same power (917. &c.).

962. The direct reference of the effects produced by the voltaic pile at
the place of experimental decomposition to the chemical affinities active
at the place of excitation (891. 917.), gives a very simple and natural
view of the cause why the bodies (or _ions_) evolved pass in certain
directions; for it is only when they pass in those directions that their
forces can consist with and compensate (in direction at least) the superior
forces which are dominant at the place where the action of the whole is
determined. If, for instance, in a voltaic circuit, the activity of which
is determined, by the attraction of zinc for the oxygen of water, the zinc
move from right to left, then any other _cation_ included in the circuit,
being part of an electrolyte, or forming part of it at the moment, will
also move from right to left: and as the oxygen of the water, by its
natural affinity for the zinc, moves from left to right, so any other body
of the same class with it (i.e. any other _anion_), under its government
for the time, will move from left to right.

963. This I may illustrate by reference to fig. 83, the double circle of
which may represent a complete voltaic circuit, the direction of its forces
being determined by supposing for a moment the zinc _b_ and the platina _c_
as representing plates of those metals acting upon water, _d, e_, and other
substances, but having their energy exalted so as to effect several
decompositions by the use of a battery at _a_ (989.). This supposition may
be allowed, because the action in the battery will only consist of
repetitions of what would take place between _b_ and _c_, if they really
constituted but a single pair. The zinc _b_, and the oxygen _d_, by their
mutual affinity, tend to unite; but as the oxygen is already in association
with the hydrogen _e_, and has its inherent chemical or electric powers
neutralized for the time by those of the latter, the hydrogen _e_ must
leave the oxygen _d_, and advance in the direction of the arrow head, or
else the zinc _b_ cannot move in the same direction to unite to the oxygen
_d_, nor the oxygen _d_ move in the contrary direction to unite to the zinc
_b_, the relation of the _similar_ forces of _b_ and _c_, in contrary
directions, to the _opposite_ forces of _d_ being the preventive. As the
hydrogen _e_ advances, it, on coming against the platina _c, f_, which
forms a part of the circuit, communicates its electric or chemical forces
through it to the next electrolyte in the circuit, fused chloride of lead,
_g, h_, where the chlorine must move in conformity with the direction of
the oxygen at _d_, for it has to compensate the forces disturbed in its
part of the circuit by the superior influence of those between the oxygen
and zinc at _d, b_, aided as they are by those of the battery _a_; and for
a similar reason the lead must move in the direction pointed out by the
arrow head, that it may be in right relation to the first moving body of
its own class, namely, the zinc _b_. If copper intervene in the circuit
from _i_ to _k_, it acts as the platina did before; and if another
electrolyte, as the iodide of tin, occur at _l, m_, then the iodine _l_,
being an _anion_, must move in conformity with the exciting _anion_,
namely, the oxygen _d_, and the _cation_ tin _m_ move in correspondence
with the other _cations b, e_, and _h_, that the chemical forces may be in
equilibrium as to their direction and quantity throughout the circuit.
Should it so happen that the anions in their circulation can combine with
the metals at the _anodes_ of the respective electrolytes, as would be the
case at the platina _f_ and the copper _k_, then those bodies becoming
parts of electrolytes, under the influence of the current, immediately
travel; but considering their relation to the zinc _b_, it is evidently
impossible that they can travel in any other direction than what will
accord with its course, and therefore can never tend to pass otherwise than
_from_ the anode and _to_ the cathode.

964. In such a circle as that delineated, therefore, all the known _anions_
may be grouped within, and all the _cations_ without. If any number of them
enter as _ions_ into the constitution of _electrolytes_, and, forming one
circuit, are simultaneously subject to one common current, the anions must
move in accordance with each other in one direction, and the cations in the
other. Nay, more than that, equivalent portions of these bodies must so
advance in opposite directions: for the advance of every 32.5 parts of the
zinc _b_ must be accompanied by a motion in the opposite direction of 8
parts of oxygen at _d_, of 36 parts of chlorine at _g_, of 126 parts of
iodine at _l_; and in the same direction by electro-chemical equivalents of
hydrogen, lead, copper and tin, at _e, h, k_. and _m_.

965. If the present paper be accepted as a correct expression of facts, it
will still only prove a confirmation of certain general views put forth by
Sir Humphry Davy in his Bakerian Lecture for 1806[A], and revised and
re-stated by him in another Bakerian Lecture, on electrical and chemical
changes, for the year 1826[B]. His general statement is, that "_chemical
and electrical attractions were produced by the same cause, acting in one
case on particles, in the other on masses, of matter; and that the same
property, under different modifications, was the cause of all the phenomena
exhibited by different voltaic combinations_[C]." This statement I believe
to be true; but in admitting and supporting it, I must guard myself from
being supposed to assent to all that is associated with it in the two
papers referred to, or as admitting the experiments which are there quoted
as decided proofs of the truth of the principle. Had I thought them so,
there would have been no occasion for this investigation. It may be
supposed by some that I ought to go through these papers, distinguishing
what I admit from what I reject, and giving good experimental or
philosophical reasons for the judgment in both cases. But then I should be
equally bound to review, for the same purpose, all that has been written
both for and against the necessity of metallic contact,--for and against
the origin of voltaic electricity in chemical action,--a duty which I may
not undertake in the present paper[D].

  [A] Philosophical Transactions, 1807.

  [B] Ibid. 1826, p. 383.

  [C] Ibid. 1826, p. 389.

  [D] I at one time intended to introduce here, in the form of a note, a
  table of reference to the papers of the different philosophers who
  have referred the origin of the electricity in the voltaic pile to
  contact, or to chemical action, or to both; but on the publication of
  the first volume of M. Becquerel's highly important and valuable
  Traité de l'Electricité et du Magnétisme, I thought it far better to
  refer to that work for these references, and the views held by the
  authors quoted. See pages 86, 91, 104, 110, 112, 117, 118, 120, 151,
  152, 224, 227, 228, 232, 233, 252, 255, 257, 258, 290, &c.--July 3rd,
  1834.


¶ ii. _On the Intensity necessary for Electrolyzation._

966. It became requisite, for the comprehension of many of the conditions
attending voltaic action, to determine positively, if possible, whether
electrolytes could resist the action of an electric current when beneath a
certain intensity? whether the intensity at which the current ceased to act
would be the same for all bodies? and also whether the electrolytes thus
resisting decomposition would conduct the electric current as a metal does,
after they ceased to conduct as electrolytes, or would act as perfect
insulators?

967. It was evident from the experiments described (904. 906.) that
different bodies were decomposed with very different facilities, and
apparently that they required for their decomposition currents of different
intensities, resisting some, but giving way to others. But it was needful,
by very careful and express experiments, to determine whether a current
could really pass through, and yet not decompose an electrolyte (910.).

968. An arrangement (fig. 84.) was made, in which two glass vessels
contained the same dilute sulphuric acid, sp. gr. 1.25. The plate _z_ was
amalgamated zinc, in connexion, by a platina wire _a_, with the platina
plate _e_; _b_ was a platina wire connecting the two platina plates PP';
_c_ was a platina wire connected with the platina plate P". On the plate
_e_ was placed a piece of paper moistened in solution of iodide of
potassium: the wire _c_ was so curved that its end could be made to rest at
pleasure on this paper, and show, by the evolution of iodine there, whether
a current was passing; or, being placed in the dotted position, it formed a
direct communication with the platina plate _e_, and the electricity could
pass without causing decomposition. The object was to produce a current by
the action of the acid on the amalgamated zinc in the first vessel A; to
pass it through the acid in the second vessel B by platina electrodes, that
its power of decomposing water might, if existing, be observed; and to
verify the existence of the current at pleasure, by decomposition at _e_,
without involving the continual obstruction to the current which would
arise from making the decomposition there constant. The experiment, being
arranged, was examined and the existence of a current ascertained by the
decomposition at _e_; the whole was then left with the end of the wire _c_
resting on the plate _e_, so as to form a constant metallic communication
there.

969. After several hours, the end of the wire _c_ was replaced on the
test-paper at _e_: decomposition occurred, and _the proof_ of a passing
current was therefore complete. The current was very feeble compared to
what it had been at the beginning of the experiment, because of a peculiar
state acquired by the metal surfaces in the second vessel, which caused
them to oppose the passing current by a force which they possess under
these circumstances (1040.). Still it was proved, by the decomposition,
that this state of the plates in the second vessel was not able entirely to
stop the current determined in the first, and that was all that was needful
to be ascertained in the present inquiry.

970. This apparatus was examined from time to time, and an electric current
always found circulating through it, until twelve days had elapsed, during
which the water in the second vessel had been constantly subject to its
action. Notwithstanding this lengthened period, not the slightest
appearance of a bubble upon either of the plates in that vessel occurred.
From the results of the experiment, I conclude that a current _had_ passed,
but of so low an intensity as to fall beneath that degree at which the
elements of water, unaided by any secondary force resulting from the
capability of combination with the matter of the electrodes, or of the
liquid surrounding them, separated from each other.

971. It may be supposed, that the oxygen and hydrogen had been evolved in
such small quantities as to have entirely dissolved in the water, and
finally to have escaped at the surface, or to have reunited into water.
That the hydrogen can be so dissolved was shown in the first vessel; for
after several days minute bubbles of gas gradually appeared upon a glass
rod, inserted to retain the zinc and platina apart, and also upon the
platina plate itself, and these were hydrogen. They resulted principally in
this way:--notwithstanding the amalgamation of the zinc, the acid exerted a
little direct action upon it, so that a small stream of hydrogen bubbles
was continually rising from its surface; a little of this hydrogen
gradually dissolved in the dilute acid, and was in part set free against
the surfaces of the rod and the plate, according to the well-known action
of such solid bodies in solutions of gases (623. &c.).

972. But if the gases had been evolved in the second vessel by the
decomposition of water, and had tended to dissolve, still there would have
been every reason to expect that a few bubbles should have appeared on the
electrodes, especially on the negative one, if it were only because of its
action as a nucleus on the solution supposed to be formed; but none
appeared even after twelve days.

973. When a few drops only of nitric acid were added to the vessel A, fig.
84, then the results were altogether different. In less than five minutes
bubbles of gas appeared on the plates P' and P" in the second vessel. To
prove that this was the effect of the electric current (which by trial at
_c_ was found at the same time to be passing,) the connexion at _c_ was
broken, the plates P'P" cleared from bubbles and left in the acid of the
vessel B, for fifteen minutes: during that time no bubbles appeared upon
them; but on restoring the communication at _c_, a minute did not elapse
before gas appeared in bubbles upon the plates. The proof, therefore, is
most full and complete, that the current excited by dilute sulphuric acid
with a little nitric acid in vessel A, has intensity enough to overcome the
chemical affinity exerted between the oxygen and hydrogen of the water in
the vessel B, whilst that excited by dilute sulphuric acid alone has _not_
sufficient intensity.

974. On using a strong solution of caustic potassa in the vessel A, to
excite the current, it was found by the decomposing effects at _e_, that
the current passed. But it had not intensity enough to decompose the water
in the vessel B; for though left for fourteen days, during the whole of
which time the current was found to be passing, still not the slightest
appearance of gas appeared on the plates P'P", nor any other signs of the
water having suffered decomposition.

975. Sulphate of soda in solution was then experimented with, for the
purpose of ascertaining with respect to it, whether a certain electrolytic
intensity was also required for its decomposition in this state, in analogy
with the result established with regard to water (974). The apparatus was
arranged as in fig. 85; P and Z are the platina and zinc plates dipping
into a solution of common salt; _a_ and _b_ are platina plates connected by
wires of platina (except in the galvanometer _g_) with P and Z; _c_ is a
connecting wire of platina, the ends of which can be made to rest either on
the plates _a, b_, or on the papers moistened in solutions which are placed
upon them; so that the passage of the current without decomposition, or
with one or two decompositions, was under ready command, as far as
arrangement was concerned. In order to change the _anodes_ and _cathodes_
at the places of decomposition, the form of apparatus fig. 86, was
occasionally adopted. Here only one platina plate, _c_, was used; both
pieces of paper on which decomposition was to be effected were placed upon
it, the wires from P and Z resting upon these pieces of paper, or upon the
plate _c_, according as the current with or without decomposition of the
solutions was required.

976. On placing solution of iodide of potassium in paper at one of the
decomposing localities, and solution of sulphate of soda at the other, so
that the electric current should pass through both at once, the solution of
iodide was slowly decomposed, yielding iodine at the _anode_ and alkali at
the _cathode_; but the solution of sulphate of soda exhibited no signs of
decomposition, neither acid nor alkali being evolved from it. On placing
the wires so that the iodide alone was subject to the action of the current
(900.), it was quickly and powerfully decomposed; but on arranging them so
that the sulphate of soda alone was subject to action, it still refused to
yield up its elements. Finally, the apparatus was so arranged under a wet
bell-glass, that it could be left for twelve hours, the current passing
during the whole time through a solution of sulphate of soda, retained in
its place by only two thicknesses of bibulous litmus and turmeric paper. At
the end of that time it was ascertained by the decomposition of iodide of
potassium at the second place of action, that the current was passing and
had passed for the twelve hours, and yet no trace of acid or alkali from
the sulphate of soda appeared.

977. From these experiments it may, I think, be concluded, that a solution
of sulphate of soda can conduct a current of electricity, which is unable
to decompose the neutral salt present; that this salt in the state of
solution, like water, requires a certain electrolytic intensity for its
decomposition; and that the necessary intensity is much higher for this
substance than for the iodide of potassium in a similar state of solution.

978. I then experimented on bodies rendered decomposable by fusion, and
first on _chloride of lead_. The current was excited by dilute sulphuric
acid without any nitric acid between zinc and platina plates, fig. 87, and
was then made to traverse a little chloride of lead fused upon glass at
_a_, a paper moistened in solution of iodide of potassium at _b_, and a
galvanometer at _g_. The metallic terminations at _a_ and _b_ were of
platina. Being thus arranged, the decomposition at _b_ and the deflection
at _g_ showed that an electric current was passing, but there was no
appearance of decomposition at _a_, not even after a _metallic_
communication at _b_ was established. The experiment was repeated several
times, and I am led to conclude that in this case the current has not
intensity sufficient to cause the decomposition of the chloride of lead;
and further, that, like water (974.), fused chloride of lead can conduct an
electric current having an intensity below that required to effect
decomposition.

979. _Chloride of silver_ was then placed at _a_, fig. 87, instead of
chloride of lead. There was a very ready decomposition of the solution of
iodide of potassium at _b_, and when metallic contact was made there, very
considerable deflection of the galvanometer needle at _g_. Platina also
appeared to be dissolved at the anode of the fused chloride at _a_, and
there was every appearance of a decomposition having been effected there.

980. A further proof of decomposition was obtained in the following manner.
The platina wires in the fused chloride at _a_ were brought very near
together (metallic contact having been established at _b_), and left so;
the deflection at the galvanometer indicated the passage of a current,
feeble in its force, but constant. After a minute or two, however, the
needle would suddenly be violently affected, and indicate a current as
strong as if metallic contact had taken place at _a_. This I actually found
to be the case, for the silver reduced by the action of the current
crystallized in long delicate spiculæ, and these at last completed the
metallic communication; and at the same time that they transmitted a more
powerful current than the fused chloride, they proved that electro-chemical
decomposition of that chloride had been going on. Hence it appears, that
the current excited by dilute sulphuric acid between zinc and platina, has
an intensity above that required to electrolyze the fused chloride of
silver when placed between platina electrodes, although it has not
intensity enough to decompose chloride of lead under the same
circumstances.

981. A drop of _water_ placed at _a_ instead of the fused chlorides, showed
as in the former case (970.), that it could conduct a current unable to
decompose it, for decomposition of the solution of iodide at _b_ occurred
after some time. But its conducting power was much below that of the fused
chloride of lead (978.).

982. Fused _nitre_ at _a_ conducted much better than water: I was unable to
decide with certainty whether it was electrolyzed, but I incline to think
not, for there was no discoloration against the platina at the _cathode_.
If sulpho-nitric acid had been used in the exciting vessel, both the nitre
and the chloride of lead would have suffered decomposition like the water
(906.).

983. The results thus obtained of conduction without decomposition, and the
necessity of a certain electrolytic intensity for the separation of the
_ions_ of different electrolytes, are immediately connected with the
experiments and results given in § 10. of the Fourth Series of these
Researches (418. 423. 444. 419.). But it will require a more exact
knowledge of the nature of intensity, both as regards the first origin of
the electric current, and also the manner in which it may be reduced, or
lowered by the intervention of longer or shorter portions of bad
conductors, whether decomposable or not, before their relation can be
minutely and fully understood.

984. In the case of water, the experiments I have as yet made, appear to
show, that, when the electric current is reduced in intensity below the
point required for decomposition, then the degree of conduction is the same
whether sulphuric acid, or any other of the many bodies which can affect
its transferring power as an electrolyte, are present or not. Or, in other
words, that the necessary electrolytic intensity for water is the same
whether it be pure, or rendered a better conductor by the addition of these
substances; and that for currents of less intensity than this, the water,
whether pure or acidulated, has equal conducting power. An apparatus, fig.
84, was arranged with dilute sulphuric acid in the vessel A, and pure
distilled water in the vessel B. By the decomposition at _c_, it appeared
as if water was a _better_ conductor than dilute sulphuric acid for a
current of such low intensity as to cause no decomposition. I am inclined,
however, to attribute this apparent superiority of water to variations in
that peculiar condition of the platina electrodes which is referred to
further on in this Series (1040.), and which is assumed, as far as I can
judge, to a greater degree in dilute sulphuric acid than in pure water. The
power therefore, of acids, alkalies, salts, and other bodies in solution,
to increase conducting power, appears to hold good only in those cases
where the electrolyte subject to the current suffers decomposition, and
loses all influence when the current transmitted has too low an intensity
to affect chemical change. It is probable that the ordinary conducting
power of an electrolyte in the solid state (419.) is the same as that which
it possesses in the fluid state for currents, the tension of which is
beneath the due electrolytic intensity.

985. Currents of electricity, produced by less than eight or ten series of
voltaic elements, can be reduced to that intensity at which water can
conduct them without suffering decomposition, by causing them to pass
through three or four vessels in which water shall be successively
interposed between platina surfaces. The principles of interference upon
which this effect depends, will be described hereafter (1009. 1018.), but
the effect may be useful in obtaining currents of standard intensity, and
is probably applicable to batteries of any number of pairs of plates.

986. As there appears every reason to expect that all electrolytes will be
found subject to the law which requires an electric current of a certain
intensity for their decomposition, but that they will differ from each
other in the degree of intensity required, it will be desirable hereafter
to arrange them in a table, in the order of their electrolytic intensities.
Investigations on this point must, however, be very much extended, and
include many more bodies than have been here mentioned before such a table
can be constructed. It will be especially needful in such experiments, to
describe the nature of the electrodes used, or, if possible, to select such
as, like platina or plumbago in certain cases, shall have no power of
assisting the separation of the _ions_ to be evolved (913).

987. Of the two modes in which bodies can transmit the electric forces,
namely, that which is so characteristically exhibited by the metals, and
usually called conduction, and that in which it is accompanied by
decomposition, the first appears common to all bodies, although it occurs
with almost infinite degrees of difference; the second is at present
distinctive of the electrolytes. It is, however, just possible that it may
hereafter be extended to the metals; for their power of conducting without
decomposition may, perhaps justly, be ascribed to their requiring a very
high electrolytic intensity for their decomposition.

987-1/2. The establishment of the principle that a certain electrolytic
intensity is necessary before decomposition can be effected, is of great
importance to all those considerations which arise regarding the probable
effects of weak currents, such for instance as those produced by natural
thermo-electricity, or natural voltaic arrangements in the earth. For to
produce an effect of decomposition or of combination, a current must not
only exist, but have a certain intensity before it can overcome the
quiescent affinities opposed to it, otherwise it will be conducted,
producing no permanent chemical effects. On the other hand, the principles
are also now evident by which an opposing action can be so weakened by the
juxtaposition of bodies not having quite affinity enough to cause direct
action between them (913.), that a very weak current shall be able to raise
the sum of actions sufficiently high, and cause chemical changes to occur.

988. In concluding this division _on the intensity necessary for
electrolyzation_, I cannot resist pointing out the following remarkable
conclusion in relation to intensity generally. It would appear that when a
voltaic current is produced, having a certain intensity, dependent upon the
strength of the chemical affinities by which that current is excited
(916.), it can decompose a particular electrolyte without relation to the
quantity of electricity passed, the _intensity_ deciding whether the
electrolyte shall give way or not. If that conclusion be confirmed, then we
may arrange circumstances so that the _same quantity_ of electricity may
pass in the _same time_, in at the _same surface_, into the _same
decomposing body in the same state_, and yet, differing in intensity, will
_decompose in one case and in the other not_:--for taking a source of too
low an intensity to decompose, and ascertaining the quantity passed in a
given time, it is easy to take another source having a sufficient
intensity, and reducing the quantity of electricity from it by the
intervention of bad conductors to the same proportion as the former
current, and then all the conditions will be fulfilled which are required
to produce the result described.


¶ iii. _On associated Voltaic Circles, or the Voltaic Battery._

989. Passing from the consideration of single circles (875. &c.) to their
association in the voltaic battery, it is a very evident consequence, that
if matters are so arranged that two sets of affinities, in place of being
opposed to each other as in figg. 73. 76. (880. 891.), are made to act in
conformity, then, instead of either interfering with the other, it will
rather assist it. This is simply the case of two voltaic pairs of metals
arranged so as to form one circuit. In such arrangements the activity of
the whole is known to be increased, and when ten, or a hundred, or any
larger number of such alternations are placed in conformable association
with each other, the power of the whole becomes proportionally exalted, and
we obtain that magnificent instrument of philosophic research, the _voltaic
battery_.

990. But it is evident from the principles of definite action already laid
down, that the _quantity_ of electricity in the current cannot be increased
with the increase of the _quantity of metal_ oxidized and dissolved at each
new place of chemical action. A single pair of zinc and platina plates
throws as much electricity into the form of a current, by the oxidation of
32.5 grains of the zinc (868.) as would be circulated by the same
alteration of a thousand times that quantity, or nearly five pounds of
metal oxidized at the surface of the zinc plates of a thousand pairs placed
in regular battery order. For it is evident, that the electricity which
passes across the acid from the zinc to the platina in the first cell, and
which has been associated with, or even evolved by, the decomposition of a
definite portion of water in that cell, cannot pass from the zinc to the
platina across the acid in the second cell, without the decomposition of
the same quantity of water there, and the oxidation of the same quantity of
zinc by it (924. 949.). The same result recurs in every other cell; the
electro-chemical equivalent of water must be decomposed in each, before the
current can pass through it; for the quantity of electricity passed and the
quantity of electrolyte decomposed, _must_ be the equivalents of each
other. The action in each cell, therefore, is not to increase the quantity
set in motion in any one cell, but to aid in urging forward that quantity,
the passing of which is consistent with the oxidation of its own zinc; and
in this way it exalts that peculiar property of the current which we
endeavour to express by the term _intensity_, without increasing the
_quantity_ beyond that which is proportionate to the quantity of zinc
oxidized in any single cell of the series.

991. To prove this, I arranged ten pairs of amalgamated zinc and platina
plates with dilute sulphuric acid in the form of a battery. On completing
the circuit, all the pairs acted and evolved gas at the surfaces of the
platina. This was collected and found to be alike in quantity for each
plate; and the quantity of hydrogen evolved at any one platina plate was in
the same proportion to the quantity of metal dissolved from any one zinc
plate, as was given in the experiment with a single pair (864. &c.). It was
therefore certain, that, just as much electricity and no more had passed
through the series of ten pair of plates as had passed through, or would
have been put into motion by, any single pair, notwithstanding that ten
times the quantity of zinc had been consumed.

992. This truth has been proved also long ago in another way, by the action
of the evolved current on a magnetic needle; the deflecting power of one
pair of plates in a battery being equal to the deflecting power of the
whole, provided the wires used be sufficiently large to carry the current
of the single pair freely; but the _cause_ of this equality of action could
not be understood whilst the definite action and evolution of electricity
(783. 869.) remained unknown.

993. The superior decomposing power of a battery over a single pair of
plates is rendered evident in two ways. Electrolytes held together by an
affinity so strong as to resist the action of the current from a single
pair, yield up their elements to the current excited by many pairs; and
that body which is decomposed by the action of one or of few pairs of
metals, &c., is resolved into its _ions_ the more readily as it is acted
upon by electricity urged forward by many alternations.

994. Both these effects are, I think, easily understood. Whatever
_intensity_ may be, (and that must of course depend upon the nature of
electricity, whether it consist of a fluid or fluids, or of vibrations of
an ether, or any other kind or condition of matter,) there seems to be no
difficulty in comprehending that the _degree_ of intensity at which a
current of electricity is evolved by a first voltaic element, shall be
increased when that current is subjected to the action of a second voltaic
element, acting in conformity and possessing equal powers with the first:
and as the decompositions are merely opposed actions, but exactly of the
same kind as those which generate the current (917.), it seems to be a
natural consequence, that the affinity which can resist the force of a
single decomposing action may be unable to oppose the energies of many
decomposing actions, operating conjointly, as in the voltaic battery.

995. That a body which can give way to a current of feeble intensity,
should give way more freely to one of stronger force, and yet involve no
contradiction to the law of definite electrolytic action, is perfectly
consistent. All the facts and also the theory I have ventured to put forth,
tend to show that the act of decomposition opposes a certain force to the
passage of the electric current; and, that this obstruction should be
overcome more or less readily, in proportion to the greater or less
intensity of the decomposing current, is in perfect consistency with all
our notions of the electric agent.

996. I have elsewhere (947.) distinguished the chemical action of zinc and
dilute sulphuric acid into two portions; that which, acting effectually on
the zinc, evolves hydrogen at once upon its surface, and that which,
producing an arrangement of the chemical forces throughout the electrolyte
present, (in this case water,) tends to take oxygen from it, but cannot do
so unless the electric current consequent thereon can have free passage,
and the hydrogen be delivered elsewhere than against the zinc. The electric
current depends altogether upon the second of these; but when the current
can pass, by favouring the electrolytic action it tends to diminish the
former and increase the latter portion.

997. It is evident, therefore, that when ordinary zinc is used in a voltaic
arrangement, there is an enormous waste of that power which it is the
object to throw into the form of an electric current; a consequence which
is put in its strongest point of view when it is considered that three
ounces and a half of zinc, properly oxidized, can circulate enough
electricity to decompose nearly one ounce of water, and cause the evolution
of about 2100 cubic inches of hydrogen gas. This loss of power not only
takes place during the time the electrodes of the battery are in
communication, being then proportionate to the quantity of hydrogen evolved
against the surface of any one of the zinc plates, but includes also _all_
the chemical action which goes on when the extremities of the pile are not
in communication.

998. This loss is far greater with ordinary zinc than with the pure metal,
as M. De la Rive has shown[A]. The cause is, that when ordinary zinc is
acted upon by dilute sulphuric acid, portions of copper, lead, cadmium, or
other metals which it may contain, are set free upon its surface; and
these, being in contact with the zinc, form small but very active voltaic
circles, which cause great destruction of the zinc and evolution of
hydrogen, apparently upon the zinc surface, but really upon the surface of
these incidental metals. In the same proportion as they serve to discharge
or convey the electricity back to the zinc, do they diminish its power of
producing an electric current which shall extend to a greater distance
across the acid, and be discharged only through the copper or platina plate
which is associated with it for the purpose of forming a voltaic apparatus.

  [A] Quarterly Journal of Science, 1831, p. 388; or Bibliothèque
  Universelle, 1830, p. 391.

999. All these evils are removed by the employment of an amalgam of zinc in
the manner recommended by Mr. Kemp[A], or the use of the amalgamated zinc
plates of Mr. Sturgeon (863.), who has himself suggested and objected to
their application in galvanic batteries; for he says, "Were it not on
account of the brittleness and other inconveniences occasioned by the
incorporation of the mercury with the zinc, amalgamation of the zinc
surfaces in galvanic batteries would become an important improvement; for
the metal would last much longer, and remain bright for a considerable
time, even for several successive hours; essential considerations in the
employment of this apparatus[B]."

  [A] Jameson's Edinburgh Journal, October 1828.

  [B] Recent Experimental Researches, p. 42, &c. Mr. Sturgeon is of
  course unaware of the definite production of electricity by chemical
  action, and is in fact quoting the experiment as the strongest
  argument _against_ the chemical theory of galvanism.

1000. Zinc so prepared, even though impure, does not sensibly decompose the
water of dilute sulphuric acid, but still has such affinity for the oxygen,
that the moment a metal which, like copper or platina, has little or no
affinity, touches it in the acid, action ensues, and a powerful and
abundant electric current is produced. It is probable that the mercury acts
by bringing the surface, in consequence of its fluidity, into one uniform
condition, and preventing those differences in character between one spot
and another which are necessary for the formation of the minute voltaic
circuits referred to (998.). If any difference does exist at the first
moment, with regard to the proportion of zinc and mercury, at one spot on
the _surface_, as compared with another, that spot having the least mercury
is first acted on, and, by solution of the zinc, is soon placed in the same
condition as the other parts, and the whole plate rendered superficially
uniform. One part cannot, therefore, act as a discharger to another; and
hence _all_ the chemical power upon the water at its surface is in that
equable condition (949.), which, though it tends to produce an electric
current through the liquid to another plate of metal which can act as a
discharger (950.), presents no irregularities by which any one part, having
weaker affinities for oxygen, can act as a discharger to another. Two
excellent and important consequences follow upon this state of the metal.
The first is, that _the full equivalent_ of electricity is obtained for the
oxidation of a certain quantity of zinc; the second, that a battery
constructed with the zinc so prepared, and charged with dilute sulphuric
acid, is active only whilst the electrodes are connected, and ceases to act
or be acted upon by the acid the instant the communication is broken.

1001. I have had a small battery of ten pairs of plates thus constructed,
and am convinced that arrangements of this kind will be very important,
especially in the development and illustration of the philosophical
principles of the instrument. The metals I have used are amalgamated zinc
and platina, connected together by being soldered to platina wires, the
whole apparatus having the form of the couronne des tasses. The liquid used
was dilute sulphuric acid of sp. gr. 1.25. No action took place upon the
metals except when the electrodes were in communication, and then the
action upon the zinc was only in proportion to the decomposition in the
experimental cell; for when the current was retarded there, it was retarded
also in the battery, and no waste of the powers of the metal was incurred.

1002. In consequence of this circumstance, the acid in the cells remained
active for a very much longer time than usual. In fact, time did not tend
to lower it in any sensible degree: for whilst the metal was preserved to
be acted upon at the proper moment, the acid also was preserved almost at
its first strength. Hence a constancy of action far beyond what can be
obtained by the use of common zinc.

1003. Another excellent consequence was the renewal, during the interval of
rest, between two experiments of the first and most efficient state. When
an amalgamated zinc and a platina plate, immersed in dilute sulphuric acid,
are first connected, the current is very powerful, but instantly sinks very
much in force, and in some cases actually falls to only an eighth or a
tenth of that first produced (1036.). This is due to the acid which is in
contact with the zinc becoming neutralized by the oxide formed; the
continued quick oxidation of the metal being thus prevented. With ordinary
zinc, the evolution of gas at its surface tends to mingle all the liquid
together, and thus bring fresh acid against the metal, by which the oxide
formed there can be removed. With the amalgamated zinc battery, at every
cessation of the current, the saline solution against the zinc is gradually
diffused amongst the rest of the liquid; and upon the renewal of contact at
the electrodes, the zinc plates are found most favourably circumstanced for
the production of a ready and powerful current.

1004. It might at first be imagined that amalgamated zinc would be much
inferior in force to common zinc, because, of the lowering of its energy,
which the mercury might be supposed to occasion over the whole of its
surface; but this is not the case. When the electric currents of two pairs
of platina and zinc plates were opposed, the difference being that one of
the zincs was amalgamated and the other not, the current from the
amalgamated zinc was most powerful, although no gas was evolved against it,
and much was evolved at the surface of the unamalgamated metal. Again, as
Davy has shown[A], if amalgamated and unamalgamated zinc be put in contact,
and dipped into dilute sulphuric acid, or other exciting fluids, the former
is positive to the latter, i.e. the current passes from the amalgamated
zinc, through the fluid, to the unprepared zinc. This he accounts for by
supposing that "there is not any inherent and specific property in each
metal which gives it the electrical character, but that it depends upon its
peculiar state--on that form of aggregation which fits it for chemical
change."

  [A] Philosophical Transactions, 1826, p. 405.

1005. The superiority of the amalgamated zinc is not, however, due to any
such cause, but is a very simple consequence of the state of the fluid in
contact with it; for as the unprepared zinc acts directly and alone upon
the fluid, whilst that which is amalgamated does not, the former (by the
oxide it produces) quickly neutralizes the acid in contact with its
surface, so that the progress of oxidation is retarded, whilst at the
surface of the amalgamated zinc, any oxide formed is instantly removed by
the free acid present, and the clean metallic surface is always ready to
act with full energy upon the water. Hence its superiority (1037.).
1006. The progress of improvement in the voltaic battery and its
applications, is evidently in the contrary direction at present to what it
was a few years ago; for in place of increasing the number of plates, the
strength of acid, and the extent altogether of the instrument, the change
is rather towards its first state of simplicity, but with a far more
intimate knowledge and application of the principles which govern its force
and action. Effects of decomposition can now be obtained with ten pairs of
plates (417.), which required five hundred or a thousand pairs for their
production in the first instance. The capability of decomposing fused
chlorides, iodides, and other compounds, according to the law before
established (380. &c.), and the opportunity of collecting certain of the
products, without any loss, by the use of apparatus of the nature of those
already described (789. 814. &c.), render it probable that the voltaic
battery may become a useful and even economical manufacturing instrument;
for theory evidently indicates that an equivalent of a rare substance may
be obtained at the expense of three or four equivalents of a very common
body, namely, zinc: and practice seems thus far to justify the expectation.
In this point of view I think it very likely that plates of platina or
silver may be used instead of plates of copper with advantage, and that
then the evil arising occasionally from solution of the copper, and its
precipitation on the zinc, (by which the electromotive power of the zinc is
so much injured,) will be avoided (1047.).


¶ iv. _On the Resistance of an Electrolyte to Electrolytic Action, and on
Interpositions._

1007. I have already illustrated, in the simplest possible form of
experiment (891. 910.), the resistance established at the place of
decomposition to the force active at the exciting place. I purpose
examining the effects of this resistance more generally; but it is rather
with reference to their practical interference with the action and
phenomena of the voltaic battery, than with any intention at this time to
offer a strict and philosophical account of their nature. Their general and
principal cause is the resistance of the chemical affinities to be
overcome; but there are numerous other circumstances which have a joint
influence with these forces (1034. 1040. &c.), each of which would require
a minute examination before a correct account of the whole could be given.

1008. As it will be convenient to describe the experiments in a form
different to that in which they were made, both forms shall first be
explained. Plates of platina, copper, zinc, and other metals, about three
quarters of an inch wide and three inches long, were associated together in
pairs by means of platina wires to which they were soldered, fig. 88, the
plates of one pair being either alike or different, as might be required.
These were arranged in glasses, fig. 89, so as to form Volta's crown of
cups. The acid or fluid in the cups never covered the whole of any plate;
and occasionally small glass rods were put into the cups, between the
plates, to prevent their contact. Single plates were used to terminate the
series and complete the connexion with a galvanometer, or with a
decomposing apparatus (899. 968. &c.), or both. Now if fig. 90 be examined
and compared with fig. 91, the latter may be admitted as representing the
former in its simplest condition; for the cups i, ii, and iii of the
former, with their contents, are represented by the cells i, ii, and iii of
the latter, and the metal plates Z and P of the former by the similar
plates represented Z and P in the latter. The only difference, in fact,
between the apparatus, fig. 90, and the trough represented fig. 91, is that
twice the quantity of surface of contact between the metal and acid is
allowed in the first to what would occur in the second.

1009. When the extreme plates of the arrangement just described, fig. 90,
are connected metallically through the galvanometer _g_, then the whole
represents a battery consisting of two pairs of zinc and platina plates
urging a current forward, which has, however, to decompose water unassisted
by any direct chemical affinity before it can be transmitted across the
cell iii, and therefore before it can circulate. This decomposition of
water, which is opposed to the passage of the current, may, as a matter of
convenience, be considered as taking place either against the surfaces of
the two platina plates which constitute the electrodes in the cell in, or
against the two surfaces of that platina plate which separates the cells ii
and iii, fig. 91, from each other. It is evident that if that plate were
away, the battery would consist of two pairs of plates and two cells,
arranged in the most favourable position for the production of a current.
The platina plate therefore, which being introduced as at _x_, has oxygen
evolved at one surface and hydrogen at the other (that is, if the
decomposing current passes), may be considered as the cause of any
obstruction arising from the decomposition of water by the electrolytic
action of the current; and I have usually called it the interposed plate.

1010. In order to simplify the conditions, dilute sulphuric acid was first
used in all the cells, and platina for the interposed plates; for then the
initial intensity of the current which tends to be formed is constant,
being due to the power which zinc has of decomposing water; and the
opposing force of decomposition is also constant, the elements of the water
being unassisted in their separation at the interposed plates by any
affinity or secondary action at the electrodes (744.), arising either from
the nature of the plate itself or the surrounding fluid.

1011. When only one voltaic pair of zinc and platina plates was used, the
current of electricity was entirely stopped to all practical purposes by
interposing one platina plate, fig. 92, i.e. by requiring of the current
that it should decompose water, and evolve both its elements, before it
should pass. This consequence is in perfect accordance with the views
before given (910. 917. 973.). For as the whole result depends upon the
opposition of forces at the places of electric excitement and
electro-decomposition, and as water is the substance to be decomposed at
both before the current can move, it is not to be expected that the zinc
should have such powerful attraction for the oxygen, as not only to be able
to take it from its associated hydrogen, but leave such a surplus of force
as, passing to the second place of decomposition, should be there able to
effect a second separation of the elements of water. Such an effect would
require that the force of attraction between zinc and oxygen should under
the circumstances be _at least_ twice as great as the force of attraction
between the oxygen and hydrogen.

1012. When two pairs of zinc and platina exciting plates were used, the
current was also practically stopped by one interposed platina plate, fig.
93. There was a very feeble effect of a current at first, but it ceased
almost immediately. It will be referred to, with many other similar
effects, hereafter (1017.).

1013. Three pairs of zinc and platina plates, fig. 94, were able to produce
a current which could pass an interposed platina plate, and effect the
electrolyzation of water in cell iv. The current was evident, both by the
continued deflection of the galvanometer, and the production of bubbles of
oxygen and hydrogen at the electrodes in cell iv. Hence the accumulated
surplus force of three plates of zinc, which are active in decomposing
water, is more than equal, when added together, to the force with which
oxygen and hydrogen are combined in water, and is sufficient to cause the
separation of these elements from each other.

1014. The three pairs of zinc and platina plates were now opposed by two
intervening platina plates, fig. 95. In this case the current was stopped.

1015. Four pairs of zinc and platina plates were also neutralized by two
interposed platina plates, fig. 96.

1016. Five pairs of zinc and platina, with two interposed platina plates,
fig. 97, gave a feeble current; there was permanent deflection at the
galvanometer, and decomposition in the cells vi and vii. But the current
was very feeble; very much less than when all the intermediate plates were
removed and the two extreme ones only retained: for when they were placed
six inches asunder in one cell, they gave a powerful current. Hence five
exciting pairs, with two interposed obstructing plates, do not give a
current at all comparable to that of a single unobstructed pair.

1017. I have already said that a _very feeble current_ passed when the
series included one interposed platina and two pairs of zinc and platina
plates (1012.). A similarly feeble current passed in every case, and even
when only one exciting pair and four intervening platina plates were used,
fig. 98, a current passed which could be detected at _x_, both by chemical
action on the solution of iodide of potassium, and by the galvanometer.
This current I believe to be due to electricity reduced in intensity below
the point requisite for the decomposition of water (970. 984.); for water
can conduct electricity of such low intensity by the same kind of power
which it possesses in common with metals and charcoal, though it cannot
conduct electricity of higher intensity without suffering decomposition,
and then opposing a new force consequent thereon. With an electric current
of, or under this intensity, it is probable that increasing the number of
interposed platina plates would not involve an increased difficulty of
conduction.

1018. In order to obtain an idea of the additional interfering power of
each added platina plate, six voltaic pairs and four intervening platinas
were arranged as in fig. 99; a very feeble current then passed (985.
1017.). When one of the platinas was removed so that three intervened, a
current somewhat stronger passed. With two intervening platinas a still
stronger current passed; and with only one intervening platina a very fair
current was obtained. But the effect of the successive plates, taken in the
order of their interposition, was very different, as might be expected; for
the first retarded the current more powerfully than the second, and the
second more than the third.

1019. In these experiments both amalgamated and unamalgamated zinc were
used, but the results generally were the same.

1020. The effects of retardation just described were altered altogether
when changes were made in the _nature of the liquid_ used between the
plates, either in what may be called the _exciting_ or the _retarding_
cells. Thus, retaining the exciting force the same, by still using pure
dilute sulphuric acid for that purpose, if a little nitric acid were added
to the liquid in the _retarding_ cells, then the transmission of the
current was very much facilitated. For instance, in the experiment with one
pair of exciting plates and one intervening plate (1011.), fig. 92, when a
few drops of nitric acid were added to the contents of cell ii, then the
current of electricity passed with considerable strength (though it soon
fell from other causes (1036; 1040.),) and the same increased effect was
produced by the nitric acid when many interposed plates were used.

1021. This seems to be a consequence of the diminution of the difficulty of
decomposing water when its hydrogen, instead of being absolutely expelled,
as in the former cases, is transferred to the oxygen of the nitric acid,
producing a secondary result at the _cathode_ (752.); for in accordance
with the chemical views of the electric current and its action already
advanced (913.), the water, instead of opposing a resistance to
decomposition equal to the full amount of the force of mutual attraction
between its oxygen and hydrogen, has that force counteracted in part, and
therefore diminished by the attraction of the hydrogen at the _cathode_ for
the oxygen of the nitric acid which surrounds it, and with which it
ultimately combines instead of being evolved in its free state.

1022. When a little nitric acid was put into the exciting cells, then again
the circumstances favouring the transmission of the current were
strengthened, for the _intensity_ of the current itself was increased by
the addition (906.). When therefore a little nitric acid was added to both
the _exciting_ and the _retarding_ cells, the current of electricity passed
with very considerable freedom.

1023. When dilute muriatic acid was used, it produced and transmitted a
current more easily than pure dilute sulphuric acid, but not so readily as
dilute nitric acid. As muriatic acid appears to be decomposed more freely
than water (765.), and as the affinity of zinc for chlorine is very
powerful, it might be expected to produce a current more intense than that
from the use of dilute sulphuric acid; and also to transmit it more freely
by undergoing decomposition at a lower intensity (912.).

1024. In relation to the effect of these interpositions, it is necessary to
state that they do not appear to be at all dependent upon the size of the
electrodes, or their distance from each other in the acid, except that when
a current _can pass_, changes in these facilitate or retard its passage.
For on repeating the experiment with one intervening and one pair of
exciting plates (1011.), fig. 92, and in place of the interposed plate P
using sometimes a mere wire, and sometimes very large plates (1008.), and
also changing the terminal exciting plates Z and P, so that they were
sometimes wires only and at others of great size, still the results were
the same as those already obtained.

1025. In illustration of the effect of distance, an experiment like that
described with two exciting pairs and one intervening plate (1012.), fig.
93, was arranged so that the distance between the plates in the third cell
could be increased to six or eight inches, or diminished to the thickness
of a piece of intervening bibulous paper. Still the result was the same in
both cases, the effect not being sensibly greater, when the plates were
merely separated by the paper, than when a great way apart; so that the
principal opposition to the current in this case does not depend upon the
_quantity_ of intervening electrolytic conductor, but on the _relation of
its elements to the intensity of the current_, or to the chemical nature of
the electrodes and the surrounding fluids.

1026. When the acid was sulphuric acid, _increasing its strength_ in any of
the cells, caused no change in the effects; it did not produce a more
intense current in the exciting cells (908.), or cause the current produced
to traverse the decomposing cells more freely. But if to very weak
sulphuric acid a few drops of nitric acid were added, then either one or
other of those effects could be produced; and, as might be expected in a
case like this, where the exciting or conducting action bore a _direct_
reference to the acid itself, increasing the strength of this (the nitric
acid), also increased its powers.

1027. The _nature of the interposed plate_ was now varied to show its
relation to the phenomena either of excitation or retardation, and
amalgamated zinc was first substituted for platina. On employing one
voltaic pair and one interposed zinc plate, fig. 100, there was as powerful
a current, apparently, as if the interposed zinc plate was away. Hydrogen
was evolved against P in cell ii, and against the side of the second zinc
in cell i; but no gas appeared against the side of the zinc in cell ii, nor
against the zinc in cell i.

1028. On interposing two amalgamated zinc plates, fig. 101, instead of one,
there was still a powerful current, but interference had taken place. On
using three intermediate zinc plates, fig. 102, there was still further
retardation, though a good current of electricity passed.

1029. Considering the retardation as due to the inaction of the amalgamated
zinc upon the dilute acid, in consequence of the slight though general
effect of diminished chemical power produced by the mercury on the surface,
and viewing this inaction as the circumstance which rendered it necessary
that each plate should have its tendency to decompose water assisted
slightly by the electric current, it was expected that plates of the metal
in the unamalgamated state would probably not require such assistance, and
would offer no sensible impediment to the passing of the current. This
expectation was fully realized in the use of two and three interposed
unamalgamated plates. The electric current passed through them as freely as
if there had been no such plates in the way. They offered no obstacle,
because they could decompose water without the current; and the latter had
only to give direction to a part of the forces, which would have been
active whether it had passed or not.

1030. Interposed plates of copper were then employed. These seemed at first
to occasion no obstruction, but after a few minutes the current almost
entirely ceased. This effect appears due to the surfaces taking up that
peculiar condition (1010.) by which they tend to produce a reverse current;
for when one or more of the plates were turned round, which could easily be
effected with the couronne des tasses form of experiment, fig. 90, then the
current was powerfully renewed for a few moments, and then again ceased.
Plates of platina and copper, arranged as a voltaic pile with dilute
sulphuric acid, could not form a voltaic trough competent to act for more
than a _few_ minutes, because of this peculiar counteracting effect.

1031. All these effects of retardation, exhibited by decomposition against
surfaces for which the evolved elements have more or less affinity, or are
altogether deficient in attraction, show generally, though beautifully, the
chemical relations and source of the current, and also the balanced state
of the affinities at the places of excitation and decomposition. In this
way they add to the mass of evidence in favour of the identity of the two;
for they demonstrate, as it were, the antagonism of the _chemical powers_
at the electromotive part with the _chemical powers_ at the interposed
parts; they show that the first are _producing_ electric effects, and the
second _opposing_ them; they bring the two into direct relation; they prove
that either can determine the other, thus making what appears to be cause
and effect convertible, and thereby demonstrating that both chemical and
electrical action are merely two exhibitions of one single agent or power
(916. &c.).

1032. It is quite evident, that as water and other electrolytes can conduct
electricity without suffering decomposition (986.), when the electricity is
of sufficiently low intensity, it may not be asserted as absolutely true in
all cases, that whenever electricity passes through an electrolyte, it
produces a definite effect of decomposition. But the quantity of
electricity which can pass in a given time through an electrolyte without
causing decomposition, is so small as to bear no comparison to that
required in a case of very moderate decomposition, and with electricity
above the intensity required for electrolyzation, I have found no sensible
departure as yet from the law of _definite electrolytic action_ developed
in the preceding series of these Researches (783. &c.).

1033. I cannot dismiss this division of the present Paper without making a
reference to the important experiments of M. Aug. De la Rive on the effects
of interposed plates[A]. As I have had occasion to consider such plates
merely as giving rise to new decompositions, and in that way only causing
obstruction to the passage of the electric current, I was freed from the
necessity of considering the peculiar effects described by that
philosopher. I was the more willing to avoid for the present touching upon
these, as I must at the same time have entered into the views of Sir
Humphry Davy upon the same subject[B] and also those of Marianini[C] and
Hitter[D], which are connected with it.

  [A] Annales de Chimie, tom. xxviii. p 190; and Mémoires de Génève.

  [B] Philosophical Transactions, 1826, p. 413.

  [C] Annales de Chimie, tom. xxxiii. pp. 117, 119, &c.

  [D] Journal de Physique, tom. lvii. pp. 319, 350.


¶ v. _General Remarks on the active Voltaic Battery._

1034. When the ordinary voltaic battery is brought into action, its very
activity produces certain effects, which re-act upon it, and cause serious
deterioration of its power. These render it an exceedingly inconstant
instrument as to the _quantity_ of effect which it is capable of producing.
They are already, in part, known and understood; but as their importance,
and that of certain other coincident results, will be more evident by
reference to the principles and experiments already stated and described, I
have thought it would be useful, in this investigation of the voltaic pile,
to notice them briefly here.

1035. When the battery is in action, it causes such substances to be formed
and arranged in contact with the plates as very much weaken its power, or
even tend to produce a counter current. They are considered by Sir Humphry
Davy as sufficient to account for the phenomena of Ritter's secondary
piles, and also for the effects observed by M.A. De la Rive with interposed
platina plates[A].

  [A] Philosophical Transactions, 1826, p. 113.

1036. I have already referred to this consequence (1003.), as capable, in
some cases, of lowering the force of the current to one-eighth or one-tenth
of what it was at the first moment, and have met with instances in which
its interference was very great. In an experiment in which one voltaic pair
and one interposed platina plate were used with dilute sulphuric acid in
the cells fig. 103, the wires of communication were so arranged, that the
end of that marked 3 could be placed at pleasure upon paper moistened in
the solution of iodide of potassium at _x_, or directly upon the platina
plate there. If, after an interval during which the circuit had not been
complete, the wire 3 were placed upon the paper, there was evidence of a
current, decomposition ensued, and the galvanometer was affected. If the
wire 3 were made to touch the metal of _p_, a comparatively strong sudden
current was produced, affecting the galvanometer, but lasting only for a
moment; the effect at the galvanometer ceased, and if the wire 3 were
placed on the paper at _x_, no signs of decomposition occurred. On raising
the wire 3, and breaking the circuit altogether for a while, the apparatus
resumed its first power, requiring, however, from five to ten minutes for
this purpose; and then, as before, on making contact between 3 and _p_,
there was again a momentary current, and immediately all the effects
apparently ceased.

1037. This effect I was ultimately able to refer to the state of the film
of fluid in contact with the zinc plate in cell i. The acid of that film is
instantly neutralized by the oxide formed; the oxidation of the zinc
cannot, of course, go on with the same facility as before; and the chemical
action being thus interrupted, the voltaic action diminishes with it. The
time of the rest was required for the diffusion of the liquid, and its
replacement by other acid. From the serious influence of this cause in
experiments with single pairs of plates of different metals, in which I was
at one time engaged, and the extreme care required to avoid it, I cannot
help feeling a strong suspicion that it interferes more frequently and
extensively than experimenters are aware of, and therefore direct their
attention to it.

1038. In considering the effect in delicate experiments of this source of
irregularity of action, in the voltaic apparatus, it must be remembered
that it is only that very small portion of matter which is directly in
contact with the oxidizable metal which has to be considered with reference
to the change of its nature; and this portion is not very readily displaced
from its position upon the surface of the metal (582. 605.), especially if
that metal be rough and irregular. In illustration of this effect, I will
quote a remarkable experiment. A burnished platina plate (569.) was put
into hot strong sulphuric acid for an instant only: it was then put into
distilled water, moved about in it, taken out, and wiped dry: it was put
into a second portion of distilled water, moved about in it, and again
wiped: it was put into a third portion of distilled water, in which it was
moved about for nearly eight seconds; it was then, without wiping, put into
a fourth portion of distilled water, where it was allowed to remain five
minutes. The two latter portions of water were then tested for sulphuric
acid; the third gave no sensible appearance of that substance, but the
fourth gave indications which were not merely evident, but abundant for the
circumstances under which it had been introduced. The result sufficiently
shows with what difficulty that portion of the substance which is in
_contact_ with the metal leaves it; and as the contact of the fluid formed
against the plate in the voltaic circuit must be as intimate and as perfect
as possible, it is easy to see how quickly and greatly it must vary from
the general fluid in the cells, and how influential in diminishing the
force of the battery this effect must be.

1039. In the ordinary voltaic pile, the influence of this effect will occur
in all variety of degrees. The extremities of a trough of twenty pairs of
plates of Wollaston's construction were connected with the
volta-electrometer, fig. 66. (711.), of the Seventh Series of these
Researches, and after five minutes the number of bubbles of gas issuing
from the extremity of the tube, in consequence of the decomposition of the
water, noted. Without moving the plates, the acid between the copper and
zinc was agitated by the introduction of a feather. The bubbles were
immediately evolved more rapidly, above twice the number being produced in
the same portion of time as before. In this instance it is very evident
that agitation by a feather must have been a very imperfect mode of
restoring the acid in the cells against the plates towards its first equal
condition; and yet imperfect as the means were, they more than doubled the
power of the battery. The _first effect_ of a battery which is known to be
so superior to the degree of action which the battery can sustain, is
almost entirely due to the favourable condition of the acid in contact with
the plates.

1040. A _second_ cause of diminution in the force of the voltaic battery,
consequent upon its own action, is that extraordinary state of the surfaces
of the metals (969.) which was first described, I believe, by Ritter[A], to
which he refers the powers of his secondary piles, and which has been so
well experimented upon by Marianini, and also by A. De la Rive. If the
apparatus, fig. 103. (1096.), be left in action for an hour or two, with
the wire 3 in contact with the plate _p_, so as to allow a free passage for
the current, then, though the contact be broken for ten or twelve minutes,
still, upon its renewal, only a feeble current will pass, not at all equal
in force to what might be expected. Further, if P^{1} and P^{2} be
connected by a metal wire, a powerful momentary current will pass from
P^{2} to P^{1} through the acid, and therefore in the reverse direction to
that produced by the action of the zinc in the arrangement; and after this
has happened, the general current can pass through the whole of the system
as at first, but by its passage again restores the plates P^{2} and P^{1}
into the former opposing condition. This, generally, is the fact described
by Ritter, Marianini, and De la Rive. It has great opposing influence on
the action of a pile, especially if the latter consist of but a small
number of alternations, and has to pass its current through many
interpositions. It varies with the solution in which the interposed plates
are immersed, with the intensity of the current, the strength of the pile,
the time of action, and especially with accidental discharges of the plates
by inadvertent contacts or reversions of the plates during experiments, and
must be carefully watched in every endeavour to trace the source, strength,
and variations of the voltaic current. Its effect was avoided in the
experiments already described (1036. &c.), by making contact between the
plates P^{1} and P^{2} before the effect dependent upon the state of the
solution in contact with the zinc plate was observed, and by other
precautions.

  [A] Journal de Physique, lvii. p. 349.

1041. When an apparatus like fig. 98. (1017.) with several platina plates
was used, being connected with a battery able to force a current through
them, the power which they acquired, of producing a reversed current, was
very considerable.

1042. _Weak and exhausted charges_ should never be used at the same time
with _strong and fresh ones_ in the different cells of a trough, or the
different troughs of a battery: the fluid in all the cells should be alike,
else the plates in the weaker cells, in place of assisting, retard the
passage of the electricity generated in, and transmitted across, the
stronger cells. Each zinc plate so circumstanced has to be assisted in
decomposing power before the whole current can pass between it and the
liquid. So, that, if in a battery of fifty pairs of plates, ten of the
cells contain a weaker charge than the others, it is as if ten decomposing
plates were opposed to the transit of the current of forty pairs of
generating plates (1031.). Hence a serious loss of force, and hence the
reason why, if the ten pairs of plates were removed, the remaining forty
pairs would be much more powerful than the whole fifty.

1043. Five similar troughs, of ten pairs of plates each, were prepared,
four of them with a good uniform charge of acid, and the fifth with the
partially neutralized acid of a used battery. Being arranged in right
order, and connected with a volta-electrometer (711.), the whole fifty
pairs of plates yielded 1.1 cubic inch of oxygen and hydrogen in one
minute: but on moving one of the connecting wires so that only the four
well-charged troughs should be included in the circuit, they produced with
the same volta-electrometer 8.4 cubical inches of gas in the same time.
Nearly seven-eighths of the power of the four troughs had been lost,
therefore, by their association with the fifth trough.

1044. The same battery of fifty pairs of plates, after being thus used, was
connected with a volta-electrometer (711.), so that by quickly shifting the
wires of communication, the current of the whole of the battery, or of any
portion of it, could be made to pass through the instrument for given
portions of time in succession. The whole of the battery evolved 0.9 of a
cubic inch of oxygen and hydrogen in half a minute; the forty plates
evolved 4.6 cubic inches in the same time; the whole then evolved 1 cubic
inch in the half-minute; the ten weakly charged evolved 0.4 of a cubic inch
in the time given: and finally the whole evolved 1.15 cubic inch in the
standard time. The order of the observations was that given: the results
sufficiently show the extremely injurious effect produced by the mixture of
strong and weak charges in the same battery[A].

  [A] The gradual increase in the action of the whole fifty pairs of
  plates was due to the elevation of temperature in the weakly charged
  trough by the passage of the current, in consequence of which the
  exciting energies of the fluid within were increased.

1045. In the same manner associations of _strong and weak_ pairs of plates
should be carefully avoided. A pair of copper and platina plates arranged
in _accordance_ with a pair of zinc and platina plates in dilute sulphuric
acid, were found to stop the action of the latter, or even of two pairs of
the latter, as effectually almost as an interposed plate of platina
(1011.), or as if the copper itself had been platina. It, in fact, became
an interposed decomposing plate, and therefore a retarding instead of an
assisting pair.

1046. The _reversal_, by accident or otherwise, of the plates in a battery
has an exceedingly injurious effect. It is not merely the counteraction of
the current which the reversed plates can produce, but their effect also in
retarding even as indifferent plates, and requiring decomposition to be
effected upon their surface, in _accordance_ with the course of the
current, before the latter can pass. They oppose the current, therefore, in
the first place, as interposed platina plates would do (1011-1018.); and to
this they add a force of opposition as counter-voltaic plates. I find that,
in a series of four pairs of zinc and platina plates in dilute sulphuric
acid, if one pair be reversed, it very nearly neutralizes the power of the
whole.

1047. There are many other causes of reaction, retardation, and
irregularity in the voltaic battery. Amongst them is the not unusual one of
precipitation of copper upon the zinc in the cells, the injurious effect of
which has before been adverted to (1006.). But their interest is not
perhaps sufficient to justify any increase of the length of this paper,
which is rather intended to be an investigation of the theory of the
voltaic pile than a particular account of its practical application[A].

  [A] For further practical results relating to these points of the
  philosophy of the voltaic battery, see Series X. § 17.
  1163.--1160.--_Dec. 1838._

_Note_.--Many of the views and experiments in this Series of my
Experimental Researches will be seen at once to be corrections and
extensions of the theory of electro-chemical decomposition, given in the
Fifth and Seventh Series of these Researches. The expressions I would now
alter are those which concern the independence of the evolved elements in
relation to the poles or electrodes, and the reference of their evolution
to powers entirely internal (524. 537. 661.). The present paper fully shows
my present views; and I would refer to paragraphs 891. 904. 910. 917. 918.
947. 963. 1007. 1031. &c., as stating what they are. I hope this note will
be considered as sufficient in the way of correction at present; for I
would rather defer revising the whole theory of electro-chemical
decomposition until I can obtain clearer views of the way in which the
power under consideration can appear at one time as associated with
particles giving them their chemical attraction, and at another as free
electricity (493. 957.).--M.F.

_Royal Institution,
March 31st, 1834._




NINTH SERIES.


§ 15. _On the influence by induction of an Electric Current on itself:--and
on the inductive action of Electric Currents generally._

Received December 18, 1834,--Read January 29, 1835.


1048. The following investigations relate to a very remarkable inductive
action of electric currents, or of the different parts of the same current
(74.), and indicate an immediate connexion between such inductive action
and the direct transmission of electricity through conducting bodies, or
even that exhibited in the form of a spark.

1049. The inquiry arose out of a fact communicated to me by Mr. Jenkin,
which is as follows. If an ordinary wire of short length be used as the
medium of communication between the two plates of an electromotor
consisting of a single pair of metals, no management will enable the
experimenter to obtain an electric shock from this wire; but if the wire
which surrounds an electro-magnet be used, a shock is felt each time the
contact with the electromotor is broken, provided the ends of the wire be
grasped one in each hand.

1050. Another effect is observed at the same time, which has long been
known to philosophers, namely, that a bright electric spark occurs at the
place of disjunction.

1051. A brief account of these results, with some of a corresponding
character which I had observed in using long wires, was published in the
Philosophical Magazine for 1834[A]; and I added to them some observations
on their nature. Further investigations led me to perceive the inaccuracy
of my first notions, and ended in identifying these effects with the
phenomena of induction which I had been fortunate enough to develop in the
First Series of these Experimental Researches (1.-59.)[B]. Notwithstanding
this identity, the extension and the results supply, lead me to believe
that they will be found worthy of the attention of the Royal Society.

  [A] Vol. v. pp. 349, 444.

  [B] Philosophical Transactions, 1832, p. 126.

1052. The _electromotor_ used consisted of a cylinder of zinc introduced
between the two parts of a double cylinder of copper, and preserved from
metallic contact in the usual way by corks. The zinc cylinder was eight
inches high and four inches in diameter. Both it and the copper cylinder
were supplied with stiff wires, surmounted by cups containing mercury; and
it was at these cups that the contacts of wires, helices, or
electro-magnets, used to complete the circuit, were made or broken. These
cups I will call G and E throughout the rest of this paper (1079.).

1053. Certain _helices_ were constructed, some of which it will be
necessary to describe. A pasteboard tube had four copper wires, one
twenty-fourth of an inch in thickness, wound round it, each forming a helix
in the same direction from end to end: the convolutions of each wire were
separated by string, and the superposed helices prevented from touching by
intervening calico. The lengths of the wires forming the helices were 48,
49.5, 48, and 45 feet. The first and third wires were united together so as
to form one consistent helix of 96 feet in length; and the second and
fourth wires were similarly united to form a second helix, closely
interwoven with the first, and 94.5 feet in length. These helices may be
distinguished by the numbers i and ii. They were carefully examined by a
powerful current of electricity and a galvanometer, and found to have no
communication with each other.

1054. Another helix was constructed upon a similar pasteboard tube, two
lengths of the same copper wire being used, each forty-six feet long. These
were united into one consistent helix of ninety-two feet, which therefore
was nearly equal in value to either of the former helices, but was not in
close inductive association with them. It may be distinguished by the
number iii.

1055. A fourth helix was constructed of very thick copper wire, being
one-fifth of an inch in diameter; the length of wire used was seventy-nine
feet, independent of the straight terminal portions.

1056. The principal _electro-magnet_ employed consisted of a cylindrical
bar of soft iron twenty-five inches long, and one inch and three quarters
in diameter, bent into a ring, so that the ends nearly touched, and
surrounded by three coils of thick copper wire, the similar ends of which
were fastened together; each of these terminations was soldered to a copper
rod, serving as a conducting continuation of the wire. Hence any electric
current sent through the rods was divided in the helices surrounding the
ring, into three parts, all of which, however, moved in the same direction.
The three wires may therefore be considered as representing one wire, of
thrice the thickness of the wire really used.

1057. Other electro-magnets could be made at pleasure by introducing a soft
iron rod into any of the helices described (1053, &c.).

1058. The _galvanometer_ which I had occasion to use was rough in its
construction, having but one magnetic needle, and not at all delicate in
its indications.

1059. The effects to be considered _depend on the conductor_ employed to
complete the communication between the zinc and copper plates of the
electromotor; and I shall have to consider this conductor under four
different forms: as the helix of an electro-magnet (1056); as an ordinary
helix (1053, &c.); as a _long_ extended wire, having its course such that
the parts can exert little or no mutual influence; and as a _short_ wire.
In all cases the conductor was of copper.

1060. The peculiar effects are best shown by the _electro-magnet_ (1056.).
When it was used to complete the communication at the electromotor, there
was no sensible spark on _making_ contact, but on _breaking_ contact there
was a very large and bright spark, with considerable combustion of the
mercury. Then, again, with respect to the shock: if the hands were
moistened in salt and water, and good contact between them and the wires
retained, no shock could be felt upon _making_ contact at the electromotor,
but a powerful one on _breaking_ contact.

1061. When the _helix_ i or iii (1053, &c.) was used as the connecting
conductor, there was also a good spark on breaking contact, but none
(sensibly) on making contact. On trying to obtain the shock from these
helices, I could not succeed at first. By joining the similar ends of i and
ii so as to make the two helices equivalent to one helix, having wire of
double thickness, I could just obtain the sensation. Using the helix of
thick wire (1055.) the shock was distinctly obtained. On placing the tongue
between two plates of silver connected by wires with the parts which the
hands had heretofore touched (1064.), there was a powerful shock on
_breaking_ contact, but none on _making_ contact.

1062. The power of producing these phenomena exists therefore in the simple
helix, as in the electro-magnet, although by no means in the same high
degree.

1063. On putting a bar of soft iron into the helix, it became an
electro-magnet (1057.), and its power was instantly and greatly raised. On
putting a bar of copper into the helix, no change was produced, the action
being that of the helix alone. The two helices i and ii, made into one
helix of twofold length of wire, produced a greater effect than either i or
ii alone.

1064. On descending from the helix to the mere _long wire_, the following
effects were obtained, A copper wire, 0.18 of an inch in diameter, and 132
feet in length, was laid out upon the floor of the laboratory, and used as
the connecting conductor (1059.); it gave no sensible spark on making
contact, but produced a bright one on breaking contact, yet not so bright
as that from the helix (1061.) On endeavouring to obtain the electric shock
at the moment contact was broken, I could not succeed so as to make it pass
through the hands; but by using two silver plates fastened by small wires
to the extremity of the principal wire used, and introducing the tongue
between those plates, I succeeded in obtaining powerful shocks upon the
tongue and gums, and could easily convulse a flounder, an eel, or a frog.
None of these effects could be obtained directly from the electromotor,
i.e. when the tongue, frog, or fish was in a similar, and therefore
comparative manner, interposed in the course of the communication between
the zinc and copper plates, separated everywhere else by the acid used to
excite the combination, or by air. The bright spark and the shock, produced
only on breaking contact, are therefore effects of the same kind as those
produced in a higher degree by the helix, and in a still higher degree by
the electro-magnet.

1065. In order to compare an extended wire with a helix, the helix i,
containing ninety-six feet, and ninety-six feet of the same-sized wire
lying on the floor of the laboratory, were used alternately as conductors:
the former gave a much brighter spark at the moment of disjunction than the
latter. Again, twenty-eight feet of copper wire were made up into a helix,
and being used gave a good spark on disjunction at the electromotor; being
then suddenly pulled out and again employed, it gave a much smaller spark
than before, although nothing but its spiral arrangement had been changed.

1066. As the superiority of a helix over a wire is important to the
philosophy of the effect, I took particular pains to ascertain the fact
with certainty. A wire of copper sixty-seven feet long was bent in the
middle so as to form a double termination which could be communicated with
the electromotor; one of the halves of this wire was made into a helix and
the other remained in its extended condition. When these were used
alternately as the connecting wire, the helix half gave by much the
strongest spark. It even gave a stronger spark than when it and the
extended wire were used conjointly as a double conductor.

1067. When a _short wire_ is used, _all_ these effects disappear. If it be
only two or three inches long, a spark can scarcely be perceived on
breaking the junction. If it be ten or twelve inches long and moderately
thick, a small spark may be more easily obtained. As the length is
increased, the spark becomes proportionately brighter, until from extreme
length the resistance offered by the metal as a conductor begins to
interfere with the principal result.

1068. The effect of elongation was well shown thus: 114 feet of copper
wire, one-eighteenth of an inch in diameter, were extended on the floor and
used as a conductor; it remained cold, but gave a bright spark on breaking
contact. Being crossed so that the two terminations were in contact near
the extremities, it was again used as a conductor, only twelve inches now
being included in the circuit: the wire became very hot from the greater
quantity of electricity passing through it, and yet the spark on breaking
contact was scarcely visible. The experiment was repeated with a wire
one-ninth of an inch in diameter and thirty-six feet long with the same
results.

1069. That the effects, and also the action, in all these forms of the
experiment are identical, is evident from the manner in which the former
can be gradually raised from that produced by the shortest wire to that of
the most powerful electro-magnet: and this capability of examining what
will happen by the most powerful apparatus, and then experimenting for the
same results, or reasoning from them, with the weaker arrangements, is of
great advantage in making out the true principles of the phenomena.

1070. The action is evidently dependent upon the wire which serves as a
conductor; for it varies as that wire varies in its length or arrangement.
The shortest wire may be considered as exhibiting the full effect of spark
or shock which the electromotor can produce by its own direct power; all
the additional force which the arrangements described can excite being due
to some affection of the current, either permanent or momentary, in the
wire itself. That it is a _momentary_ effect, produced only at the instant
of breaking contact, will be fully proved (1089. 1100.).

1071. No change takes place in the quantity or intensity of the current
during the time the latter is _continued_, from the moment after contact is
made, up to that previous to disunion, except what depends upon the
increased obstruction offered to the passage of the electricity by a long
wire as compared to a short wire. To ascertain this point with regard to
_quantity_, the helix i (1053.) and the galvanometer (1055.) were both made
parts of the metallic circuit used to connect the plates of a small
electromotor, and the deflection at the galvanometer was observed; then a
soft iron core was put into the helix, and as soon as the momentary effect
was over, and the needle had become stationary, it was again observed, and
found to stand exactly at the same division as before. Thus the quantity
passing through the wire when the current was continued was the same either
with or without the soft iron, although the peculiar effects occurring at
the moment of disjunction were very different in degree under such
variation of circumstances.

1072. That the quality of _intensity_ belonging to the constant current did
not vary with the circumstances favouring the peculiar results under
consideration, so as to yield an explanation of those results, was
ascertained in the following manner. The current excited by an electromotor
was passed through short wires, and its intensity tried by subjecting
different substances to its electrolyzing power (912. 966. &c.); it was
then passed through the wires of the powerful electro-magnet (1056.), and
again examined with respect to its intensity by the same means and found
unchanged. Again, the constancy of the _quantity_ passed in the above
experiment (1071.) adds further proof that the intensity could not have
varied; for had it been increased upon the introduction of the soft iron,
there is every reason to believe that the quantity passed in a given time
would also have increased.

1073. The fact is, that under many variations of the experiments, the
permanent current _loses_ in force as the effects upon breaking contact
become _exalted_. This is abundantly evident in the comparative experiments
with long and short wires (1068.); and is still more strikingly shown by
the following variation. Solder an inch or two in length of fine platina
wire (about one-hundredth of an inch in diameter) on to one end of the long
communicating wire, and also a similar length of the same platina wire on
to one end of the short communication; then, in comparing the effects of
these two communications, make and break contact between the platina
terminations and the mercury of the cup G or E (1079.). When the short wire
is used, the platina will be _ignited by the constant current_, because of
the quantity of electricity, but the spark on breaking contact will be
hardly visible; on using the longer communicating wire, which by
obstructing will diminish the current, the platina will remain cold whilst
the current passes, but give a bright spark at the moment it ceases: thus
the strange result is obtained of a diminished spark and shock from the
strong current, and increased effects from the weak one. Hence the spark
and shock at the moment of disjunction, although resulting from great
intensity and quantity, of the current _at that moment_, are no direct
indicators or measurers of the intensity or quantity of the constant
current previously passing, and by which they are ultimately produced.

       *       *       *       *       *

1074. It is highly important in using the spark as an indication, by its
relative brightness, of these effects, to bear in mind certain
circumstances connected with its production and appearance (958.). An
ordinary electric spark is understood to be the bright appearance of
electricity passing suddenly through an interval of air, or other badly
conducting matter. A voltaic spark is sometimes of the same nature, but,
generally, is due to the ignition and even combustion of a minute portion
of a good conductor; and that is especially the case when the electromotor
consists of but one or few pairs of plates. This can be very well observed
if either or both of the metallic surfaces intended to touch be solid and
pointed. The moment they come in contact the current passes; it heats,
ignites, and even burns the touching points, and the appearance is as if
the spark passed on making contact, whereas it is only a case of ignition
by the current, contact being previously made, and is perfectly analogous
to the ignition of a fine platina wire connecting the extremities of a
voltaic battery.

1075. When mercury constitutes one or both of the surfaces used, the
brightness of the spark is greatly increased. But as this effect is due to
the action on, and probable combustion of, the metal, such sparks must only
be compared with other sparks also taken from mercurial surfaces, and not
with such as may be taken, for instance, between surfaces of platina or
gold, for then the appearances are far less bright, though the same
quantity of electricity be passed. It is not at all unlikely that the
commonly occurring circumstance of combustion may affect even the duration
of the light; and that sparks taken between mercury, copper, or other
combustible bodies, will continue for a period sensibly longer than those
passing between platina or gold.

1076. When the end of a short clean copper wire, attached to one plate of
an electromotor, is brought down carefully upon a surface of mercury
connected with the other plate, a spark, almost continuous, can be
obtained. This I refer to a succession of effects of the following nature:
first, contact,--then ignition of the touching points,--recession of the
mercury from the mechanical results of the heat produced at the place of
contact, and the electro-magnetic condition of the parts at the moment[A],
--breaking of the contact and the production of the peculiar intense effect
dependent thereon,--renewal of the contact by the returning surface of the
undulating mercury,--and then a repetition of the same series of effects,
and that with such rapidity as to present the appearance of a continued
discharge. If a long wire or an electro-magnet be used as the connecting
conductor instead of a short wire, a similar appearance may be produced by
tapping the vessel containing the mercury and making it vibrate; but the
sparks do not usually follow each other so rapidly as to produce an
apparently continuous spark, because of the time required, when the long
wire or electro-magnet is used, both for the full development of the
current (1101. 1106.) and for its complete cessation.

  [A] Quarterly Journal of Science, vol. xii, p. 420.

1077. Returning to the phenomena in question, the first thought that arises
in the mind is, that the electricity circulates with something like
_momentum or inertia_ in the wire, and that thus a long wire produces
effects at the instant the current is stopped, which a short wire cannot
produce. Such an explanation is, however, at once set aside by the fact,
that the same length of wire produces the effects in very different
degrees, according as it is simply extended, or made into a helix, or forms
the circuit of an electro-magnet (1069.). The experiments to be adduced
(1089.) will still more strikingly show that the idea of momentum cannot
apply.

1078. The bright spark at the electromotor, and the shock in the arms,
appeared evidently to be due to _one_ current in the long wire, divided
into two parts by the double channel afforded through the body and through
the electromotor; for that the spark was evolved at the place of
disjunction with the electromotor, not by any direct action of the latter,
but by a force immediately exerted in the wire of communication, seemed to
be without doubt (1070.). It followed, therefore, that by using a better
conductor in place of the human body, the _whole_ of this extra current
might be made to pass at that place; and thus be separated from that which
the electromotor could produce by its immediate action, and its _direction_
be examined apart from any interference of the original and originating
current. This was found to be true; for on connecting the ends of the
principal wire together by a cross wire two or three feet in length,
applied just where the hands had felt the shock, the whole of the extra
current passed by the new channel, and then no better spark than one
producible by a short wire was obtained on disjunction at the electromotor.

1079. The _current_ thus separated was examined by galvanometers and
decomposing apparatus introduced into the course of this wire. I will
always speak of it as the current in the cross wire or wires, so that no
mistake, as to its place or origin, may occur. In the wood-cut, Z and C
represent the zinc and copper plates of the electromotor; G and E the cups
of mercury where contact is made or broken (1052.); A and B the
terminations of D, the long wire, the helix or the electro-magnet, used to
complete the circuit; N and P are the cross wires, which can either be
brought into contact at _x_, or else have a galvanometer (1058.) or an
electrolyzing apparatus (312. 316.) interposed there.

[Illustration]

The production of the _shock_ from the current in the cross wire, whether D
was a long extended wire, or a helix, or an electro-magnet, has been
already described (1060. 1061. 1064.).

1080. The _spark_ of the cross-wire current could be produced at _x_ in the
following manner: D was made an electro-magnet; the metallic extremities at
_x_ were held close together, or rubbed lightly against each other, whilst
contact was broken at G or E. When the communication was perfect at _x_,
little or no spark appeared at G or E. When the condition of vicinity at
_x_ was favourable for the result required, a bright spark would pass there
at the moment of disjunction, _none_ occurring at G and E: this spark was
the luminous passage of the extra current through the cross-wires. When
there was no contact or passage of current at _x_, then the spark appeared
at G or E, the extra current forcing its way through the electromotor
itself. The same results were obtained by the use of the helix or the
extended wire at D in place of the electro-magnet.

1081. On introducing a fine platina wire at _x_, and employing the
electro-magnet at D, no visible effects occurred as long as contact was
continued; but on breaking contact at G or E, the fine wire was instantly
ignited and fused. A longer or thicker wire could be so adjusted at _x_ as
to show ignition, without fusion, every time the contact was broken at G
or E.

1082. It is rather difficult to obtain this effect with helices or wires,
and for very simple reasons: with the helices i, ii, or iii, there was such
retardation of the electric current, from the length of wire used, that a
full inch of platina wire one-fiftieth of an inch in diameter could be
retained ignited at the cross-wires during the _continuance of contact_, by
the portion of electricity passing through it. Hence it was impossible to
distinguish the particular effects at the moments of making or breaking
contact from this constant effect. On using the thick wire helix (1055.),
the same results ensued.

1083. Proceeding upon the known fact that electric currents of great
quantity but low intensity, though able to ignite thick wires, cannot
produce that effect upon thin ones, I used a very fine platina wire at _x_,
reducing its diameter until a spark appeared at G or E, when contact was
broken there. A quarter of an inch of such wire might be introduced at _x_
without being ignited by the _continuance_ of contact at G or E; but when
contact was broken at either place, this wire became red-hot; proving, by
this method, the production of the induced current at that moment.

1084. _Chemical decomposition_ was next effected by the cross-wire current,
an electro-magnet being used at D, and a decomposing apparatus, with
solution of iodide of potassium in paper (1079.), employed at _x_. The
conducting power of the connecting system A B D was sufficient to carry all
the primary current, and consequently no chemical action took place at _x_
during the _continuance_ of contact at G and E; but when contact was
broken, there was instantly decomposition at _x_. The iodine appeared
against the wire N, and not against the wire P; thus demonstrating that the
current through the cross-wires, when contact was broken, was in the
_reverse direction_ to that marked by the arrow, or that which the
electromotor would have sent through it.

1085. In this experiment a bright spark occurs at the place of disjunction,
indicating that only a small part of the extra current passed the apparatus
at _x_, because of the small conducting power of the latter.

1086. I found it difficult to obtain the chemical effects with the simple
helices and wires, in consequence of the diminished inductive power of
these arrangements, and because of the passage of a strong constant current
at _x_ whenever a very active electromotor was used (1082).

1087. The most instructive set of results was obtained, however, when the
_galvanometer_ was introduced at _x_. Using an electro-magnet at D, and
continuing contact, a current was then indicated by the deflection,
proceeding from P to N, in the direction of the arrow; the cross-wire
serving to carry one part of the electricity excited by the electromotor,
and that part of the arrangement marked A B D, the other and far greater
part, as indicated by the arrows. The magnetic needle was then forced back,
by pins applied upon opposite sides of its two extremities, to its natural
position when uninfluenced by a current; after which, contact being
_broken_ at G or E, it was deflected strongly in the opposite direction;
thus showing, in accordance with the chemical effects (1084), that the
extra current followed a course in the cross-wires _contrary_ to that
indicated by the arrow, i. e. contrary to the one produced by the direct
action of the electromotor[A].

  [A] It was ascertained experimentally, that if a strong current was
  passed through the galvanometer only, and the needle restrained in one
  direction as above in its natural position, when the current was
  stopped, no vibration of the needle in the opposite direction took
  place.

1088. With the _helix_ only (1061.), these effects could scarcely be
observed, in consequence of the smaller inductive force of this
arrangement, the opposed action from induction in the galvanometer wire
itself, the mechanical condition and tension of the needle from the effect
of blocking (1087.) whilst the current due to continuance of contact was
passing round it; and because of other causes. With the _extended wire_
(1064.) all these circumstances had still greater influence, and therefore
allowed less chance of success.

1089. These experiments, establishing as they did, by the quantity,
intensity, and even direction, a distinction between the primary or
generating current and the extra current, led me to conclude that the
latter was identical with the induced current described (6. 26. 74.) in the
First Series of these Researches; and this opinion I was soon able to bring
to proof, and at the same times obtained not the partial (1078.) but entire
separation of one current from the other.

1090. The double helix (1053.) was arranged so that it should form the
connecting wire between the plates of the electromotor, in being out of the
current, and its ends unconnected. In this condition it acted very well,
and gave a good spark at the time and place of disjunction. The opposite
ends of ii were then connected together so as to form an endless wire, i
remaining unchanged: but now _no spark_, or one scarcely sensible, could be
obtained from the latter at the place of disjunction. Then, again, the ends
of ii were held so nearly together that any current running round that
helix should be rendered visible as a spark; and in this manner a spark was
obtained from ii when the junction of i with the electromotor was broken,
in place of appearing at the disjoined extremity of i itself.

1091. By introducing a galvanometer or decomposing apparatus into the
circuit formed by the helix ii, I could easily obtain the deflections and
decomposition occasioned by the induced current due to the breaking contact
at helix i, or even to that occasioned by making contact of that helix with
the electromotor; the results in both cases indicating the contrary
directions of the two induced currents thus produced (26.).

1092. All these effects, except those of decomposition, were reproduced by
two extended long wires, not having the form of helices, but placed close
to each other; and thus it was proved that the _extra current_ could be
removed from the wire carrying the original current to a neighbouring wire,
and was at the same time identified, in direction and every other respect,
with the currents producible by induction (1089.). The case, therefore, of
the bright spark and shock on disjunction may now be stated thus: If a
current be established in a wire, and another wire, forming a complete
circuit, be placed parallel to the first, at the moment the current in the
first is stopped it induces a current in the _same_ direction in the
second, the first exhibiting then but a feeble spark; but if the second
wire be away, disjunction of the first wire induces a current in itself in
the same direction, producing a strong spark. The strong spark in the
single long wire or helix, at the moment of disjunction, is therefore the
equivalent of the current which would be produced in a neighbouring wire if
such second current were permitted.

1093. Viewing the phenomena as the results of the induction of electrical
currents, many of the principles of action, in the former experiments,
become far more evident and precise. Thus the different effects of short
wires, long wires, helices, and electro-magnets (1069.) may be
comprehended. If the inductive action of a wire a foot long upon a
collateral wire also a foot in length, be observed, it will be found very
small; but if the same current be sent through a wire fifty feet long, it
will induce in a neighbouring wire of fifty feet a far more powerful
current at the moment of making or breaking contact, each successive foot
of wire adding to the sum of action; and by parity of reasoning, a similar
effect should take place when the conducting wire is also that in which the
induced current is formed (74.): hence the reason why a long wire gives a
brighter spark on breaking contact than a short one (1068.), although it
carries much less electricity.

1094. If the long wire be made into a helix, it will then be still more
effective in producing sparks and shocks on breaking contact; for by the
mutual inductive action of the convolutions each aids its neighbour, and
will be aided in turn, and the sum of effect will be very greatly
increased.

1095. If an electro-magnet be employed, the effect will be still more
highly exalted; because the iron, magnetized by the power of the continuing
current, will lose its magnetism at the moment the current ceases to pass,
and in so doing will tend to produce an electric current in the wire around
it (37. 38.), in conformity with that which the cessation of current in the
helix itself also tends to produce.

1096. By applying the laws of the induction of electric currents formerly
developed (6. &c.), various new conditions of the experiments could be
devised, which by their results should serve as tests of the accuracy of
the view just given. Thus, if a long wire be doubled, so that the current
in the two halves shall have opposite actions, it ought not to give a
sensible spark at the moment of disjunction: and this proved to be the
case, for a wire forty feet long, covered with silk, being doubled and tied
closely together to within four inches of the extremities, when used in
that state, gave scarcely a perceptible spark; but being opened out and the
parts separated, it gave a very good one. The two helices i and ii being
joined at their similar ends, and then used at their other extremities to
connect the plates of the electromotor, thus constituted one long helix, of
which one half was opposed in direction to the other half: under these
circumstances it gave scarcely a sensible spark, even when the soft iron
core was within, although containing nearly two hundred feet of wire. When
it was made into one consistent helix of the same length of wire it gave a
very bright spark.

1097. Similar proofs can be drawn from the mutual inductive action of two
separate currents (1110.); and it is important for the general principles
that the consistent action of two such currents should be established.
Thus, two currents going in the same direction should, if simultaneously
stopped, aid each other by their relative influence; or if proceeding in
contrary directions, should oppose each other under similar circumstances.
I endeavoured at first to obtain two currents from two different
electromotors, and passing them through the helices i and ii, tried to
effect the disjunctions mechanically at the same moment. But in this I
could not succeed; one was always separated before the other, and in that
case produced little or no spark, its inductive power being employed in
throwing a current round the remaining complete circuit (1090.): the
current which was stopped last always gave a bright spark. If it were ever
to become needful to ascertain whether two junctions were accurately broken
at the same moment, these sparks would afford a test for the purpose,
having an infinitesimal degree of perfection.

1098. I was able to prove the points by other expedients. Two short thick
wires were selected to serve as terminations, by which contact could be
made or broken with the electromotor. The compound helix, consisting of i
and ii (1053.), was adjusted so that the extremities of the two helices
could be placed in communication with the two terminal wires, in such a
manner that the current moving through the thick wires should be divided
into two equal portions in the two helices, these portions travelling,
according to the mode of connexion, either in the same direction or in
contrary directions at pleasure. In this manner two streams could be
obtained, both of which could be stopped simultaneously, because the
disjunction could be broken at G or F by removing a single wire. When the
helices were in contrary directions, there was scarcely a sensible spark at
the place of disjunction; but when they were in accordance there was a very
bright one.

1099. The helix i was now used constantly, being sometimes associated, as
above, with helix ii in an according direction, and sometimes with helix
iii, which was placed at a little distance. The association i and ii, which
presented two currents able to affect each other by induction, because of
their vicinity, gave a brighter spark than the association i and iii, where
the two streams could not exert their mutual influence; but the difference
was not so great as I expected.

1100. Thus all the phenomena tend to prove that the effects are due to an
inductive action, occurring at the moment when the principal current is
stopped. I at one time thought they were due to an action continued during
the _whole time_ of the current, and expected that a steel magnet would
have an influence according to its position in the helix, comparable to
that of a soft iron bar, in assisting the effect. This, however, is not the
case; for hard steel, or a magnet in the helix, is not so effectual as soft
iron; nor does it make any difference how the magnet is placed in the
helix, and for very simple reasons, namely, that the effect does not depend
upon a permanent state of the core, but a _change of state_; and that the
magnet or hard steel cannot sink through such a difference of state as soft
iron, at the moment contact ceases, and therefore cannot produce an equal
effect in generating a current of electricity by induction (34. 37.).

       *       *       *       *       *

1101. As an electric current acts by induction with equal energy at the
moment of its commencement as at the moment of its cessation (10. 26.), but
in a contrary direction, the reference of the effects under examination to
an inductive action, would lead to the conclusion that corresponding
effects of an opposite nature must occur in a long wire, a helix, or an
electro-magnet, every time that _contact is made with_ the electromotor.
These effects will tend to establish a resistance for the first moment in
the long conductor, producing a result equivalent to the reverse of a shock
or a spark. Now it is very difficult to devise means fit for the
recognition of such negative results; but as it is probable that some
positive effect is produced at the time, if we knew what to expect, I think
the few facts bearing upon this subject with which I am acquainted are
worth recording.

1102. The electro-magnet was arranged with an electrolyzing apparatus at
_x_, as before described (1084.), except that the intensity of the chemical
action at the electromotor was increased until the electric current was
just able to produce the feeblest signs of decomposition whilst contact was
continued at G and E (1079.); (the iodine of course appearing against the
end of the cross wire P;) the wire N was also separated from A at _r_, so
that contact there could be made or broken at pleasure. Under these
circumstances the following set of actions was repeated several times:
contact was broken at _r_, then broken at G, next made at _r_, and lastly
renewed at G; thus any current from N to P due to _breaking_ of contact was
avoided, but any additional force to the current from P to N due to
_making_ contact could be observed. In this way it was found, that a much
greater decomposing effect (causing the evolution of iodine against P)
could be obtained by a few completions of contact than by the current which
could pass in a much longer time if the contact was _continued_. This I
attribute to the act of induction in the wire ABD at the moment of contact
rendering that wire a worse conductor, or rather retarding the passage of
the electricity through it for the instant, and so throwing a greater
quantity of the electricity which the electromotor could produce, through
the cross wire passage NP. The instant the induction ceased, ABD resumed
its full power of carrying a constant current of electricity, and could
have it highly increased, as we know by the former experiments (1060.) by
the opposite inductive action brought into activity at the moment contact
at Z or C was _broken_.

1103. A galvanometer was then introduced at _x_, and the deflection of the
needle noted whilst contact was continued at G and E: the needle was then
blocked as before in one direction (1087.), so that it should not return
when the current ceased, but remain in the position in which the current
could retain it. Contact at G or E was broken, producing of course no
visible effect; it was then renewed, and the needle was instantly
deflected, passing from the blocking pins to a position still further from
its natural place than that which the constant current could give, and thus
showing, by the temporary excess of current in this cross communication,
the temporary retardation in the circuit ABD.

1104. On adjusting a platina wire at _x_ (1081.) so that it should not be
ignited by the current passing through it whilst contact at G and E was
_continued_, and yet become red-hot by a current somewhat more powerful, I
was readily able to produce its ignition upon _making contact_, and again
upon _breaking contact_. Thus the momentary retardation in ABD on making
contact was again shown by this result, as well also as the opposite result
upon breaking contact. The two ignitions of the wire at _x_ were of course
produced by electric currents moving in opposite directions.

1105. Using the _helix_ only, I could not obtain distinct deflections at
_x_, due to the extra effect on making contact, for the reasons already
mentioned (1088.). By using a very fine platina wire there (1083.), I did
succeed in obtaining the igniting effect for making contact in the same
manner, though by no means to the same degree, as with the electro-magnet
(1104).

1106. We may also consider and estimate the effect on _making contact_, by
transferring the force of induction from the wire carrying the original
current to a lateral wire, as in the cases described (1090.); and we then
are sure, both by the chemical and galvanometrical results (1091.), that
the forces upon making and breaking contact, like action and reaction, are
equal in their strength but contrary in their direction. If, therefore, the
effect on making contact resolves itself into a mere retardation of the
current at the first moment of its existence, it must be, in its degree,
equivalent to the high exaltation of that same current at the moment
contact is broken.

1107. Thus the case, under the circumstances, is, that the intensity and
quantity of electricity moving in a current are smaller when the current
commences or is increased, and greater when it diminishes or ceases, than
they would be if the inductive action occurring at these moments did not
take place; or than they are in the original current wire if the inductive
action be transferred from that wire to a collateral one (1090.).

1108. From the facility of transference to neighbouring wires, and from the
effects generally, the inductive forces appear to be lateral, i.e. exerted
in a direction perpendicular to the direction of the originating and
produced currents: and they also appear to be accurately represented by the
magnetic curves, and closely related to, if not identical with, magnetic
forces.

1109. There can be no doubt that the current in one part of a wire can act
by induction upon other parts of the _same_ wire which are lateral to the
first, i.e. in the same vertical section (74.), or in the parts which are
more or less oblique to it (1112.), just as it can act in producing a
current in a neighbouring wire or in a neighbouring coil of the same wire.
It is this which gives the appearance of the current acting upon itself:
but all the experiments and all analogy tend to show that the elements (if
I may so say) of the currents do not act upon themselves, and so cause the
effect in question, but produce it by exciting currents in conducting
matter which is lateral to them.

1110. It is possible that some of the expressions I have used may seem to
imply, that the inductive action is essentially the action of one current
upon another, or of one element of a current upon another element of the
same current. To avoid any such conclusion I must explain more distinctly
my meaning. If an endless wire be taken, we have the means of generating a
current in it which shall run round the circuit without adding any
electricity to what was previously in the wire. As far as we can judge, the
electricity which appears as a current is the same as that which before was
quiescent in the wire; and though we cannot as yet point out the essential
condition of difference of the electricity at such times, we can easily
recognize the two states. Now when a current acts by induction upon
conducting matter lateral to it, it probably acts upon the electricity in
that conducting matter whether it be in the form of a _current_ or
_quiescent_, in the one case increasing or diminishing the current
according to its direction, in the other producing a current, and the
_amount_ of the inductive action is probably the same in both cases. Hence,
to say that the action of induction depended upon the mutual relation of
two or more currents, would, according to the restricted sense in which the
term current is understood at present (283. 517. 667.), be an error.

1111. Several of the effects, as, for instances, those with helices(1066.),
with according or counter currents (1097. 1098.), and those on the
production of lateral currents (1090.), appeared to indicate that a current
could produce an effect of induction in a neighbouring wire more readily
than in its own carrying wire, in which case it might be expected that some
variation of result would be produced if a bundle of wires were used as a
conductor instead of a single wire. In consequence the following
experiments were made. A copper wire one twenty-third of an inch in
diameter was cut into lengths of five feet each, and six of these being
laid side by side in one bundle, had their opposite extremities soldered to
two terminal pieces of copper. This arrangement could be used as a
discharging wire, but the general current could be divided into six
parallel streams, which might be brought close together, or, by the
separation of the wires, be taken more or less out of each other's
influence. A somewhat brighter spark was, I think, obtained on breaking
contact when the six wires were close together than when held asunder.

1112. Another bundle, containing twenty of these wires, was eighteen feet
long: the terminal pieces were one-fifth of an inch in diameter, and each
six inches long. This was compared with nineteen feet in length of copper
wire one-fifth of an inch in diameter. The bundle gave a smaller spark on
breaking contact than the latter, even when its strands were held together
by string: when they were separated, it gave a still smaller spark. Upon
the whole, however, the diminution of effect was not such as I expected:
and I doubt whether the results can be considered as any proof of the truth
of the supposition which gave rise to them.

1113. The inductive force by which two elements of one current (1109.
1110.) act upon each other, appears to diminish as the line joining them
becomes oblique to the direction of the current and to vanish entirely when
it is parallel. I am led by some results to suspect that it then even
passes into the repulsive force noticed by Ampère[A]; which is the cause of
the elevations in mercury described by Sir Humphry Davy[B], and which again
is probably directly connected with the quality of intensity.

  [A] Recueil d'Observations Electro-Dynamiques, p. 285.

  [B] Philosophical Transactions, 1823, p. 155.

1114. Notwithstanding that the effects appear only at the making and
breaking of contact, (the current remaining unaffected, seemingly, in the
interval,) I cannot resist the impression that there is some connected and
correspondent effect produced by this lateral action of the elements of the
electric stream during the time of its continuance (60. 242.). An action of
this kind, in fact, is evident in the magnetic relations of the parts of
the current. But admitting (as we may do for the moment) the magnetic
forces to constitute the power which produces such striking and different
results at the commencement and termination of a current, still there
appears to be a link in the chain of effects, a wheel in the physical
mechanism of the action, as yet unrecognised. If we endeavour to consider
electricity and magnetism as the results of two forces of a physical agent,
or a peculiar condition of matter, exerted in determinate directions
perpendicular to each other, then, it appears to me, that we must consider
these two states or forces as convertible into each other in a greater or
smaller degree; i.e. that an element of an electric current has not a
determinate electric force and a determinate magnetic force constantly
existing in the same ratio, but that the two forces are, to a certain
degree, convertible by a process or change of condition at present unknown
to us. How else can a current of a given intensity and quantity be able, by
its direct action, to sustain a state which, when allowed to react, (at the
cessation of the original current,) shall produce a second current, having
an intensity and quantity far greater than the generating one? This cannot
result from a direct reaction of the electric force; and if it result from
a change of electrical into magnetic force, and a reconversion back again,
it will show that they differ in something more than mere direction, as
regards _that agent_ in the conducting wire which constitutes their
immediate cause.

1115. With reference to the appearance, at different times, of the contrary
effects produced by the making and breaking contact, and their separation
by an intermediate and indifferent state, this separation is probably more
apparent than real. If the conduction of electricity be effected by
vibrations (283.), or by any other mode in which opposite forces are
successively and rapidly excited and neutralized, then we might expect a
peculiar and contrary development of force at the commencement and
termination of the periods during which the conducting action should last
(somewhat in analogy with the colours produced at the outside of an
imperfectly developed solar spectrum): and the intermediate actions,
although not sensible in the same way, may be very important and, for
instance, perhaps constitute the very essence of conductibility. It is by
views and reasons such as these, which seem to me connected with the
fundamental laws and facts of electrical science, that I have been induced
to enter, more minutely than I otherwise should have done, into the
experimental examination of the phenomena described in this paper.

1116. Before concluding, I may briefly remark, that on using a voltaic
battery of fifty pairs of plates instead of a single pair (1052.), the
effects were exactly of the same kind. The spark on making contact, for the
reasons before given, was very small (1101. 1107.); that on breaking
contact, very excellent and brilliant. The _continuous_ discharge did not
seem altered in character, whether a short wire or the powerful
electro-magnet were used as a connecting discharger.

1117. The effects produced at the commencement and end of a current, (which
are separated by an interval of time when that current is supplied from a
voltaic apparatus,) must occur at the same moment when a common electric
discharge is passed through a long wire. Whether, if happening accurately
at the same moment, they would entirely neutralize each other, or whether
they would not still give some definite peculiarity to the discharge, is a
matter remaining to be examined; but it is very probable that the peculiar
character and pungency of sparks drawn from a long wire depend in part upon
the increased intensity given at the termination of the discharge by the
inductive action then occurring.

1118. In the wire of the helix of magneto-electric machines, (as, for
instance, in Mr. Saxton's beautiful arrangement,) an important influence of
these principles of action is evidently shown. From the construction of the
apparatus the current is permitted to move in a complete metallic circuit
of great length during the first instants of its formation: it gradually
rises in strength, and is then suddenly stopped by the breaking of the
metallic circuit; and thus great intensity is given _by induction_ to the
electricity, which at that moment passes (1064. 1060.). This intensity is
not only shown by the brilliancy of the spark and the strength of the
shock, but also by the necessity which has been experienced of
well-insulating the convolutions of the helix, in which the current is
formed: and it gives to the current a force at these moments very far above
that which the apparatus could produce if the principle which forms the
subject of this paper were not called into play.

_Royal Institution,
December 8th, 1834._




TENTH SERIES.


§ 16. _On an improved form of the Voltaic Battery._ § 17. _Some practical
results respecting the construction and use of the Voltaic Battery._

Received June 16,--Read June 18, 1835.


1119. I Have lately had occasion to examine the voltaic trough practically,
with a view to improvements in its construction and use; and though I do
not pretend that the results have anything like the importance which
attaches to the discovery of a new law or principle, I still think they are
valuable, and may therefore, if briefly told, and in connexion with former
papers, be worthy the approbation of the Royal Society.

§ 16. _On an improved form of the Voltaic Battery._


1120. In a simple voltaic circuit (and the same is true of the battery) the
chemical forces which, during their activity, give power to the instrument,
are generally divided into two portions; one of these is exerted locally,
whilst the other is transferred round the circle (947. 996.); the latter
constitutes the electric current of the instrument, whilst the former is
altogether lost or wasted. The ratio of these two portions of power may be
varied to a great extent by the influence of circumstances: thus, in a
battery not closed, _all_ the action is local; in one of the ordinary
construction, _much_ is in circulation when the extremities are in
communication: and in the perfect one, which I have described (1001.),
_all_ the chemical power circulates and becomes electricity. By referring
to the quantity of zinc dissolved from the plates (865. 1120.), and the
quantity of decomposition effected in the volta-electrometer (711. 1126,)
or elsewhere, the proportions of the local and transferred actions under
any particular circumstances can be ascertained, and the efficacy of the
voltaic arrangement, or the waste of chemical power at its zinc plates, be
accurately determined.

1121. If a voltaic battery were constructed of zinc and platina, the latter
metal surrounding the former, as in the double copper arrangement, and the
whole being excited by dilute sulphuric acid, then no insulating divisions
of glass, porcelain or air would be required between the contiguous platina
surfaces; and, provided these did not touch metallically, the same acid
which, being between the zinc and platina, would excite the battery into
powerful action, would, between the two surfaces of platina, produce no
discharge of the electricity, nor cause any diminution of the power of the
trough. This is a necessary consequence of the resistance to the passage of
the current which I have shown occurs at the place of decomposition (1007.
1011.); for that resistance is fully able to stop the current, and
therefore acts as insulation to the electricity of the contiguous plates,
inasmuch as the current which tends to pass between them never has a higher
intensity than that due to the action of a single pair.

1122. If the metal surrounding the zinc be copper (1045.), and if the acid
be nitro-sulphuric acid (1020.), then a slight discharge between the two
contiguous coppers does take place, provided there be no other channel open
by which the forces may circulate; but when such a channel is permitted,
the return or back discharge of which I speak is exceedingly diminished, in
accordance with the principles laid down in the Eighth Series of these
Researches.

1123. Guided by these principles I was led to the construction of a voltaic
trough, in which the coppers, passing round both surfaces of the zincs, as
in Wollaston's construction, should not be separated from each other except
by an intervening thickness of paper, or in some other way, so as to
prevent metallic contact, and should thus constitute an instrument compact,
powerful, economical, and easy of use. On examining, however, what had been
done before, I found that the new trough was in all essential respects the
same as that invented and described by Dr. Hare, Professor in the
University of Pennsylvania, to whom I have great pleasure in referring it.

1124. Dr. Hare has fully described his trough[A]. In it the contiguous
copper plates are separated by thin veneers of wood, and the acid is poured
on to, or off, the plates by a quarter revolution of an axis, to which both
the trough containing the plates, and another trough to collect and hold
the liquid, are fixed. This arrangement I have found the most convenient of
any, and have therefore adopted it. My zinc plates were cut from rolled
metal, and when soldered to the copper plates had the form delineated, fig.
1. These were then bent over a gauge into the form fig. 2, and when packed
in the wooden box constructed to receive them, were arranged as in fig.
3[B], little plugs of cork being used to keep the zinc plates from touching
the copper plates, and a single or double thickness of cartridge paper
being interposed between the contiguous surfaces of copper to prevent them
from coming in contact. Such was the facility afforded by this arrangement,
that a trough of forty pairs of plates could be unpacked in five minutes,
and repacked again in half an hour; and the whole series was not more than
fifteen inches in length.

[Illustration: Fig. 1.]

[Illustration: Fig. 2.]

[Illustration: Fig. 3.]

  [A] Philosophical Magazine, 1824, vol. lxiii. p. 241; or Silliman's
  Journal, vol. vii. See also a previous paper by Dr. Hare, Annals of
  Philosophy, 1821, vol. i. p. 329, in which he speaks of the
  non-necessity of insulation between the coppers.

  [B] The papers between the coppers are, for the sake of distinctness,
  omitted in the figure.

1125. This trough, of forty pairs of plates three inches square, was
compared, as to the ignition of a platina wire, the discharge between
points of charcoal, the shock on the human frame, &c., with forty pairs of
four-inch plates having double coppers, and used in porcelain troughs
divided into insulating cells, the strength of the acid employed to excite
both being the same. In all these effects the former appeared quite equal
to the latter. On comparing a second trough of the new construction,
containing twenty pairs of four-inch plates, with twenty pairs of four-inch
plates in porcelain troughs, excited by acid of the same strength, the new
trough appeared to surpass the old one in producing these effects,
especially in the ignition of wire.

1126. In these experiments the new trough diminished in its energy much
more rapidly than the one on the old construction, and this was a necessary
consequence of the smaller quantity of acid used to excite it, which in the
case of the forty pairs of new construction was only one-seventh part of
that used for the forty pairs in the porcelain troughs. To compare,
therefore, both forms of the voltaic trough in their decomposing powers,
and to obtain accurate data as to their relative values, experiments of the
following kind were made. The troughs were charged with a known quantity of
acid of a known strength; the electric current was passed through a
volta-electrometer (711.) having electrodes 4 inches long and 2.3 inches in
width, so as to oppose as little obstruction as possible to the current;
the gases evolved were collected and measured, and gave the quantity of
water decomposed. Then the whole of the charge used was mixed together, and
a known part of it analyzed, by being precipitated and boiled with excess
of carbonate of soda, and the precipitate well-washed, dried, ignited, and
weighed. In this way the quantity of metal oxidized and dissolved by the
acid was ascertained; and the part removed from each zinc plate, or from
all the plates, could be estimated and compared with the water decomposed
in the volta-electrometer. To bring these to one standard of comparison, I
have reduced the results so as to express the loss at the plates in
equivalents of zinc for the equivalent of water decomposed at the
volta-electrometer: I have taken the equivalent number of water as 9, and
of zinc as 32.5, and have considered 100 cubic inches of the mixed oxygen
and hydrogen, as they were collected over a pneumatic trough, to result
from the decomposition of 12.68 grains of water.

1127. The acids used in these experiments were three,--sulphuric, nitric,
and muriatic. The sulphuric acid was strong oil of vitriol; one cubical
inch of it was equivalent to 486 grains of marble. The nitric acid was very
nearly pure; one cubical inch dissolved 150 grains of marble. The muriatic
acid was also nearly pure, and one cubical inch dissolved 108 grains of
marble. These were always mixed with water by volumes, the standard of
volume being a cubical inch.

1128. An acid was prepared consisting of 200 parts water, 4-1/2 parts
sulphuric acid, and 4 parts nitric acid; and with this both my trough
containing forty pairs of three-inch plates, and four porcelain troughs,
arranged in succession, each containing ten pairs of plates with double
coppers four inches square, were charged. These two batteries were then
used in succession, and the action of each was allowed to continue for
twenty or thirty minutes, until the charge was nearly exhausted, the
connexion with the volta-electrometer being carefully preserved during the
whole time, and the acid in the troughs occasionally mixed together. In
this way the former trough acted so well, that for each equivalent of water
decomposed in the volta-electrometer only from 2 to 2.5 equivalents of zinc
were dissolved from each plate. In four experiments the average was 2.21
equivalents for each plate, or 88.4 for the whole battery. In the
experiments with the porcelain troughs, the equivalents of consumption at
each plate were 3.51, or 141.6 for the whole battery. In a perfect voltaic
battery of forty pairs of plates (991. 1001.) the consumption would have
been one equivalent for each zinc plate, or forty for the whole.

1129. Similar experiments were made with two voltaic batteries, one
containing twenty pairs of four-inch plates, arranged as I have described
(1124.), and the other twenty pairs of four-inch plates in porcelain
troughs. The average of five experiments with the former was a consumption
of 3.7 equivalents of zinc from each plate, or 74 from the whole: the
average of three experiments with the latter was 5.5 equivalents from each
plate, or 110 from the whole: to obtain this conclusion two experiments
were struck out, which were much against the porcelain troughs, and in
which some unknown deteriorating influence was supposed to be accidentally
active. In all the experiments, care was taken not to compare _new_ and
_old_ plates together, as that would have introduced serious errors into
the conclusions (1146.).

1130. When ten pairs of the new arrangement were used, the consumption of
zinc at each plate was 6.76 equivalents, or 67.6 for the whole. With ten
pairs of the common construction, in a porcelain trough, the zinc oxidized
was, upon an average, 15.5 equivalents each plate, or 155 for the entire
trough.

1131. No doubt, therefore, can remain of the equality or even the great
superiority of this form of voltaic battery over the best previously in
use, namely, that with double coppers, in which the cells are insulated.
The insulation of the coppers may therefore be dispensed with; and it is
that circumstance which principally permits of such other alterations in
the construction of the trough as gives it its practical advantages.

1132. The advantages of this form of trough are very numerous and great. i.
It is exceedingly compact, for 100 pairs of plates need not occupy a trough
of more than three feet in length, ii. By Dr. Hare's plan of making the
trough turn upon copper pivots which rest upon copper bearings, the latter
afford _fixed_ terminations; and these I have found it very convenient to
connect with two cups of mercury, fastened in the front of the stand of the
instrument. These fixed terminations give the great advantage of arranging
an apparatus to be used in connexion with the battery _before_ the latter
is put into action, iii. The trough is put into readiness for use in an
instant, a single jug of dilute acid being sufficient for the charge of 100
pairs of four-inch plates, iv. On making the trough pass through a quarter
of a revolution, it becomes active, and the great advantage is obtained of
procuring for the experiment the effect of the _first contact_ of the zinc
and acid, which is twice or sometimes even thrice that which the battery
can produce a minute or two after (1036. 1150.). v. When the experiment is
completed, the acid can be at once poured from between the plates, so that
the battery is never left to waste during an unconnected state of its
extremities; the acid is not unnecessarily exhausted; the zinc is not
uselessly consumed; and, besides avoiding these evils, the charge is mixed
and rendered uniform, which produces a great and good result (1039.); and,
upon proceeding to a second experiment, the important effect of _first
contact_ is again obtained. vi. The saving of zinc is very great. It is not
merely that, whilst in action, the zinc performs more voltaic duty (1128.
1129.), but _all_ the destruction which takes place with the ordinary forms
of battery between the experiments is prevented. This saving is of such
extent, that I estimate the zinc in the new form of battery to be thrice as
effective as that in the ordinary form. vii. The importance of this saving
of metal is not merely that the value of the zinc is saved, but that the
battery is much lighter and more manageable; and also that the surfaces of
the zinc and copper plates may be brought much nearer to each other when
the battery is constructed, and remain so until it is worn out: the latter
is a very important advantage (1148.). viii. Again, as, in consequence of
the saving, thinner plates will perform the duty of thick ones, rolled zinc
may be used; and I have found rolled zinc superior to cast zinc in action;
a superiority which I incline to attribute to its greater purity (1144.).
ix. Another advantage is obtained in the economy of the acid used, which is
proportionate to the diminution of the zinc dissolved. x. The acid also is
more easily exhausted, and is in such small quantity that there is never
any occasion to return an old charge into use. The acid of old charges
whilst out of use, often dissolves portions of copper from the black
flocculi usually mingled with it, which are derived from the zinc; now any
portion of copper in solution in the charge does great harm, because, by
the _local_ action of the acid and zinc, it tends to precipitate upon the
latter, and diminish its voltaic efficacy (1145.). xi. By using a due
mixture of nitric and sulphuric acid for the charge (1139.), no gas is
evolved from the troughs; so that a battery of several hundred pairs of
plates may, without inconvenience, be close to the experimenter. xii. If,
during a series of experiments, the acid becomes exhausted, it can be
withdrawn, and replaced by other acid with the utmost facility; and after
the experiments are concluded, the great advantage of easily washing the
plates is at command. And it appears to me, that in place of making, under
different circumstances, mutual sacrifices of comfort, power, and economy,
to obtain a desired end, all are at once obtained by Dr. Hare's form of
trough.

1133. But there are some disadvantages which I have not yet had time to
overcome, though I trust they will finally be conquered. One is the extreme
difficulty of making a wooden trough constantly water-tight under the
alternations of wet and dry to which the voltaic instrument is subject. To
remedy this evil, Mr. Newman is now engaged in obtaining porcelain troughs.
The other disadvantage is a precipitation of copper on the zinc plates. It
appears to me to depend mainly on the circumstance that the papers between
the coppers retain acid when the trough is emptied; and that this acid
slowly acting on the copper, forms a salt, which gradually mingles with the
next charge, and is reduced on the zinc plate by the local action (1120.):
the power of the whole battery is then reduced. I expect that by using
slips of glass or wood to separate the coppers at their edges, their
contact can be sufficiently prevented, and the space between them be left
so open that the acid of a charge can be poured and washed out, and so be
removed from _every part_ of the trough when the experiments in which the
latter is used are completed.

1134. The actual superiority of the troughs which I have constructed on
this plan, I believe to depend, first and principally, on the closer
approximation of the zinc and copper surfaces;--in my troughs they are only
one-tenth of an inch apart (1148.);--and, next, on the superior quality of
the rolled zinc above the cast zinc used in the construction of the
ordinary pile. It cannot be that insulation between the contiguous coppers
is a disadvantage, but I do not find that it is any advantage; for when,
with both the forty pairs of three-inch plates and the twenty pairs of
four-inch plates, I used papers well-soaked in wax[A], these being so large
that when folded at the edges they wrapped over each other, so as to make
cells as insulating as those of the porcelain troughs, still no sensible
advantage in the chemical action was obtained.

  [A] A single paper thus prepared could insulate the electricity of a
  trough of forty pairs of plates.

1135. As, upon principle, there must be a discharge of part of the
electricity from the edges of the zinc and copper plates at the sides of
the trough, I should prefer, and intend having, troughs constructed with a
plate or plates of crown glass at the sides of the trough: the bottom will
need none, though to glaze that and the ends would be no disadvantage. The
plates need not be fastened in, but only set in their places; nor need they
be in large single pieces.


§ 17. _Some practical results respecting the construction and use of the
Voltaic Battery_ (1034. &c.).


1136. The electro-chemical philosopher is well acquainted with some
practical results obtained from the voltaic battery by MM.. Gay-Lussac and
Thenard, and given in the first forty-five pages of their 'Recherches
Physico-Chimiques'. Although the following results are generally of the
same nature, yet the advancement made in this branch of science of late
years, the knowledge of the definite action of electricity, and the more
accurate and philosophical mode of estimating the results by the
equivalents of zinc consumed, will be their sufficient justification.

1137. _Nature and strength of the acid._--My battery of forty pairs of
three-inch plates was charged with acid consisting of 200 parts water and 9
oil of vitriol. Each plate lost, in the average of the experiments, 4.66
equivalents of zinc for the equivalent of water decomposed in the
volta-electrometer, or the whole battery 186.4 equivalents of zinc. Being
charged with a mixture of 200 water and 16 of the muriatic acid, each plate
lost 3.8, equivalents of zinc for the water decomposed, or the whole
battery 152 equivalents of zinc. Being charged with a mixture of 200 water
and 8 nitric acid, each plate lost 1.85, equivalents of zinc for one
equivalent of water decomposed, or the whole battery 74.16 equivalents of
zinc. The sulphuric and muriatic acids evolved much hydrogen at the plates
in the trough; the nitric acid no gas whatever. The relative strengths of
the original acids have already been given (1127.); but a difference in
that respect makes no important difference in the results when thus
expressed by equivalents (1140.).

1138. Thus nitric acid proves to be the best for this purpose; its
superiority appears to depend upon its favouring the electrolyzation of the
liquid in the cells of the trough upon the principles already explained
(905. 973, 1022.), and consequently favouring the transmission of the
electricity, and therefore the production of transferable power (1120.).

1139. The addition of nitric acid might, consequently, be expected to
improve sulphuric and muriatic acids. Accordingly, when the same trough was
charged with a mixture of 200 water, 9 oil of vitriol, and 4 nitric acid,
the consumption of zinc was at each plate 2.786, and for the whole battery
111.5, equivalents. When the charge was 200 water, 9 oil of vitriol, and 8
nitric acid, the loss per plate was 2.26, or for the whole battery 90.4,
equivalents. When the trough was charged with a mixture of 200 water, 16
muriatic acid, and 6 nitric acid, the loss per plate was 2.11, or for the
whole battery 84.4, equivalents. Similar results were obtained with my
battery of twenty pairs of four-inch plates (1129.). Hence it is evident
that the nitric acid was of great service when mingled with the sulphuric
acid; and the charge generally used after this time for ordinary
experiments consisted of 200 water, 4-1/2 oil of vitriol, and 4 nitric
acid.

1140. It is not to be supposed that the different strengths of the acids
produced the differences above; for within certain limits I found the
electrolytic effects to be nearly as the strengths of the acids, so as to
leave the expression of force, when given in equivalents, almost constant.
Thus, when the trough was charged with a mixture of 200 water and 8 nitric
acid, each plate lost 1.854 equivalent of zinc. When the charge was 200
water and 16 nitric acid, the loss per plate was 1.82 equivalent. When it
was 200 water and 32 nitric acid, the loss was 2.1 equivalents. The
differences here are not greater than happen from unavoidable
irregularities, depending on other causes than the strength of acid.

1141. Again, when a charge consisting of 200 water, 4-1/2 oil of vitriol,
and 4 nitric acid was used, each zinc plate lost 2.16 equivalents; when the
charge with the same battery was 200 water, 9 oil of vitriol, and 8 nitric
acid, each zinc plate lost 2.26 equivalents.

1142. I need hardly say that no copper is dissolved during the regular
action of the voltaic trough. I have found that much ammonia is formed in
the cells when nitric acid, either pure or mixed with sulphuric acid, is
used. It is produced in part as a secondary result at the cathodes (663.)
of the different portions of fluid constituting the necessary electrolyte,
in the cells.

1143. _Uniformity of the charge._--This is a most important point, as I
have already shown experimentally (1042. &c.). Hence one great advantage of
Dr. Hare's mechanical arrangement of his trough.

1144. _Purity of the zinc._--If pure zinc could be obtained, it would be
very advantageous in the construction of the voltaic apparatus (998.). Most
zincs, when put into dilute sulphuric acid, leave more or less of an
insoluble matter upon the surface in the form of a crust, which contains
various metals, as copper, lead, zinc, iron, cadmium, &c., in the metallic
state. Such particles, by discharging part of the transferable power,
render it, as to the whole battery, local; and so diminish the effect. As
an indication connected with the more or less perfect action of the
battery, I may mention that no gas ought to rise from the zinc plates. The
more gas which is generated upon these surfaces, the greater is the local
action and the less the transferable force. The investing crust is also
inconvenient, by preventing the displacement and renewal of the charge upon
the surface of the zinc. Such zinc as, dissolving in the cleanest manner in
a dilute acid, dissolves also the slowest, is the best; zinc which contains
much copper should especially be avoided. I have generally found rolled
Liege or Mosselman's zinc the purest; and to the circumstance of having
used such zinc in its construction attribute in part the advantage of the
new battery (1134.).

1145. _Foulness of the zinc plates._--After use, the plates of a battery
should be cleaned from the metallic powder upon their surfaces, especially
if they are employed to obtain the laws of action of the battery itself.
This precaution was always attended to with the porcelain trough batteries
in the experiments described (1125, &c.). If a few foul plates are mingled
with many clean ones, they make the action in the different cells
irregular, and the transferable power is accordingly diminished, whilst the
local and wasted power is increased. No old charge containing copper should
be used to excite a battery.

1146. _New and old plates._--I have found voltaic batteries far more
powerful when the plates were new than when they have been used two or
three times; so that a new and an used battery cannot be compared together,
or even a battery with itself on the first and after times of use. My
trough of twenty pairs of four-inch plates, charged with acid consisting of
200 water, 4-1/2 oil of vitriol, and 4 nitric acid, lost, upon the first
time of being used, 2.82 equivalents per plate. When used after the fourth
time with the same charge, the loss was from 3.26 to 4.47 equivalents per
plate; the average being 3.7 equivalents. The first time the forty pair of
plates (1124.) were used, the loss at each plate was only 1.65 equivalent;
but afterwards it became 2.16, 2.17, 2.52. The first time twenty pair of
four-inch plates in porcelain troughs were used, they lost, per plate, only
3.7 equivalents; but after that, the loss was 5.25, 5.36, 5.9 equivalents.
Yet in all these cases the zincs had been well-cleaned from adhering
copper, &c., before each trial of power.

1147. With the rolled zinc the fall in force soon appeared to become
constant, i.e. to proceed no further. But with the cast zinc plates
belonging to the porcelain troughs, it appeared to continue, until at last,
with the same charge, each plate lost above twice as much zinc for a given
amount of action as at first. These troughs were, however, so irregular
that I could not always determine the circumstances affecting the amount of
electrolytic action.

1148. _Vicinity of the copper and zinc._--The importance of this point in
the construction of voltaic arrangements, and the greater power, as to
immediate action, which is obtained when the zinc and copper surfaces are
near to each other than when removed further apart, are well known. I find
that the power is not only greater on the instant, but also that the sum of
transferable power, in relation to the whole sum of chemical action at the
plates, is much increased. The cause of this gain is very evident. Whatever
tends to retard the circulation of the transferable force, (i.e. the
electricity,) diminishes the proportion of such force, and increases the
proportion of that which is local (996. 1120.). Now the liquid in the cells
possesses this retarding power, and therefore acts injuriously, in greater
or less proportion, according to the quantity of it between the zinc and
copper plates, i.e. according to the distances between their surfaces. A
trough, therefore, in which the plates are only half the distance asunder
at which they are placed in another, will produce more transferable, and
less local, force than the latter; and thus, because the electrolyte in the
cells can transmit the current more readily; both the intensity and
quantity of electricity is increased for a given consumption of zinc. To
this circumstance mainly I attribute the superiority of the trough I have
described (1134.).

1149. The superiority of _double coppers_ over single plates also depends
in part upon diminishing the resistance offered by the electrolyte between
the metals. For, in fact, with double coppers the sectional area of the
interposed acid becomes nearly double that with single coppers, and
therefore it more freely transfers the electricity. Double coppers are,
however, effective, mainly because they virtually double the acting surface
of the zinc, or nearly so; for in a trough with single copper plates and
the usual construction of cells, that surface of zinc which is not opposed
to a copper surface is thrown almost entirely out of voltaic action, yet
the acid continues to act upon it and the metal is dissolved, producing
very little more than local effect (947. 996). But when by doubling the
copper, that metal is opposed to the second surface of the zinc plate, then
a great part of the action upon the latter is converted into transferable
force, and thus the power of the trough as to quantity of electricity is
highly exalted.

1150. _First immersion of the plates._--The great effect produced at the
first immersion of the plates, (apart from their being new or used
(1146.),) I have attributed elsewhere to the unchanged condition of the
acid in contact with the zinc plate (1003. 1037.): as the acid becomes
neutralized, its exciting power is proportionally diminished. Hare's form
of trough secures much advantage of this kind, by mingling the liquid, and
bringing what may be considered as a fresh surface of acid against the
plates every time it is used immediately after a rest.

1151. _Number of plates._[A]--The most advantageous number of plates in a
battery used for chemical decomposition, depends almost entirely upon the
resistance to be overcome at the place of action; but whatever that
resistance may be, there is a certain number which is more economical than
either a greater or a less. Ten pairs of four-inch plates in a porcelain
trough of the ordinary construction, acting in the volta-electrometer
(1126.) upon dilute sulphuric acid of spec. grav. 1.314, gave an average
consumption of 15.4 equivalents per plate, or 154 equivalents on the whole.
Twenty pairs of the same plates, with the same acid, gave only a
consumption of 5.5 per plate, or 110 equivalents upon the whole. When forty
pairs of the same plates were used, the consumption was 3.54 equivalents
per plate, or 141.6 upon the whole battery. Thus the consumption of zinc
arranged as _twenty_ plates was more advantageous than if arranged either
as _ten_ or as _forty_.

  [A] Gay-Lussac and Thenard, Recherches Physico-Chimiques, tom. i. p. 29.

1152. Again, ten pairs of my four-inch plates (1129.) lost 6.76 each, or
the whole ten 67.6 equivalents of zinc, in effecting decomposition; whilst
twenty pairs of the same plates, excited by the same acid, lost 3.7
equivalents each, or on the whole 74 equivalents. In other comparative
experiments of numbers, ten pairs of the three inch-plates, (1125.) lost
3.725, or 37.25 equivalents upon the whole; whilst twenty pairs lost 2.53
each, or 50.6 in all; and forty pairs lost on an average 2.21, or 88.4
altogether. In both these cases, therefore, increase of numbers had not
been advantageous as to the effective production of _transferable chemical
power_ from the _whole quantity of chemical force_ active at the surfaces
of excitation (1120.).

1153. But if I had used a weaker acid or a worse conductor in the
volta-electrometer, then the number of plates which would produce the most
advantageous effect would have risen; or if I had used a better conductor
than that really employed in the volta-electrometer, I might have reduced
the number even to one; as, for instance, when a thick wire is used to
complete the circuit (865., &c.). And the cause of these variations is very
evident, when it is considered that each successive plate in the voltaic
apparatus does not add anything to the _quantity_ of transferable power or
electricity which the first plate can put into motion, provided a good
conductor be present, but tends only to exalt the _intensity_ of that
quantity, so as to make it more able to overcome the obstruction of bad
conductors (994. 1158.).

1154. _Large or small plates._[A]--The advantageous use of large or small
plates for electrolyzations will evidently depend upon the facility with
which the transferable power of electricity can pass. If in a particular
case the most effectual number of plates is known (1151.), then the
addition of more zinc would be most advantageously made in increasing the
_size_ of the plates, and not their _number_. At the same time, large
increase in the size of the plates would raise in a small degree the most
favourable number.

  [A] Gay-Lussac and Thenard, Recherches Physico-Chimiques, tom, i. p. 20.

1155. Large and small plates should not be used together in the same
battery: the small ones occasion a loss of the power of the large ones,
unless they be excited by an acid proportionably more powerful; for with a
certain acid they cannot transmit the same portion of electricity in a
given time which the same acid can evolve by action on the larger plates.

1156. _Simultaneous decompositions._--When the number of plates in a
battery much surpasses the most favourable proportion (1151--1153.), two or
more decompositions may be effected simultaneously with advantage. Thus my
forty pairs of plates (1124.) produced in one volta-electrometer 22.8 cubic
inches of gas. Being recharged exactly in the same manner, they produced in
each of two volta-electrometers 21 cubical inches. In the first experiment
the whole consumption of zinc was 88.4 equivalents, and in the second only
48.28 equivalents, for the whole of the water decomposed in both
volta-electrometers.

1157. But when the twenty pairs of four-inch plates (1129.) were tried in
a similar manner, the results were in the opposite direction. With one
volta-electrometer 52 cubic inches of gas were obtained; with two, only
14.6 cubic inches from each. The quantity of charge was not the same in
both cases, though it was of the same strength; but on rendering the
results comparative by reducing them to equivalents (1126.), it was found
that the consumption of metal in the first case was 74, and in the second
case 97, equivalents for the _whole_ of the water decomposed. These
results of course depend upon the same circumstances of retardation, &c.,
which have been referred to in speaking of the proper number of plates
(1151.).

1158. That the _transferring_, or, as it is usually called, _conducting,
power_ of an electrolyte which is to be decomposed, or other interposed
body, should be rendered as good as possible[A], is very evident (1020.
1120.). With a perfectly good conductor and a good battery, nearly all the
electricity is passed, i.e. _nearly all_ the chemical power becomes
transferable, even with a single pair of plates (807.). With an interposed
nonconductor none of the chemical power becomes transferable. With an
imperfect conductor more or less of the chemical power becomes transferable
as the circumstances favouring the transfer of forces across the imperfect
conductor are exalted or diminished: these circumstances are, actual
increase or improvement of the conducting power, enlargement of the
electrodes, approximation of the electrodes, and increased intensity of the
passing current.

  [A] Gay-Lussac and Thenard, Recherches Physico-Chimiques, tom. i. pp.
  13, 15, 22.

1159. The introduction of common spring water in place of one of the
volta-electrometers used with twenty pairs of four-inch plates (1156.)
caused such obstruction as not to allow one-fifteenth of the transferable
force to pass which would have circulated without it. Thus
fourteen-fifteenths of the available force of the battery were destroyed,
local force, (which was rendered evident by the evolution of gas from the
being converted into zincs,) and yet the platina electrodes in the water
were three inches long, nearly an inch wide, and not a quarter of an inch
apart.

1160. These points, i.e. the increase of conducting power, the enlargement
of the electrodes, and their approximation, should be especially attended
to in _volta-electrometers_. The principles upon which their utility depend
are so evident that there can be no occasion for further development of
them here.

_Royal Institution,
October 11, 1834._




ELEVENTH SERIES.

§ 18. _On Induction._ ¶ i. _Induction an action of contiguous particles._
¶ ii. _Absolute charge of matter._ ¶ iii. _Electrometer and inductive
apparatus employed._ ¶ iv. _Induction in curved lines._ ¶ v. _Specific
inductive capacity._ ¶ vi. _General results as to induction._

Received November 30,--Read December 21, 1837.


¶ i. _Induction an action of contiguous particles._


1161. The science of electricity is in that state in which every part of it
requires experimental investigation; not merely for the discovery of new
effects, but what is just now of far more importance, the development of
the means by which the old effects are produced, and the consequent more
accurate determination of the first principles of action of the most
extraordinary and universal power in nature:--and to those philosophers who
pursue the inquiry zealously yet cautiously, combining experiment with
analogy, suspicious of their preconceived notions, paying more respect to a
fact than a theory, not too hasty to generalize, and above all things,
willing at every step to cross-examine their own opinions, both by
reasoning and experiment, no branch of knowledge can afford so fine and
ready a field for discovery as this. Such is most abundantly shown to be
the case by the progress which electricity has made in the last thirty
years: Chemistry and Magnetism have successively acknowledged its
over-ruling influence; and it is probable that every effect depending upon
the powers of inorganic matter, and perhaps most of those related to
vegetable and animal life, will ultimately be found subordinate to it.

1162. Amongst the actions of different kinds into which electricity has
conventionally been subdivided, there is, I think, none which excels, or
even equals in importance, that called _Induction_. It is of the most
general influence in electrical phenomena, appearing to be concerned in
every one of them, and has in reality the character of a first, essential,
and fundamental principle. Its comprehension is so important, that I think
we cannot proceed much further in the investigation of the laws of
electricity without a more thorough understanding of its nature; how
otherwise can we hope to comprehend the harmony and even unity of action
which doubtless governs electrical excitement by friction, by chemical
means, by heat, by magnetic influence, by evaporation, and even by the
living being?

1163. In the long-continued course of experimental inquiry in which I have
been engaged, this general result has pressed upon me constantly, namely,
the necessity of admitting two forces, or two forms or directions of a
force (516. 517.), combined with the impossibility of separating these two
forces (or electricities) from each other, either in the phenomena of
statical electricity or those of the current. In association with this, the
impossibility under any circumstances, as yet, of absolutely charging
matter of any kind with one or the other electricity only, dwelt on my
mind, and made me wish and search for a clearer view than any that I was
acquainted with, of the way in which electrical powers and the particles of
matter are related; especially in inductive actions, upon which almost all
others appeared to rest.

1164. When I discovered the general fact that electrolytes refused to yield
their elements to a current when in the solid state, though they gave them
forth freely if in the liquid condition (380. 394. 402.), I thought I saw
an opening to the elucidation of inductive action, and the possible
subjugation of many dissimilar phenomena to one law. For let the
electrolyte be water, a plate of ice being coated with platina foil on its
two surfaces, and these coatings connected with any continued source of the
two electrical powers, the ice will charge like a Leyden arrangement,
presenting a case of common induction, but no current will pass. If the ice
be liquefied, the induction will fall to a certain degree, because a
current can now pass; but its passing is dependent upon a _peculiar
molecular arrangement_ of the particles consistent with the transfer of the
elements of the electrolyte in opposite directions, the degree of discharge
and the quantity of elements evolved being exactly proportioned to each
other (377. 783.). Whether the charging of the metallic coating be effected
by a powerful electrical machine, a strong and large voltaic battery, or a
single pair of plates, makes no difference in the principle, but only in
the degree of action (360). Common induction takes place in each case if
the electrolyte be solid, or if fluid, chemical action and decomposition
ensue, provided opposing actions do not interfere; and it is of high
importance occasionally thus to compare effects in their extreme degrees,
for the purpose of enabling us to comprehend the nature of an action in its
weak state, which may be only sufficiently evident to us in its stronger
condition (451.). As, therefore, in the electrolytic action, _induction_
appeared to be the _first_ step, and _decomposition_ the _second_ (the
power of separating these steps from each other by giving the solid or
fluid condition to the electrolyte being in our hands); as the induction
was the same in its nature as that through air, glass, wax, &c. produced by
any of the ordinary means; and as the whole effect in the electrolyte
appeared to be an action of the particles thrown into a peculiar or
polarized state, I was led to suspect that common induction itself was in
all cases an _action of contiguous particles_[A], and that electrical
action at a distance (i.e. ordinary inductive action) never occurred except
through the influence of the intervening matter.

  [A] The word _contiguous_ is perhaps not the best that might have been
  used here and elsewhere; for as particles do not touch each other it
  is not strictly correct. I was induced to employ it, because in its
  common acceptation it enabled me to state the theory plainly and with
  facility. By contiguous particles I mean those which are next.--_Dec.
  1838._

1165. The respect which I entertain towards the names of Epinus, Cavendish,
Poisson, and other most eminent men, all of whose theories I believe
consider induction as an action at a distance and in straight lines, long
indisposed me to the view I have just stated; and though I always watched
for opportunities to prove the opposite opinion, and made such experiments
occasionally as seemed to bear directly on the point, as, for instance, the
examination of electrolytes, solid and fluid, whilst under induction by
polarized light (951. 955.), it is only of late, and by degrees, that the
extreme generality of the subject has urged me still further to extend my
experiments and publish my view. At present I believe ordinary induction in
all cases to be an action of contiguous particles consisting in a species
of polarity, instead of being an action of either particles or masses at
sensible distances; and if this be true, the distinction and establishment
of such a truth must be of the greatest consequence to our further progress
in the investigation of the nature of electric forces. The linked condition
of electrical induction with chemical decomposition; of voltaic excitement
with chemical action; the transfer of elements in an electrolyte; the
original cause of excitement in all cases; the nature and relation of
conduction and insulation of the direct and lateral or transverse action
constituting electricity and magnetism; with many other things more or less
incomprehensible at present, would all be affected by it, and perhaps
receive a full explication in their reduction under one general law.

1166. I searched for an unexceptionable test of my view, not merely in the
accordance of known facts with it, but in the consequences which would flow
from it if true; especially in those which would not be consistent with the
theory of action at a distance. Such a consequence seemed to me to present
itself in the direction in which inductive action could be exerted. If in
straight lines only, though not perhaps decisive, it would be against my
view; but if in curved lines also, that would be a natural result of the
action of contiguous particles, but, as I think, utterly incompatible with
action at a distance, as assumed by the received theories, which, according
to every fact and analogy we are acquainted with, is always in straight
lines.

1167. Again, if induction be an action of contiguous particles, and also
the first step in the process of electrolyzation (1164. 919.), there seemed
reason to expect some particular relation of it to the different kinds of
matter through which it would be exerted, or something equivalent to a
_specific electric induction_ for different bodies, which, if it existed,
would unequivocally prove the dependence of induction on the particles; and
though this, in the theory of Poisson and others, has never been supposed
to be the case, I was soon led to doubt the received opinion, and have
taken great pains in subjecting this point to close experimental
examination.

1168. Another ever-present question on my mind has been, whether
electricity has an actual and independent existence as a fluid or fluids,
or was a mere power of matter, like what we conceive of the attraction of
gravitation. If determined either way it would be an enormous advance in
our knowledge; and as having the most direct and influential bearing on my
notions, I have always sought for experiments which would in any way tend
to elucidate that great inquiry. It was in attempts to prove the existence
of electricity separate from matter, by giving an independent charge of
either positive or negative power only, to some one substance, and the
utter failure of all such attempts, whatever substance was used or whatever
means of exciting or _evolving_ electricity were employed, that first drove
me to look upon induction as an action of the particles of matter, each
having _both_ forces developed in it in exactly equal amount. It is this
circumstance, in connection with others, which makes me desirous of placing
the remarks on absolute charge first, in the order of proof and argument,
which I am about to adduce in favour of my view, that electric induction is
an action of the contiguous particles of the insulating medium or
_dielectric_[A].

  [A] I use the word _dielectric_ to express that substance through or
  across which the electric forces are acting.--_Dec. 1838._


¶ ii. _On the absolute charge of matter._

1169. Can matter, either conducting or non-conducting, be charged with one
electric force independently of the other, in any degree, either in a
sensible or latent state?

1170. The beautiful experiments of Coulomb upon the equality of action of
_conductors_, whatever their substance, and the residence of _all_ the
electricity upon their surfaces[A], are sufficient, if properly viewed, to
prove that _conductors cannot be bodily charged_; and as yet no means of
communicating electricity to a conductor so as to place its particles in
relation to one electricity, and not at the same time to the other in
exactly equal amount, has been discovered.

  [A] Mémoires de l'Académie, 1786, pp. 67. 69. 72; 1787, p. 452.

1171. With regard to electrics or non-conductors, the conclusion does not
at first seem so clear. They may easily be electrified bodily, either by
communication (1247.) or excitement; but being so charged, every case in
succession, when examined, came out to be a case of induction, and not of
absolute charge. Thus, glass within conductors could easily have parts not
in contact with the conductor brought into an excited state; but it was
always found that a portion of the inner surface of the conductor was in an
opposite and equivalent state, or that another part of the glass itself was
in an equally opposite state, an _inductive_ charge and not an _absolute_
charge having been acquired.

1172. Well-purified oil of turpentine, which I find to be an excellent
liquid insulator for most purposes, was put into a metallic vessel, and,
being insulated, an endeavour was made to charge its particles, sometimes
by contact of the metal with the electrical machine, and at others by a
wire dipping into the fluid within; but whatever the mode of communication,
no electricity of one kind only was retained by the arrangement, except
what appeared on the exterior surface of the metal, that portion being
present there only by an inductive action through the air to the
surrounding conductors. When the oil of turpentine was confined in glass
vessels, there were at first some appearances as if the fluid did receive
an absolute charge of electricity from the charging wire, but these were
quickly reduced to cases of common induction jointly through the fluid, the
glass, and the surrounding air.

1173. I carried these experiments on with air to a very great extent. I had
a chamber built, being a cube of twelve feet. A slight cubical wooden frame
was constructed, and copper wire passed along and across it in various
directions, so as to make the sides a large net-work, and then all was
covered in with paper, placed in close connexion with the wires, and
supplied in every direction with bands of tin foil, that the whole might be
brought into good metallic communication, and rendered a free conductor in
every part. This chamber was insulated in the lecture-room of the Royal
Institution; a glass tube about six feet in length was passed through its
side, leaving about four feet within and two feet on the outside, and
through this a wire passed from the large electrical machine (290.) to the
air within. By working the machine, the air in this chamber could be
brought into what is considered a highly electrified state (being, in fact,
the same state as that of the air of a room in which a powerful machine is
in operation), and at the same time the outside of the insulated cube was
everywhere strongly charged. But putting the chamber in communication with
the perfect discharging train described in a former series (292.), and
working the machine so as to bring the air within to its utmost degree of
charge if I quickly cut off the connexion with the machine, and at the same
moment or instantly after insulated the cube, the air within had not the
least power to communicate a further charge to it. If any portion of the
air was electrified, as glass or other insulators may be charged (1171.),
it was accompanied by a corresponding opposite action _within_ the cube,
the whole effect being merely a case of induction. Every attempt to charge
air bodily and independently with the least portion of either electricity
failed.

1174 I put a delicate gold-leaf electrometer within the cube, and then
charged the whole by an _outside_ communication, very strongly, for some
time together; but neither during the charge or after the discharge did the
electrometer or air within show the least signs of electricity. I charged
and discharged the whole arrangement in various ways, but in no case could
I obtain the least indication of an absolute charge; or of one by induction
in which the electricity of one kind had the smallest superiority in
quantity over the other. I went into the cube and lived in it, and using
lighted candles, electrometers, and all other tests of electrical states, I
could not find the least influence upon them, or indication of any thing
particular given by them, though all the time the outside of the cube was
powerfully charged, and large sparks and brushes were darting off from
every part of its outer surface. The conclusion I have come to is, that
non-conductors, as well as conductors, have never yet had an absolute and
independent charge of one electricity communicated to them, and that to all
appearance such a state of matter is impossible.

1175. There is another view of this question which may be taken under the
supposition of the existence of an electric fluid or fluids. It may be
impossible to have one fluid or state in a free condition without its
producing by induction the other, and yet possible to have cases in which
an isolated portion of matter in one condition being uncharged, shall, by a
change of state, evolve one electricity or the other: and though such
evolved electricity might immediately induce the opposite state in its
neighbourhood, yet the mere evolution of one electricity without the other
in the _first instance_, would be a very important fact in the theories
which assume a fluid or fluids; these theories as I understand them
assigning not the slightest reason why such an effect should not occur.

1176. But on searching for such cases I cannot find one. Evolution by
friction, as is well known, gives both powers in equal proportion. So does
evolution by chemical action, notwithstanding the great diversity of bodies
which may be employed, and the enormous quantity of electricity which can
in this manner be evolved (371. 376. 861. 868. 961.). The more promising
cases of change of state, whether by evaporation, fusion, or the reverse
processes, still give both forms of the power in _equal_ proportion; and
the cases of splitting of mica and other crystals, the breaking of sulphur,
&c., are subject to the same law of limitation.

1177. As far as experiment has proceeded, it appears, therefore, impossible
either to evolve or make disappear one electric force without equal and
corresponding change in the other. It is also equally impossible
experimentally to charge a portion of matter with one electric force
independently of the other. Charge always implies _induction_, for it can
in no instance be effected without; and also the presence of the _two_
forms of power, equally at the moment of the development and afterwards.
There is no _absolute_ charge of matter with one fluid; no latency of a
single electricity. This though a negative result is an exceedingly
important one, being probably the consequence of a natural impossibility,
which will become clear to us when we understand the true condition and
theory of the electric power.

1178. The preceding considerations already point to the following
conclusions: bodies cannot be charged absolutely, but only relatively, and
by a principle which is the same with that of _induction_. All _charge_ is
sustained by induction. All phenomena of _intensity_ include the principle
of induction. All _excitation_ is dependent on or directly related to
induction. All _currents_ involve previous intensity and therefore previous
induction. INDUCTION appears to be the essential function both the first
development and the consequent phenomena of electricity.


¶ iii. _Electrometer and inductive apparatus employed._

1179. Leaving for a time the further consideration of the preceding facts
until they can be collated with other results bearing directly on the great
question of the nature of induction, I will now describe the apparatus I
have had occasion to use; and in proportion to the importance of the
principles sought to be established is the necessity of doing this so
clearly, as to leave no doubt of the results behind.

1180. _Electrometer._--The measuring instrument I have employed has been
the torsion balance electrometer of Coulomb, constructed, generally,
according to his directions[A], but with certain variations and additions,
which I will briefly describe. The lower part was a glass cylinder eight
inches in height and eight inches in diameter; the tube for the torsion
thread was seventeen inches in length. The torsion thread itself was not of
metal, but glass, according to the excellent suggestion of the late Dr.
Ritchie[B]. It was twenty inches in length, and of such tenuity that when
the shell-lac lever and attached ball, &c. were connected with it, they
made about ten vibrations in a minute. It would bear torsion through four
revolutions or 1440°, and yet, when released, return accurately to its
position; probably it would have borne considerably more than this without
injury. The repelled ball was of pith, gilt, and was 0.3 of an inch in
diameter. The horizontal stem or lever supporting it was of shell-lac,
according to Coulomb's direction, the arm carrying the ball being 2.4
inches long, and the other only 1.2 inches: to this was attached the vane,
also described by Coulomb, which I found to answer admirably its purpose of
quickly destroying vibrations. That the inductive action within the
electrometer might be uniform in all positions of the repelled ball and in
all states of the apparatus, two bands of tin foil, about an inch wide
each, were attached to the inner surface of the glass cylinder, going
entirely round it, at the distance of 0.4 of an inch from each other, and
at such a height that the intermediate clear surface was in the same
horizontal plane with the lever and ball. These bands were connected with
each other and with the earth, and, being perfect conductors, always
exerted a uniform influence on the electrified balls within, which the
glass surface, from its irregularity of condition at different times, I
found, did not. For the purpose of keeping the air within the electrometer
in a constant state as to dryness, a glass dish, of such size as to enter
easily within the cylinder, had a layer of fused potash placed within it,
and this being covered with a disc of fine wire-gauze to render its
inductive action uniform at all parts, was placed within the instrument at
the bottom and left there.

  [A] Mémoires de l'Académie, 1785, p. 570.

  [B] Philosophical Transactions, 1830.

1181. The moveable ball used to take and measure the portion of electricity
under examination, and which may be called the _repelling_, or the
_carrier_, ball, was of soft alder wood, well and smoothly gilt. It was
attached to a fine shell-lac stem, and introduced through a hole into the
electrometer according to Coulomb's method: the stem was fixed at its upper
end in a block or vice, supported on three short feet; and on the surface
of the glass cover above was a plate of lead with stops on it, so that when
the carrier ball was adjusted in its right position, with the vice above
bearing at the same time against these stops, it was perfectly easy to
bring away the carrier-ball and restore it to its place again very
accurately, without any loss of time.

1182. It is quite necessary to attend to certain precautions respecting
these balls. If of pith alone they are bad; for when very dry, that
substance is so imperfect a conductor that it neither receives nor gives a
charge freely, and so, after contact with a charged conductor, it is liable
to be in an uncertain condition. Again, it is difficult to turn pith so
smooth as to leave the ball, even when gilt, so free from irregularities of
form, as to retain its charge undiminished for a considerable length of
time. When, therefore, the balls are finally prepared and gilt they should
be examined; and being electrified, unless they can hold their charge with
very little diminution for a considerable time, and yet be discharged
instantly and perfectly by the touch of an uninsulated conductor, they
should be dismissed.

1183. It is, perhaps, unnecessary to refer to the graduation of the
instrument, further than to explain how the observations were made. On a
circle or ring of paper on the outside of the glass cylinder, fixed so as
to cover the internal lower ring of tinfoil, were marked four points
corresponding to angles of 90°; four other points exactly corresponding to
these points being marked on the upper ring of tinfoil within. By these and
the adjusting screws on which the whole instrument stands, the glass
torsion thread could be brought accurately into the centre of the
instrument and of the graduations on it. From one of the four points on the
exterior of the cylinder a graduation of 90° was set off, and a
corresponding graduation was placed upon the upper tinfoil on the opposite
side of the cylinder within; and a dot being marked on that point of the
surface of the repelled ball nearest to the side of the electrometer, it
was easy, by observing the line which this dot made with the lines of the
two graduations just referred to, to ascertain accurately the position of
the ball. The upper end of the glass thread was attached, as in Coulomb's
original electrometer, to an index, which had its appropriate graduated
circle, upon which the degree of torsion was ultimately to be read off.

1184. After the levelling of the instrument and adjustment of the glass
thread, the blocks which determine the place of the _carrier ball_ are to
be regulated (1181.) so that, when the carrier arrangement is placed
against them, the centre of the ball may be in the radius of the instrument
corresponding to 0° on the lower graduation or that on the side of the
electrometer, and at the same level and distance from the centre as the
_repelled ball_ on the suspended torsion lever. Then the torsion index is
to be turned until the ball connected with it (the repelled ball) is
accurately at 30°, and finally the graduated arc belonging to the torsion
index is to be adjusted so as to bring 0° upon it to the index. This state
of the instrument was adopted as that which gave the most direct expression
of the experimental results, and in the form having fewest variable errors;
the angular distance of 30° being always retained as the standard distance
to which the balls were in every case to be brought, and the whole of the
torsion being read off at once on the graduated circle above. Under these
circumstances the distance of the balls from each other was not merely the
same in degree, but their position in the instrument, and in relation to
every part of it, was actually the same every time that a measurement was
made; so that all irregularities arising from slight difference of form and
action in the instrument and the bodies around were avoided. The only
difference which could occur in the position of anything within, consisted
in the deflexion of the torsion thread from a vertical position, more or
less, according to the force of repulsion of the balls; but this was so
slight as to cause no interfering difference in the symmetry of form within
the instrument, and gave no error in the amount of torsion force indicated
on the graduation above.

1185. Although the constant angular distance of 30° between the centres of
the balls was adopted, and found abundantly sensible, for all ordinary
purposes, yet the facility of rendering the instrument far more sensible by
diminishing this distance was at perfect command; the results at different
distances being very easily compared with each other either by experiment,
or, as they are inversely as the squares of the distances, by calculation.

1186. The Coulomb balance electrometer requires experience to be
understood; but I think it a very valuable instrument in the hands of those
who will take pains by practice and attention to learn the precautions
needful in its use. Its insulating condition varies with circumstances, and
should be examined before it is employed in experiments. In an ordinary and
fair condition, when the balls were so electrified as to give a repulsive
torsion force of 100° at the standard distance of 30°, it took nearly four
hours to sink to 50° at the same distance; the average loss from 400° to
300° being at the rate of 2°.7 per minute, from 300° to 200° of 1°.7 per
minute, from 200° to 100° of 1°.3 per minute, and from 100° to 50° of 0°.87
per minute. As a complete measurement by the instrument may be made in much
less than a minute, the amount of loss in that time is but small, and can
easily be taken into account.

1187. _The inductive apparatus._--My object was to examine inductive action
carefully when taking place through different media, for which purpose it
was necessary to subject these media to it in exactly similar
circumstances, and in such quantities as should suffice to eliminate any
variations they might present. The requisites of the apparatus to be
constructed were, therefore, that the inducing surfaces of the conductors
should have a constant form and state, and be at a constant distance from
each other; and that either solids, fluids, or gases might be placed and
retained between these surfaces with readiness and certainty, and for any
length of time.

1188. The apparatus used may be described in general terms as consisting of
two metallic spheres of unequal diameter, placed, the smaller within the
larger, and concentric with it; the interval between the two being the
space through which the induction was to take place. A section of it is
given (Plate VII. fig. 104.) on a scale of one-half: _a, a_ are the two
halves of a brass sphere, with an air-tight joint at _b_, like that of the
Magdeburg hemispheres, made perfectly flush and smooth inside so as to
present no irregularity; _c_ is a connecting piece by which the apparatus
is joined to a good stop-cock _d_, which is itself attached either to the
metallic foot _e_, or to an air-pump. The aperture within the hemisphere at
_f_ is very small: _g_ is a brass collar fitted to the upper hemisphere,
through which the shell-lac support of the inner ball and its stem passes;
_h_ is the inner ball, also of brass; it screws on to a brass stem _i_,
terminated above by a brass ball B, _l, l_ is a mass of shell-lac, moulded
carefully on to _i_, and serving both to support and insulate it and its
balls _h_, B. The shell-lac stem _l_ is fitted into the socket _g_, by a
little ordinary resinous cement, more fusible than shell-lac, applied at
_mm_ in such a way as to give sufficient strength and render the apparatus
air-tight there, yet leave as much as possible of the lower part of the
shell-lac stem untouched, as an insulation between the ball _h_ and the
surrounding sphere _a, a_. The ball _h_ has a small aperture at _n_, so
that when the apparatus is exhausted of one gas and filled with another,
the ball _h_ may itself also be exhausted and filled, that no variation of
the gas in the interval _o_ may occur during the course of an experiment.

1189. It will be unnecessary to give the dimensions of all the parts, since
the drawing is to a scale of one-half: the inner ball has a diameter 2.33
inches, and the surrounding sphere an internal diameter of 3.57 inches.
Hence the width of the intervening space, through which the induction is to
take place, is 0.62 of an inch; and the extent of this place or plate, i.e.
the surface of a medium sphere, may be taken as twenty-seven square inches,
a quantity considered as sufficiently large for the comparison of different
substances. Great care was taken in finishing well the inducing surfaces of
the ball _h_ and sphere _a, a_; and no varnish or lacquer was applied to
them, or to any part of the metal of the apparatus.

1190. The attachment and adjustment of the shell-lac stem was a matter
requiring considerable care, especially as, in consequence of its cracking,
it had frequently to be renewed. The best lac was chosen and applied to the
wire _i_, so as to be in good contact with it everywhere, and in perfect
continuity throughout its own mass. It was not smaller than is given by
scale in the drawing, for when less it frequently cracked within a few
hours after it was cold. I think that very slow cooling or annealing
improved its quality in this respect. The collar _g_ was made as thin as
could be, that the lac might be as wide there as possible. In order that at
every re-attachment of the stem to the upper hemisphere the ball _h_ might
have the same relative position, a gauge _p_ (fig. 105.) was made of wood,
and this being applied to the ball and hemisphere whilst the cement at _m_
was still soft, the bearings of the ball at _qq_, and the hemisphere at
_rr_, were forced home, and the whole left until cold. Thus all difficulty
in the adjustment of the ball in the sphere was avoided.

1191. I had occasion at first to attach the stem to the socket by other
means, as a band of paper or a plugging of white silk thread; but these
were very inferior to the cement, interfering much with the insulating
power of the apparatus.

1192. The retentive power of this apparatus was, when in good condition,
better than that of the electrometer (1186.), i.e. the proportion of loss
of power was less. Thus when the apparatus was electrified, and also the
balls in the electrometer, to such a degree, that after the inner ball had
been in contact with the top _k_ of the ball of the apparatus, it caused a
repulsion indicated by 600° of torsion force, then in falling from 600° to
400° the average loss was 8°.6 per minute; from 400° to 300° the average
loss was 2°.6 per minute; from 300° to 200° it was 1°.7 per minute; from
200° to 170° it was 1° per minute. This was after the apparatus had been
charged for a short time; at the first instant of charging there is an
apparent loss of electricity, which can only be comprehended hereafter
(1207. 1250.).

1193. When the apparatus loses its insulating power suddenly, it is almost
always from a crack near to or within the brass socket. These cracks are
usually transverse to the stem. If they occur at the part attached by
common cement to the socket, the air cannot enter, and thus constituting
vacua, they conduct away the electricity and lower the charge, as fast
almost as if a piece of metal had been introduced there. Occasionally stems
in this state, being taken out and cleared from the common cement, may, by
the careful application of the heat of a spirit-lamp, be so far softened
and melted as to restore the perfect continuity of the parts; but if that
does not succeed in replacing things in a good condition, the remedy is a
new shell-lac stem.

1194. The apparatus when in order could easily be exhausted of air and
filled with any given gas; but when that gas was acid or alkaline, it could
not properly be removed by the air-pump, and yet required to be perfectly
cleared away. In such cases the apparatus was opened and emptied of gas;
and with respect to the inner ball _h_, it was washed out two or three
times with distilled water introduced at the screw-hole, and then being
heated above 212°, air was blown through to render the interior perfectly
dry.

1195. The inductive apparatus described is evidently a Leyden phial, with
the advantage, however, of having the _dielectric_ or insulating medium
changed at pleasure. The balls _h_ and B, with the connecting wire _i_,
constitute the charged conductor, upon the surface of which all the
electric force is resident by virtue of induction (1178.). Now though the
largest portion of this induction is between the ball _h_ and the
surrounding sphere _aa_, yet the wire _i_ and the ball B determine a part
of the induction from their surfaces towards the external surrounding
conductors. Still, as all things in that respect remain the same, whilst
the medium within at _oo_, may be varied, any changes exhibited by the
whole apparatus will in such cases depend upon the variations made in the
interior; and these were the changes I was in search of, the negation or
establishment of such differences being the great object of my inquiry. I
considered that these differences, if they existed, would be most
distinctly set forth by having two apparatus of the kind described,
precisely similar in every respect; and then, _different insulating media_
being within, to charge one and measure it, and after dividing the charge
with the other, to observe what the ultimate conditions of both were. If
insulating media really had any specific differences in favouring or
opposing inductive action through them, such differences, I conceived,
could not fail of being developed by such a process.

1196. I will wind up this description of the apparatus, and explain the
precautions necessary to their use, by describing the form and order of the
experiments made to prove their equality when both contained common air. In
order to facilitate reference I will distinguish the two by the terms App.
i. and App. ii.

1197. The electrometer is first to be adjusted and examined (1184.), and
the app. i. and ii. are to be perfectly discharged. A Leyden phial is to be
charged to such a degree that it would give a spark of about one-sixteenth
or one-twentieth of an inch in length between two balls of half an inch
diameter; and the carrier ball of the electrometer being charged by this
phial, is to be introduced into the electrometer, and the lever ball
brought by the motion of the torsion index against it; the charge is thus
divided between the balls, and repulsion ensues. It is useful then to bring
the repelled ball to the standard distance of 30° by the motion of the
torsion index, and observe the force in degrees required for this purpose;
this force will in future experiments be called _repulsion of the balls_.

1198. One of the inductive apparatus, as, for instance, app. i., is now to
be charged from the Leyden phial, the latter being in the state it was in
when used to charge the balls; the carrier ball is to be brought into
contact with the top of its upper ball (_k_, fig. 104.), then introduced
into the electrometer, and the repulsive force (at the distance of 30°)
measured. Again, the carrier should be applied to the app. i. and the
measurement repeated; the apparatus i. and ii. are then to be joined, so as
to _divide_ the charge, and afterwards the force of each measured by the
carrier ball, applied as before, and the results carefully noted. After
this both i. and ii. are to be discharged; then app. ii. charged, measured,
divided with app. i., and the force of each again measured and noted. If in
each case the half charges of app. i. and ii. are equal, and are together
equal to the whole charge before division, then it may be considered as
proved that the two apparatus are precisely equal in power, and fit to be
used in cases of comparison between different insulating media or
_dielectrics_.

1199. But the _precautions_ necessary to obtain accurate results are
numerous. The apparatus i. and ii. must always be placed on a thoroughly
uninsulating medium. A mahogany table, for instance, is far from
satisfactory in this respect, and therefore a sheet of tinfoil, connected
with an extensive discharging train (292.), is what I have used. They must
be so placed also as not to be too near each other, and yet equally exposed
to the inductive influence of surrounding objects; and these objects,
again, should not be disturbed in their position during an experiment, or
else variations of induction upon the external ball B of the apparatus may
occur, and so errors be introduced into the results. The carrier ball, when
receiving its portion of electricity from the apparatus, should always be
applied at the same part of the ball, as, for instance, the summit _k_, and
always in the same way; variable induction from the vicinity of the head,
hands, &c. being avoided, and the ball after contact being withdrawn
upwards in a regular and constant manner.

1200. As the stem had occasionally to be changed (1190.), and the change
might occasion slight variations in the position of the ball within, I made
such a variation purposely, to the amount of an eighth of an inch (which is
far more than ever could occur in practice), but did not find that it
sensibly altered the relation of the apparatus, or its inductive condition
_as a whole_. Another trial of the apparatus was made as to the effect of
dampness in the air, one being filled with very dry air, and the other with
air from over water. Though this produced no change in the result, except
an occasional tendency to more rapid dissipation, yet the precaution was
always taken when working with gases (1290.) to dry them perfectly.

1201. It is essential that the interior of the apparatus should be
perfectly free from _dust or small loose particles_, for these very rapidly
lower the charge and interfere on occasions when their presence and action
would hardly be expected. To breathe on the interior of the apparatus and
wipe it out quietly with a clean silk handkerchief, is an effectual way of
removing them; but then the intrusion of other particles should be
carefully guarded against, and a dusty atmosphere should for this and
several other reasons be avoided.

1202. The shell-lac stem requires occasionally to be well-wiped, to remove,
in the first instance, the film of wax and adhering matter which is upon
it; and afterwards to displace dirt and dust which will gradually attach to
it in the course of experiments. I have found much to depend upon this
precaution, and a silk handkerchief is the best wiper.

1203. But wiping and some other circumstances tend to give a charge to the
surface of the shell-lac stem. This should be removed, for, if allowed to
remain, it very seriously affects the degree of charge given to the carrier
ball by the apparatus (1232.). This condition of the stem is best observed
by discharging the apparatus, applying the carrier ball to the stem,
touching it with the finger, insulating and removing it, and examining
whether it has received any charge (by induction) from the stem; if it has,
the stem itself is in a charged state. The best method of removing the
charge I have found to be, to cover the finger with a single fold of a silk
handkerchief, and breathing on the stem, to wipe it immediately after with
the finger; the ball B and its connected wire, &c. being at the same time
_uninsulated_: the wiping place of the silk must not be changed; it then
becomes sufficiently damp not to excite the stem, and is yet dry enough to
leave it in a clean and excellent insulating condition. If the air be
dusty, it will be found that a single charge of the apparatus will bring on
an electric state of the outside of the stem, in consequence of the
carrying power of the particles of dust; whereas in the morning, and in a
room which has been left quiet, several experiments can be made in
succession without the stem assuming the least degree of charge.

1204. Experiments should not be made by candle or lamp light except with
much care, for flames have great and yet unsteady powers of affecting and
dissipating electrical charges.

1205. As a final observation on the state of the apparatus, they should
retain their charges well and uniformly, and alike for both, and at the
same time allow of a perfect and instantaneous discharge, giving afterwards
no charge to the carrier ball, whatever part of the ball B it may be
applied to (1218.).

1206. With respect to the balance electrometer, all the precautions that
need be mentioned, are, that the carrier ball is to be preserved during the
first part of an experiment in its electrified state, the loss of
electricity which would follow upon its discharge being avoided; and that
in introducing it into the electrometer through the hole in the glass plate
above, care should be taken that it do not touch, or even come near to, the
edge of the glass.

1207. When the whole charge in one apparatus is divided between the two,
the gradual fall, apparently from dissipation, in the apparatus which has
_received_ the half charge is greater than in the one _originally_ charged.
This is due to a peculiar effect to be described hereafter (1250. 1251.),
the interfering influence of which may be avoided to a great extent by
going through the steps of the process regularly and quickly; therefore,
after the original charge has been measured, in app. i. for instance, i.
and ii. are to be symmetrically joined by their balls B, the carrier
touching one of these balls at the same time; it is first to be removed,
and then the apparatus separated from each other; app. ii. is next quickly
to be measured by the carrier, then app. i.; lastly, ii. is to be
discharged, and the discharged carrier applied to it to ascertain whether
any residual effect is present (1205.), and app. i. being discharged is
also to be examined in the same manner and for the same purpose.

1208. The following is an example of the division of a charge by the two
apparatus, air being the dielectric in both of them. The observations are
set down one under the other in the order in which they were taken, the
left-hand numbers representing the observations made on app. i., and the
right-hand numbers those on app. ii. App. i. is that which was originally
charged, and after two measurements, the charge was divided with app. ii.

App. i.    App. ii.
     Balls 160°

      . . . .   0°
254°  . . . .
250   . . . .
divided and instantly taken
      . . . . 122
124   . . . .
  1   . . . .     after being discharged.
      . . . .   2 after being discharged.

1209. Without endeavouring to allow for the loss which must have been
gradually going on during the time of the experiment, let us observe the
results of the numbers as they stand. As 1° remained in app. i. in an
undischargeable state, 249° may be taken as the utmost amount of the
transferable or divisible charge, the half of which is 124°.5. As app. ii.
was free of charge in the first instance, and immediately after the
division was found with 122°, this amount _at least_ may be taken as what
it had received. On the other hand 124° minus 1°, or 123°, may be taken as
the half of the transferable charge retained by app. i. Now these do not
differ much from each other, or from 124°.5, the half of the full amount of
transferable charge; and when the gradual loss of charge evident in the
difference between 254° and 250° of app. i. is also taken into account,
there is every reason to admit the result as showing an equal division of
charge, _unattended by any disappearance of power_ except that due to
dissipation.

1210. I will give another result, in which app. ii. was first charged, and
where the residual action of that apparatus was greater than in the former
case.

App. i.    App. ii.
     Balls 150°

      . . . . 152°
      . . . . 148
divided and instantly taken
  70° . . . .
      . . . .  78
      . . . .   5 immediately after discharge.
   0  . . . .     immediately after discharge.

1211. The transferable charge being 148° - 5°, its half is 71°.5, which is
not far removed from 70°, the half charge of i.; or from 73°, the half
charge of ii.: these half charges again making up the sum of 143°, or just
the amount of the whole transferable charge. Considering the errors of
experiment, therefore, these results may again be received as showing that
the apparatus were equal in inductive capacity, or in their powers of
receiving charges.

1212. The experiments were repeated with charges of negative electricity
with the same general results.

1213. That I might be sure of the sensibility and action of the apparatus,
I made such a change in one as ought upon principle to increase its
inductive force, i.e. I put a metallic lining into the lower hemisphere of
app. i., so as to diminish the thickness of the intervening air in that
part, from 0.62 to 0.435 of an inch: this lining was carefully shaped and
rounded so that it should not present a sudden projection within at its
edge, but a gradual transition from the reduced interval in the lower part
of the sphere to the larger one in the upper.

1214. This change immediately caused app. i. to produce effects indicating
that it had a greater aptness or capacity for induction than app. ii. Thus,
when a transferable charge in app. ii. of 469° was divided with app. i.,
the former retained a charge of 225°, whilst the latter showed one of 227°,
i.e. the former had lost 244° in communicating 227° to the latter: on the
other hand, when app. i. had a transferable charge in it of 381° divided by
contact with app. ii., it lost 181° only, whilst it gave to app. ii. as
many as 194:--the sum of the divided forces being in the first instance
_less_, and in the second instance _greater_ than the original undivided
charge. These results are the more striking, as only one-half of the
interior of app. i. was modified, and they show that the instruments are
capable of bringing out differences in inductive force from amongst the
errors of experiment, when these differences are much less than that
produced by the alteration made in the present instance.


¶ iv. _Induction in curved lines._

1215. Amongst those results deduced from the molecular view of induction
(1166.), which, being of a peculiar nature, are the best tests of the truth
or error of the theory, the expected action in curved lines is, I think,
the most important at present; for, if shown to take place in an
unexceptionable manner, I do not see how the old theory of action at a
distance and in straight lines can stand, or how the conclusion that
ordinary induction is an action of contiguous particles can be resisted.

1216. There are many forms of old experiments which might be quoted as
favourable to, and consistent with the view I have adopted. Such are most
cases of electro-chemical decomposition, electrical brushes, auras, sparks,
&c.; but as these might be considered equivocal evidence, inasmuch as they
include a current and discharge, (though they have long been to me
indications of prior molecular action (1230.)) I endeavoured to devise such
experiments for first proofs as should not include transfer, but relate
altogether to the pure simple inductive action of statical electricity.

1217. It was also of importance to make these experiments in the simplest
possible manner, using not more than one insulating medium or dielectric at
a time, lest differences of slow conduction should produce effects which
might erroneously be supposed to result from induction in curved lines. It
will be unnecessary to describe the steps of the investigation minutely; I
will at once proceed to the simplest mode of proving the facts, first in
air and then in other insulating media.

1218. A cylinder of solid shell-lac, 0.9 of an inch in diameter and seven
inches in length, was fixed upright in a wooden foot (fig. 106.): it was
made concave or cupped at its upper extremity so that a brass ball or other
small arrangement could stand upon it. The upper half of the stem having
been excited _negatively_ by friction with warm flannel, a brass ball, B, 1
inch in diameter, was placed on the top, and then the whole arrangement
examined by the carrier ball and Coulomb's electrometer (1180. &c.). For
this purpose the balls of the electrometer were charged _positively_ to
about 360°, and then the carrier being applied to various parts of the ball
B, the two were uninsulated whilst in contact or in position, then
insulated[A], separated, and the charge of the carrier examined as to its
nature and force. Its electricity was always positive, and its force at the
different positions _a, b, c, d,_ &c. (figs. 106. and 107.) observed in
succession, was as follows:

at _a_        above 1000°
   _b_ it was        149
   _c_               270
   _d_               512
   _b_               130

  [A] It can hardly be necessary for me to say here, that whatever
  general state the carrier ball acquired in any place where it was
  uninsulated and then insulated, it retained on removal from that
  place, notwithstanding that it might pass through other places that
  would have given to it, if uninsulated, a different condition.

1219. To comprehend the full force of these results, it must first be
understood, that all the charges of the ball B and the carrier are charges
by induction, from the action of the excited surface of the shell-lac
cylinder; for whatever electricity the ball B received by _communication_
from the shell-lac, either in the first instance or afterwards, was removed
by the uninsulating contacts, only that due to induction remaining; and
this is shown by the charges taken from the ball in this its uninsulated
state being always positive, or of the contrary character to the
electricity of the shell-lac. In the next place, the charges at _a_, _c_,
and _d_ were of such a nature as might be expected from an inductive action
in straight lines, but that obtained at _b_ is _not so_: it is clearly a
charge by induction, but _induction_ in _a curved line_; for the carrier
ball whilst applied to _b_, and after its removal to a distance of six
inches or more from B, could not, in consequence of the size of B, be
connected by a straight line with any part of the excited and inducing
shell-lac.

1220. To suppose that the upper part of the _uninsulated_ ball B, should in
some way be retained in an electrified state by that portion of the surface
of the ball which is in sight of the shell-lac, would be in opposition to
what we know already of the subject. Electricity is retained upon the
surface of conductors only by induction (1178.); and though some persons
may not be prepared as yet to admit this with respect to insulated
conductors, all will as regards uninsulated conductors like the ball B; and
to decide the matter we have only to place the carrier ball at _e_ (fig.
107.), so that it shall not come in contact with B, uninsulate it by a
metallic rod descending perpendicularly, insulate it, remove it, and
examine its state; it will be found charged with the same kind of
electricity as, and even to a _higher degree_ (1224.) than, if it had been
in contact with the summit of B.

1221. To suppose, again, that induction acts in some way _through or
across_ the metal of the ball, is negatived by the simplest considerations;
but a fact in proof will be better. If instead of the ball B a small disc
of metal be used, the carrier may be charged at, or above the middle of its
upper surface: but if the plate be enlarged to about 1-1/2 or 2 inches in
diameter, C (fig. 108.), then no charge will be given to the carrier at
_f_, though when applied nearer to the edge at _g_, or even _above the
middle_ at _h_, a charge will be obtained; and this is true though the
plate may be a mere thin film of gold-leaf. Hence it is clear that the
induction is not _through_ the metal, but through the surrounding air or
_dielectric_, and that in curved lines.

1222. I had another arrangement, in which a wire passing downwards through
the middle of the shell-lac cylinder to the earth, was connected with the
ball B (fig. 109.) so as to keep it in a constantly uninsulated state. This
was a very convenient form of apparatus, and the results with it were the
same as those just described.

1223. In another case the ball B was supported by a shell-lac stem,
independently of the excited cylinder of shell-lac, and at half an inch
distance from it; but the effects were the same. Then the brass ball of a
charged Leyden jar was used in place of the excited shell-lac to produce
induction; but this caused no alteration of the phenomena. Both positive
and negative inducing charges were tried with the same general results.
Finally, the arrangement was inverted in the air for the purpose of
removing every possible objection to the conclusions, but they came out
exactly the same.

1224. Some results obtained with a brass hemisphere instead of the ball B
were exceedingly interesting, It was 1.36 of an inch in diameter, (fig.
110.), and being placed on the top of the excited shell-lac cylinder, the
carrier ball was applied, as in the former experiments (1218.), at the
respective positions delineated in the figure. At _i_ the force was 112°,
at _k_ 108°, at _l_ 65°, at _m_ 35°; the inductive force gradually
diminishing, as might have been expected, to this point. But on raising the
carrier to the position _n_, the charge increased to 87°; and on raising it
still higher to _o_, the charge still further increased to 105°: at a
higher point still, _p_, the charge taken was smaller in amount, being 98°,
and continued to diminish for more elevated positions. Here the induction
fairly turned a corner. Nothing, in fact, can better show both the curved
lines or courses of the inductive action, disturbed as they are from their
rectilineal form by the shape, position, and condition of the metallic
hemisphere; and also a _lateral tension,_ so to speak, of these lines on
one another:--all depending, as I conceive, on induction being an action of
the contiguous particles of the dielectric, which being thrown into a state
of polarity and tension, are in mutual relation by their forces in all
directions.

1225. As another proof that the whole of these actions were inductive I may
state a result which was exactly what might be expected, namely, that if
uninsulated conducting matter was brought round and near to the excited
shell-lac stem, then the inductive force was directed towards it, and could
not be found on the top of the hemisphere. Removing this matter the lines
of force resumed their former direction. The experiment affords proofs of
the lateral tension of these lines, and supplies a warning to remove such
matter in repeating the above investigation.

1226. After these results on curved inductive action in air I extended the
experiments to other gases, using first carbonic acid and then hydrogen:
the phenomena were precisely those already described. In these experiments
I found that if the gases were confined in vessels they required to be very
large, for whether of glass or earthenware, the conducting power of such
materials is so great that the induction of the excited shell-lac cylinder
towards them is as much as if they were metal; and if the vessels be small,
so great a portion of the inductive force is determined towards them that
the lateral tension or mutual repulsion of the lines of force before spoken
of, (1224.) by which their inflexion is caused, is so much relieved in
other directions, that no inductive charge will be given to the carrier
ball in the positions _k, l, m, n, o, p_ (fig. 110.). A very good mode of
making the experiment is to let large currents of the gases ascend or
descend through the air, and carry on the experiments in these currents.

1227. These experiments were then varied by the substitution of a liquid
dielectric, namely, _oil of turpentine_, in place of air and gases. A dish
of thin glass well-covered with a film of shell-lac (1272.), which was
found by trial to insulate well, had some highly rectified oil of
turpentine put into it to the depth of half an inch, and being then placed
upon the top of the brass hemisphere (fig. 110.), observations were made
with the carrier ball as before (1224.). The results were the same, and the
circumstance of some of the positions being within the fluid and some
without, made no sensible difference.

1228. Lastly, I used a few solid dielectrics for the same purpose, and with
the same results. These were shell-lac, sulphur, fused and cast borate of
lead, flint glass well-covered with a film of lac, and spermaceti. The
following was the form of experiment with sulphur, and all were of the same
kind. A square plate of the substance, two inches in extent and 0.6 of an
inch in thickness, was cast with a small hole or depression in the middle
of one surface to receive the carrier ball. This was placed upon the
surface of the metal hemisphere (fig. 112.) arranged on the excited lac as
in former cases, and observations were made at _n, o, p_, and _q_. Great
care was required in these experiments to free the sulphur or other solid
substance from any charge it might previously have received. This was done
by breathing and wiping (1203.), and the substance being found free from
all electrical excitement, was then used in the experiment; after which it
was removed and again examined, to ascertain that it had received no
charge, but had acted really as a dielectric. With all these precautions
the results were the same: and it is thus very satisfactory to obtain the
curved inductive action through _solid bodies_, as any possible effect from
the translation of charged particles in fluids or gases, which some persons
might imagine to be the case, is here entirely negatived.

1229. In these experiments with solid dielectrics, the degree of charge
assumed by the carrier ball at the situations _n, o, p_ (fig. 112.), was
decidedly greater than that given to the ball at the same places when air
only intervened between it and the metal hemisphere. This effect is
consistent with what will hereafter be found to be the respective relations
of these bodies, as to their power of facilitating induction through them
(1269. 1273. 1277.).

1230. I might quote _many_ other forms of experiment, some old and some
new, in which induction in curved or contorted lines takes place, but think
it unnecessary after the preceding results; I shall therefore mention but
two. If a conductor A, (fig. 111.) be electrified, and an uninsulated
metallic ball B, or even a plate, provided the edges be not too thin, be
held before it, a small electrometer at _c_ or at _d_, uninsulated, will
give signs of electricity, opposite in its nature to that of A, and
therefore caused by induction, although the influencing and influenced
bodies cannot be joined by a right line passing through the air. Or if, the
electrometers being removed, a point be fixed at the back of the ball in
its uninsulated state as at C, this point will become luminous and
discharge the conductor A. The latter experiment is described by
Nicholson[A], who, however, reasons erroneously upon it. As to its
introduction here, though it is a case of discharge, the discharge is
preceded by induction, and that induction must be in curved lines.

  [A] Encyclopædia Britannica, vol. vi. p. 504.

1231. As argument against the received theory of induction and in favour of
that which I have ventured to put forth, I cannot see how the preceding
results can be avoided. The effects are clearly inductive effects produced
by electricity, not in currents but in its statical state, and this
induction is exerted in lines of force which, though in many experiments
they may be straight, are here curved more or less according to
circumstances. I use the term _line of inductive force_ merely as a
temporary conventional mode of expressing the direction of the power in
cases of induction; and in the experiments with the hemisphere (1224.), it
is curious to see how, when certain lines have terminated on the under
surface and edge of the metal, those which were before lateral to them
_expand and open out from each other_, some bending round and terminating
their action on the upper surface of the hemisphere, and others meeting, as
it were, above in their progress outwards, uniting their forces to give an
increased charge to the carrier ball, at an _increased distance_ from the
source of power, and influencing each other so as to cause a second flexure
in the contrary direction from the first one. All this appears to me to
prove that the whole action is one of contiguous particles, related to each
other, not merely in the lines which they may be conceived to form through
the dielectric, between the _inductric_ and the _inducteous_ surfaces
(1483.), but in other lateral directions also. It is this which gives an
effect equivalent to a lateral repulsion or expansion in the lines of force
I have spoken of, and enables induction to turn a corner (1304.). The
power, instead of being like that of gravity, which causes particles to act
on each other through straight lines, whatever other particles may be
between them, is more analogous to that of a series of magnetic needles, or
to the condition of the particles considered as forming the whole of a
straight or a curved magnet. So that in whatever way I view it, and with
great suspicion of the influence of favourite notions over myself, I cannot
perceive how the ordinary theory applied to explain induction can be a
correct representation of that great natural principle of electrical
action.

1232. I have had occasion in describing the precautions necessary in the
use of the inductive apparatus, to refer to one founded on induction in
curved lines (1203.); and after the experiments already described, it will
easily be seen how great an influence the shell-lac stem may exert upon the
charge of the carrier ball when applied to the apparatus (1218.), unless
that precaution be attended to.

1233. I think it expedient, next in the course of these experimental
researches, to describe some effects due to _conduction_, obtained with
such bodies as glass, lac, sulphur, &c., which had not been anticipated.
Being understood, they will make us acquainted with certain precautions
necessary in investigating the great question of specific inductive
capacity.

1234. One of the inductive apparatus already described (1187, &c.) had a
hemispherical cup of shell-lac introduced, which being in the interval
between the inner bull and the lower hemisphere, nearly occupied the space
there; consequently when the apparatus was charged, the lac was the
dielectric or insulating medium through which the induction took place in
that part. When this apparatus was first charged with electricity (1198.)
up to a certain intensity, as 400°, measured by the COULOMB'S electrometer
(1180.), it sank much faster from that degree than if it had been
previously charged to a higher point, and had gradually fallen to 400°; or
than it would do if the charge were, by a second application, raised up
again to 400°; all other things remaining the same. Again, if after having
been charged for some time, as fifteen or twenty minutes, it was suddenly
and perfectly discharged, even the stem having all electricity removed from
it (1203.), then the apparatus being left to itself, would gradually
recover a charge, which in nine or ten minutes would rise up to 50° or 60°,
and in one instance to 80°.

1235. The electricity, which in these cases returned from an apparently
latent to a sensible state, was always of the same kind as that which had
been given by the charge. The return took place at both the inducing
surfaces; for if after the perfect discharge of the apparatus the whole was
insulated, as the inner ball resumed a positive state the outer sphere
acquired a negative condition.

1236. This effect was at once distinguished from that produced by the
excited stem acting in curved lines of induction (1203. 1232.), by the
circumstance that all the returned electricity could be perfectly and
instantly discharged. It appeared to depend upon the shell-lac within, and
to be, in some way, due to electricity evolved from it in consequence of a
previous condition into which it had been brought by the charge of the
metallic coatings or balls.

1237. To examine this state more accurately, the apparatus, with the
hemispherical cup of shell-lac in it, was charged for about forty-five
minutes to above 600° with positive electricity at the balls _h_ and B.
(fig. 104.) above and within. It was then discharged, opened, the shell-lac
taken out, and its state examined; this was done by bringing the carrier
ball near the shell-lac, uninsulating it, insulating it, and then observing
what charge it had acquired. As it would be a charge by induction, the
state of the ball would indicate the opposite state of electricity in that
surface of the shell-lac which had produced it. At first the lac appeared
quite free from any charge; but gradually its two surfaces assumed opposite
states of electricity, the concave surface, which had been next the inner
and positive ball; assuming a positive state, and the convex surface, which
had been in contact with the negative coating, acquiring a negative state;
these states gradually increased in intensity for some time.

1238. As the return action was evidently greatest instantly after the
discharge, I again put the apparatus together, and charged it for fifteen
minutes as before, the inner ball positively. I then discharged it,
instantly removing the upper hemisphere with the interior ball, and,
leaving the shell-lac cup in the lower uninsulated hemisphere, examined its
inner surface by the carrier ball as before (1237.). In this way I found
the surface of the shell-lac actually _negative_, or in the reverse state
to the ball which had been in it; this state quickly disappeared, and was
succeeded by a positive condition, gradually increasing in intensity for
some time, in the same manner as before. The first negative condition of
the surface opposite the positive charging ball is a natural consequence of
the state of things, the charging ball being in contact with the shell-lac
only in a few points. It does not interfere with the general result and
peculiar state now under consideration, except that it assists in
illustrating in a very marked manner the ultimate assumption by the
surfaces of the shell-lac of an electrified condition, similar to that of
the metallic surfaces opposed to or against them.

1239. _Glass_ was then examined with respect to its power of assuming this
peculiar state. I had a thick flint-glass hemispherical cup formed, which
would fit easily into the space _o_ of the lower hemisphere (1188. 1189.);
it had been heated and varnished with a solution of shell-lac in alcohol,
for the purpose of destroying the conducting power of the vitreous surface
(1254.). Being then well-warmed and experimented with, I found it could
also assume the _same state_, but not apparently to the same degree, the
return action amounting in different cases to quantities from 6° to 18°.

1240. _Spermaceti_ experimented with in the same manner gave striking
results. When the original charge had been sustained for fifteen or twenty
minutes at about 500°, the return charge was equal to 95° or 100°, and was
about fourteen minutes arriving at the maximum effect. A charge continued
for not more than two or three seconds was here succeeded by a return
charge of 50° or 60°. The observations formerly made (1234.) held good with
this substance. Spermaceti, though it will insulate a low charge for some
time, is a better conductor than shell-lac, glass, and sulphur; and this
conducting power is connected with the readiness with which it exhibits the
particular effect under consideration.

1241. _Sulphur._--I was anxious to obtain the amount of effect with this
substance, first, because it is an excellent insulator, and in that respect
would illustrate the relation of the effect to the degree of conducting
power possessed by the dielectric (1247.); and in the next place, that I
might obtain that body giving the smallest degree of the effect now under
consideration for the investigation of the question of specific inductive
capacity (1277.).

1242. With a good hemispherical cup of sulphur cast solid and sound, I
obtained the return charge, but only to an amount of 17° or 18°. Thus glass
and sulphur, which are bodily very bad conductors of electricity, and
indeed almost perfect insulators, gave very little of this return charge.

1243. I tried the same experiment having _air_ only in the inductive
apparatus. After a continued high charge for some time I could obtain a
little effect of return action, but it was ultimately traced to the
shell-lac of the stem.

1244. I sought to produce something like this state with one electric power
and without induction; for upon the theory of an electric fluid or fluids,
that did not seem impossible, and then I should have obtained an absolute
charge (1169. 1177.), or something equivalent to it. In this I could not
succeed. I excited the outside of a cylinder of shell-lac very highly for
some time, and then quickly discharging it (1203.), waited and watched
whether any return charge would appear, but such was not the case. This is
another fact in favour of the inseparability of the two electric forces
(1177.), and another argument for the view that induction and its
concomitant phenomena depend upon a polarity of the particles of matter.

1245. Although inclined at first to refer these effects to a peculiar
masked condition of a certain portion of the forces, I think I have since
correctly traced them to known principles of electrical action. The effects
appear to be due to an actual penetration of the charge to some distance
within the electric, at each of its two surfaces, by what we call
_conduction_; so that, to use the ordinary phrase, the electric forces
sustaining the induction are not upon the metallic surfaces only, but upon
and within the dielectric also, extending to a greater or smaller depth
from the metal linings. Let _c_ (fig. 113.) be the section of a plate of
any dielectric, _a_ and _b_ being the metallic coatings; let _b_ be
uninsulated, and _a_ be charged positively; after ten or fifteen minutes,
if _a_ and _b_ be discharged, insulated, and immediately examined, no
electricity will appear in them; but in a short time, upon a second
examination, they will appear charged in the same way, though not to the
same degree, as they were at first. Now suppose that a portion of the
positive force has, under the coercing influence of all the forces
concerned, penetrated the dielectric and taken up its place at the line
_p_, a corresponding portion of the negative force having also assumed its
position at the line _n_; that in fact the electric at these two parts has
become charged positive and negative; then it is clear that the induction
of these two forces will be much greater one towards the other, and less in
an external direction, now that they are at the small distance _np_ from
each other, than when they were at the larger interval _ab_. Then let _a_
and _b_ be discharged; the discharge destroys or neutralizes all external
induction, and the coatings are therefore found by the carrier ball
unelectrified; but it also removes almost the whole of the forces by which
the electric charge was driven into the dielectric, and though probably a
part of that charge goes forward in its passage and terminates in what we
call discharge, the greater portion returns on its course to the surfaces
of _c_, and consequently to the conductors _a_ and _b_, and constitutes the
recharge observed.

1246. The following is the experiment on which I rest for the truth of this
view. Two plates of spermaceti, _d_ and, _f_ (fig. 114.), were put together
to form the dielectric, _a_ and _b_ being the metallic coatings of this
compound plate, as before. The system was charged, then discharged,
insulated, examined, and found to give no indications of electricity to the
carrier ball. The plates _d_ and _f_were then separated from each other,
and instantly _a_ with _d_ was found in a positive state, and _b_ with _f_
in a negative state, nearly all the electricity being in the linings _a_
and _b_. Hence it is clear that, of the forces sought for, the positive was
in one-half of the compound plate and the negative in the other half; for
when removed bodily with the plates from each other's inductive influence,
they appeared in separate places, and resumed of necessity their power of
acting by induction on the electricity of surrounding bodies. Had the
effect depended upon a peculiar relation of the contiguous particles of
matter only, then each half-plate, _d_ and _f_, should have shown positive
force on one surface and negative on the other.

1247. Thus it would appear that the best solid insulators, such as
shell-lac, glass, and sulphur, have conductive properties to such an
extent, that electricity can penetrate them bodily, though always subject
to the overruling condition of induction (1178.). As to the depth to which
the forces penetrate in this form of charge of the particles,
theoretically, it should be throughout the mass, for what the charge of the
metal does for the portion of dielectric next to it, should be close by the
charged dielectric for the portion next beyond it again; but probably in
the best insulators the sensible charge is to a very small depth only in
the dielectric, for otherwise more would disappear in the first instance
whilst the original charge is sustained, less time would be required for
the assumption of the particular state, and more electricity would
re-appear as return charge.

1248. The condition of _time_ required for this penetration of the charge
is important, both as respects the general relation of the cases to
conduction, and also the removal of an objection that might otherwise
properly be raised to certain results respecting specific inductive
capacities, hereafter to be given (1269. 1277.)

1249. It is the assumption for a time of this charged state of the glass
between the coatings in the Leyden jar, which gives origin to a well-known
phenomenon, usually referred to the diffusion of electricity over the
uncoated portion of the glass, namely, the _residual charge_. The extent of
charge which can spontaneously be recovered by a large battery, after
perfect uninsulation of both surfaces, is very considerable, and by far the
largest portion of this is due to the return of electricity in the manner
described. A plate of shell-lac six inches square, and half an inch thick,
or a similar plate of spermaceti an inch thick, being coated on the sides
with tinfoil as a Leyden arrangement, will show this effect exceedingly
well.

       *       *       *       *       *

1250. The peculiar condition of dielectrics which has now been described,
is evidently capable of producing an effect interfering with the results
and conclusions drawn from the use of the two inductive apparatus, when
shell-lac, glass, &c. is used in one or both of them (1192. 1207.), for
upon dividing the charge in such cases according to the method described
(1198. 1207.), it is evident that the apparatus just receiving its half
charge must fall faster in its tension than the other. For suppose app. i.
first charged, and app. ii. used to divide with it; though both may
actually lose alike, yet app. i., which has been diminished one-half, will
be sustained by a certain degree of return action or charge (1234.), whilst
app. ii. will sink the more rapidly from the coming on of the particular
state. I have endeavoured to avoid this interference by performing the
whole process of comparison as quickly as possible, and taking the force of
app. ii. immediately after the division, before any sensible diminution of
the tension arising from the assumption of the peculiar state could be
produced; and I have assumed that as about three minutes pass between the
first charge of app. i. and the division, and three minutes between the
division and discharge, when the force of the non-transferable electricity
is measured, the contrary tendencies for those periods would keep that
apparatus in a moderately steady and uniform condition for the latter
portion of time.

1251. The particular action described occurs in the shell-lac of the stems,
as well as in the _dielectric_ used within the apparatus. It therefore
constitutes a cause by which the outside of the stems may in some
operations become charged with electricity, independent of the action of
dust or carrying particles (1203.).


¶ v. _On specific induction, or specific inductive capacity._

1252. I now proceed to examine the great question of specific inductive
capacity, i.e. whether different dielectric bodies actually do possess any
influence over the degree of induction which takes place through them. If
any such difference should exist, it appeared to me not only of high
importance in the further comprehension of the laws and results of
induction, but an additional and very powerful argument for the theory I
have ventured to put forth, that the whole depends upon a molecular action,
in contradistinction to one at sensible distances.

The question may be stated thus: suppose A an electrified plate of metal
suspended in the air, and B and C two exactly similar plates, placed
parallel to and on each side of A at equal distances and uninsulated; A
will then induce equally towards B and C. If in this position of the plates
some other dielectric than air, as shell-lac, be introduced between A and
C, will the induction between them remain the same? Will the relation of C
and B to A be unaltered, notwithstanding the difference of the dielectrics
interposed between them?[A]

  [A] Refer for the practical illustration of this statement to the
  supplementary note commencing 1307, &c.--_Dec. 1838._

1253. As far as I recollect, it is assumed that no change will occur under
such variation of circumstances, and that the relations of B find C to A
depend entirely upon their distance. I only remember one experimental
illustration of the question, and that is by Coulomb[A], in which he shows
that a wire surrounded by shell-lac took exactly the same quantity of
electricity from a charged body as the same wire in air. The experiment
offered to me no proof of the truth of the supposition: for it is not the
mere films of dielectric substances surrounding the charged body which have
to be examined and compared, but the _whole mass_ between that body and the
surrounding conductors at which the induction terminates. Charge depends
upon induction (1171. 1178.); and if induction is related to the particles
of the surrounding dielectric, then it is related to _all_ the particles of
that dielectric inclosed by the surrounding conductors, and not merely to
the few situated next to the charged body. Whether the difference I sought
for existed or not, I soon found reason to doubt the conclusion that might
be drawn from Coulomb's result; and therefore had the apparatus made,
which, with its use, has been already described (1187, &c.), and which
appears to me well-suited for the investigation of the question.

  [A] Mémoires de l'Académie, 1787, pp. 452, 453.

1254. Glass, and many bodies which might at first be considered as very fit
to test the principle, proved exceedingly unfit for that purpose. Glass,
principally in consequence of the alkali it contains, however well-warmed
and dried it may be, has a certain degree of conducting power upon its
surface, dependent upon the moisture of the atmosphere, which renders it
unfit for a test experiment. Resin, wax, naphtha, oil of turpentine, and
many other substances were in turn rejected, because of a slight degree of
conducting power possessed by them; and ultimately shell-lac and sulphur
were chosen, after many experiments, as the dielectrics best fitted for the
investigation. No difficulty can arise in perceiving how the possession of
a feeble degree of conducting power tends to make a body produce effects,
which would seem to indicate that it had a greater capability of allowing
induction through it than another body perfect in its insulation. This
source of error has been that which I have found most difficult to obviate
in the proving experiments.

       *       *       *       *       *

1255. _Induction through shell-lac._--As a preparatory experiment, I first
ascertained generally that when a part of the surface of a thick plate of
shell-lac was excited or charged, there was no sensible difference in the
character of the induction sustained by that charged part, whether exerted
through the air in the one direction, or through the shell-lac of the plate
in the other; provided the second surface of the plate had not, by contact
with conductors, the action of dust, or any other means, become charged
(1203.). Its solid condition enabled it to retain the excited particles in
a permanent position, but that appeared to be all; for these particles
acted just as freely through the shell-lac on one side as through the air
on the other. The same general experiment was made by attaching a disc of
tinfoil to one side of the shell-lac plate, and electrifying it, and the
results were the same. Scarcely any other solid substance than shell-lac
and sulphur, and no liquid substance that I have tried, will bear this
examination. Glass in its ordinary state utterly fails; yet it was
essentially necessary to obtain this prior degree of perfection in the
dielectric used, before any further progress could be made in the principal
investigation.

1256. _Shell-lac and air_ were compared in the first place. For this
purpose a thick hemispherical cup of shell-lac was introduced into the
lower hemisphere of one of the inductive apparatus (1187, &c.), so as
nearly to fill the lower half of the space _o, o_ (fig. 104.) between it
and the inner ball; and then charges were divided in the manner already
described (1198. 1207.), each apparatus being used in turn to receive the
first charge before its division by the other. As the apparatus were known
to have equal inductive power when air was in both (1209. 1211.), any
differences resulting from the introduction of the shell-lac would show a
peculiar action in it, and if unequivocally referable to a specific
inductive influence, would establish the point sought to be sustained. I
have already referred to the precautions necessary in making the
experiments (1199, &c.); and with respect to the error which might be
introduced by the assumption of the peculiar state, it was guarded against,
as far as possible, in the first place, by operating quickly (1248); and,
afterwards, by using that dielectric as glass or sulphur, which assumed the
peculiar state most slowly, and in the least degree (1239. 1241.).

1257. The shell-lac hemisphere was put into app. i., and app. ii. left
filled with air. The results of an experiment in which the charge through
air was divided and reduced by the shell-lac app. were as follows:

App. i. Lac.   App. ii. Air.
        Balls 255°.

    0°  . . . .
        . . . .  304°
        . . . .  297
      Charge divided.
   113  . . . .
        . . . .  121
     0  . . . .      after being discharged.
        . . . .    7 after being discharged.

1258. Here 297°, minus 7°, or 290°, may be taken as the divisible charge of
app. ii. (the 7° being fixed stem action (1203. 1232.)), of which 145° is
the half. The lac app. i. gave 113° as the power or tension it had acquired
after division; and the air app. ii. gave 121°, minus 7°, or 114°, as the
force it possessed from what it retained of the divisible charge of 290°.
These two numbers should evidently be alike, and they are very nearly so,
indeed far within the errors of experiment and observation, but these
numbers differ very much from 145°, or the force which the half charge
would have had if app. i. had contained air instead of shell-lac; and it
appears that whilst in the division the induction through the air has lost
176° of force, that through the lac has only gained 113°.

1259. If this difference be assumed as depending entirely on the greater
facility possessed by shell-lac of allowing or causing inductive action
through its substance than that possessed by air, then this capacity for
electric induction would be inversely as the respective loss and gain
indicated above; and assuming the capacity of the air apparatus as 1, that
of the shell-lac apparatus would be 176/113 or 1.55.

1260. This extraordinary difference was so unexpected in its amount, as to
excite the greatest suspicion of the general accuracy of the experiment,
though the perfect discharge of app. i. after the division, showed that the
113° had been taken and given up readily. It was evident that, if it really
existed, it ought to produce corresponding effects in the reverse order;
and that when induction through shell-lac was converted into induction
through air, the force or tension of the whole ought to be _increased_. The
app. i. was therefore charged in the first place, and its force divided
with app. ii. The following were the results:

App. i. Lac.   App. ii. Air.
        . . . .    0°
  215°  . . . .
  204   . . . .
      Charge divided.
        . . . .  118
  118   . . . .
        . . . .    0 after being discharged.
    0   . . . .      after being discharged.

1261. Here 204° must be the utmost of the divisible charge. The app. i. and
app. ii. present 118° as their respective forces; both now much _above_ the
half of the first force, or 102°, whereas in the former case they were
below it. The lac app. i. has lost only 86°, yet it has given to the air
app. ii. 118°, so that the lac still appears much to surpass the air, the
capacity of the lac app. i. to the air app. ii. being as 1.37 to 1.

1262. The difference of 1.55 and 1.37 as the expression of the capacity for
the induction of shell-lac seems considerable, but is in reality very
admissible under the circumstances, for both are in error in _contrary
directions_. Thus in the last experiment the charge fell from 215° to 204°
by the joint effects of dissipation and absorption (1192. 1250.), during
the time which elapsed in the electrometer operations, between the
applications of the carrier ball required to give those two results. Nearly
an equal time must have elapsed between the application of the carrier
which gave the 204° result, and the division of the charge between the two
apparatus; and as the fall in force progressively decreases in amount
(1192.), if in this case it be taken at 6° only, it will reduce the whole
transferable charge at the time of division to 198° instead of 204°; this
diminishes the loss of the shell-lac charge to 80° instead of 86°; and then
the expression of specific capacity for it is increased, and, instead of
1.37, is 1.47 times that of air.

1263. Applying the same correction to the former experiment in which air
was _first_ charged, the result is of the _contrary_ kind. No shell-lac
hemisphere was then in the apparatus, and therefore the loss would be
principally from dissipation, and not from absorption: hence it would be
nearer to the degree of loss shown by the numbers 304° and 297°, and being
assumed as 6° would reduce the divisible charge to 284°. In that case the
air would have lost 170°, and communicated only 113° to the shell-lac; and
the relative specific capacity of the latter would appear to be 1.50, which
is very little indeed removed from 1.47, the expression given by the second
experiment when corrected in the same way.

1264. The shell-lac was then removed from app. i. and put into app. ii. and
the experiments of division again made. I give the results, because I think
the importance of the point justifies and even requires them.

App. i. Air.   App. ii. Lac.
        Balls 200°.

        . . . .    0°.
  286°  . . . .
  283   . . . .
      Charge divided.
        . . . .  110
  109   . . . .
        . . . .    0.25 after discharge.
Trace   . . . .         after discharge.

Here app. i. retained 109°, having lost 174° in communicating 110° to app.
ii.; and the capacity of the air app. is to the lac app., therefore, as 1
to 1.58. If the divided charge be corrected for an assumed loss of only 3°,
being the amount of previous loss in the same time, it will make the
capacity of the shell-lac app. 1.55 only.

1265. Then app. ii. was charged, and the charge divided thus:

App. i. Air.   App. ii. Lac,
    0°   . . . .
         . . . .  250°
         . . . .  251
      Charge divided.
  146    . . . .
         . . . .  149
a little . . . .          after discharge.
         . . . . a little after discharge.

Here app. i. acquired a charge of 146°, while app. ii. lost only 102° in
communicating that amount of force; the capacities being, therefore, to
each other as 1 to 1.43. If the whole transferable charge be corrected for
a loss of 4° previous to division, it gives the expression of l.49 for the
capacity of the shell-lac apparatus.

1266. These four expressions of 1.47, 1.50, 1.55, and 1.49 for the power of
the shell-lac apparatus, through the different variations of the
experiment, are very near to each other; the average is close upon 1.5,
which may hereafter be used as the expression of the result. It is a very
important result; and, showing for this particular piece of shell-lac a
decided superiority over air in allowing or causing the act of induction,
it proved the growing necessity of a more close and rigid examination of
the whole question.

1267. The shell-lac was of the best quality, and had been carefully
selected and cleaned; but as the action of any conducting particles in it
would tend, virtually, to diminish the quantity or thickness of the
dielectric used, and produce effects as if the two inducing surfaces of the
conductors in that apparatus were nearer together than in the one with air
only, I prepared another shell-lac hemisphere, of which the material had
been dissolved in strong spirit of wine, the solution filtered, and then
carefully evaporated. This is not an easy operation, for it is difficult to
drive off the last portions of alcohol without injuring the lac by the heat
applied; and unless they be dissipated, the substance left conducts too
well to be used in these experiments. I prepared two hemispheres this way,
one of them unexceptionable; and with it I repeated the former experiments
with all precautions. The results were exactly of the same kind; the
following expressions for the capacity of the shell-lac apparatus, whether
it were app. i. or ii., being given directly by the experiments, 1.46,
1.50, 1.52, 1.51; the average of these and several others being very nearly
1.5.

1268. As a final check upon the general conclusion, I then actually brought
the surfaces of the air apparatus, corresponding to the place of the
shell-lac in its apparatus, nearer together, by putting a metallic lining
into the lower hemisphere of the one not containing the lac (1213.). The
distance of the metal surface from the carrier ball was in this way
diminished from 0.62 of an inch to 0.435 of an inch, whilst the interval
occupied by the lac in the other apparatus remained O.62 of an inch as
before. Notwithstanding this change, the lac apparatus showed its former
superiority; and whether it or the air apparatus was charged first, the
capacity of the lac apparatus to the air apparatus was by the experimental
results as 1.45 to 1.

1269. From all the experiments I have made, and their constant results, I
cannot resist the conclusion that shell-lac does exhibit a case of
_specific inductive capacity_. I have tried to check the trials in every
way, and if not remove, at least estimate, every source of error. That the
final result is not due to common conduction is shown by the capability of
the apparatus to retain the communicated charge; that it is not due to the
conductive power of inclosed small particles, by which they could acquire a
polarized condition as conductors, is shown by the effects of the shell-lac
purified by alcohol; and, that it is not due to any influence of the
charged state, formerly described (1250.), first absorbing and then
evolving electricity, is indicated by the _instantaneous_ assumption and
discharge of those portions of the power which are concerned in the
phenomena, that instantaneous effect occurring in these cases, as in all
others of ordinary induction, by charged conductors. The latter argument is
the more striking in the case where the air apparatus is employed to divide
the charge with the lac apparatus, for it obtains its portion of
electricity in an _instant_, and yet is charged far above the _mean_.

1270. Admitting for the present the general fact sought to be proved; then
1.5, though it expresses the capacity of the apparatus containing the
hemisphere of shell-lac, by no means expresses the relation of lac to air.
The lac only occupies one-half of the space _o, o_, of the apparatus
containing it, through which the induction is sustained; the rest is filled
with air, as in the other apparatus; and if the effect of the two upper
halves of the globes be abstracted, then the comparison of the shell-lac
powers in the lower half of the one, with the power of the air in the lower
half of the other, will be as 2:1; and even this must be less than the
truth, for the induction of the upper part of the apparatus, i.e. of the
wire and ball B. (fig. 104.) to external objects, must be the same in both,
and considerably diminish the difference dependent upon, and really
producible by, the influence of the shell-lac within.

       *       *       *       *       *

1271. _Glass._--I next worked with glass as the dielectric. It involved the
possibility of conduction on its surface, but it excluded the idea of
conducting particles within its substance (1267.) other than those of its
own mass. Besides this it does not assume the charged state (1239.) so
readily, or to such an extent, as shell-lac.

1272. A thin hemispherical cup of glass being made hot was covered with a
coat of shell-lac dissolved in alcohol, and after being dried for many
hours in a hot place, was put into the apparatus and experimented with. It
exhibited effects so slight, that, though they were in the direction
indicating a superiority of glass over air, they were allowed to pass as
possible errors of experiment; and the glass was considered as producing no
sensible effect.

1273. I then procured a thick hemispherical flint glass cup resembling that
of shell-lac (1239.), but not filling up the space _o, o_, so well. Its
average thickness was 0.4 of an inch, there being an additional thickness
of air, averaging 0.22 of an inch, to make up the whole space of 0.62 of an
inch between the inductive metallic surfaces. It was covered with a film of
shell-lac as the former was, (1272.) and being made very warm, was
introduced into the apparatus, also warmed, and experiments made with it as
in the former instances (1257. &c.). The general results were the same as
with shell-lac, i.e. glass surpassed air in its power of favouring
induction through it. The two best results as respected the state of the
apparatus for retention of charge, &c., gave, when the air apparatus was
charged first 1.336, and when the glass apparatus was charged first 1.45,
as the specific inductive capacity for glass, both being without
correction. The average of nine results, four with the glass apparatus
first charged, and five with the air apparatus first charged, gave 1.38 as
the power of the glass apparatus; 1.22 and 1.46 being the minimum and
maximum numbers with all the errors of experiment upon them. In all the
experiments the glass apparatus took up its inductive charge instantly, and
lost it as readily (1269.); and during the short time of each experiment,
acquired the peculiar state in a small degree only, so that the influence
of this state, and also of conduction upon the results, must have been
small.

1274. Allowing specific inductive capacity to be proved and active in this
case, and 1.38 as the expression for the glass apparatus, then the specific
inductive capacity of flint glass will be above 1.76, not forgetting that
this expression is for a piece of glass of such thickness as to occupy not
quite two-thirds of the space through which the induction is sustained
(1253. 1273.).

       *       *       *       *       *

1275. _Sulphur._--The same hemisphere of this substance was used in app.
ii. as was formerly referred to (1242.). The experiments were well made,
i.e. the sulphur itself was free from charge both before and after each
experiment, and no action from the stem appeared (1203. 1232.), so that no
correction was required on that account. The following are the results when
the air apparatus was first charged and divided:

App. i. Air,   App. ii. Sulphur.
        Balls 280°.

    0°  . . . .
        . . . .    0°
  438   . . . .
  434   . . . .
      Charge divided.
        . . . .  162
  164   . . . .
        . . . .  160
  162   . . . .
        . . . .    0 after discharge.
    0   . . . .      after discharge.

Here app. i. retained 164°, having lost 276° in communicating 162° to app.
ii., and the capacity of the air apparatus is to that of the sulphur
apparatus as 1 to 1.66.

1276. Then the sulphur apparatus was charged first, thus:

        . . . .    0°
    0°  . . . .
        . . . .  395
        . . . .  388
      Charge divided.
  237   . . . .
        . . . .  238
    0   . . . .      after discharge.
        . . . .    0 after discharge.

Here app. ii. retained 238°, and gave up 150° in communicating a charge of
237° to app. i., and the capacity of the air apparatus is to that of the
sulphur apparatus as 1 to 1.58. These results are very near to each other,
and we may take the mean 1.62 as representing the specific inductive
capacity of the sulphur apparatus; in which case the specific inductive
capacity of sulphur itself as compared to air = 1 (1270.) will be about or
above 2.24.

1277. This result with sulphur I consider as one of the most
unexceptionable. The substance when fused was perfectly clear, pellucid,
and free from particles of dirt (1267.), so that no interference of small
conducting bodies confused the result. The substance when solid is an
excellent insulator, and by experiment was found to take up, with great
slowness, that state (1244. 1242.) which alone seemed likely to disturb the
conclusion. The experiments themselves, also, were free from any need of
correction. Yet notwithstanding these circumstances, so favourable to the
exclusion of error, the result is a higher specific inductive capacity for
sulphur than for any other body as yet tried; and though this may in part
be clue to the sulphur being in a better shape, i.e. filling up more
completely the space _o, o_, (fig. 104.) than the cups of shell-lac and
glass, still I feel satisfied that the experiments altogether fully prove
the existence of a difference between dielectrics as to their power of
favouring an inductive action through them; which difference may, for the
present, be expressed by the term _specific inductive capacity_.

1278. Having thus established the point in the most favourable cases that I
could anticipate, I proceeded to examine other bodies amongst solids,
liquids, and gases. These results I shall give with all convenient brevity.

       *       *       *       *       *

1279. _Spermaceti._--A good hemisphere of spermaceti being tried as to
conducting power whilst its two surfaces were still in contact with the
tinfoil moulds used in forming it, was found to conduct sensibly even
whilst warm. On removing it from the moulds and using it in one of the
apparatus, it gave results indicating a specific inductive capacity between
1.3 and 1.6 for the apparatus containing it. But as the only mode of
operation was to charge the air apparatus, and then after a quick contact
with the spermaceti apparatus, ascertain what was left in the former
(1281.), no great confidence can be placed in the results. They are not in
opposition to the general conclusion, but cannot be brought forward as
argument in favour of it.

       *       *       *       *       *

1280. I endeavoured to find some liquids which would insulate well, and
could be obtained in sufficient quantity for these experiments. Oil of
turpentine, native naphtha rectified, and the condensed oil gas fluid,
appeared by common experiments to promise best as to insulation. Being left
in contact with fused carbonate of potassa, chloride of lime, and quick
lime for some days and then filtered, they were found much injured in
insulating power; but after distillation acquired their best state, though
even then they proved to be conductors when extensive metallic contact was
made with them.

1281. _Oil of turpentine rectified._--I filled the lower half of app. i.
with the fluid: and as it would not hold a charge sufficiently to enable me
first to measure and then divide it, I charged app. ii. containing air, and
dividing its charge with app. i. by a quick contact, measured that
remaining in app. ii.: for, theoretically, if a quick contact would divide
up to equal tension between the two apparatus, yet without sensible loss
from the conducting power of app. i.; and app. ii. were left charged to a
degree of tension above half the original charge, it would indicate that
oil of turpentine had less specific inductive capacity than air; or, if
left charged below that mean state of tension, it would imply that the
fluid had the greater inductive capacity. In an experiment of this kind,
app. ii. gave as its charge 390° before division with app. i., and 175°
afterwards, which is less than the half of 390°. Again, being at 176°
before division, it was 79° after, which is also less than half the divided
charge. Being at 79°, it was a third time divided, and then fell to 36°,
less than the half of 79°. Such are the best results I could obtain; they
are not inconsistent with the belief that oil of turpentine has a greater
specific capacity than air, but they do not prove the fact, since the
disappearance of more than half the charge may be due to the conducting
power merely of the fluid.

1282. _Naphtha._--This liquid gave results similar in their nature and
direction to those with oil of turpentine.

       *       *       *       *       *

1283. A most interesting class of substances, in relation to specific
inductive capacity, now came under review, namely, the gases or aëriform
bodies. These are so peculiarly constituted, and are bound together by so
many striking physical and chemical relations, that I expected some
remarkable results from them: air in various states was selected for the
first experiments.

1284. _Air, rare and dense._--Some experiments of division (1208.) seemed
to show that dense and rare air were alike in the property under
examination. A simple and better process was to attach one of the apparatus
to an air-pump, to charge it, and then examine the tension of the charge
when the air within was more or less rarefied. Under these circumstances it
was found, that commencing with a certain charge, that charge did not
change in its tension or force as the air was rarefied, until the
rarefaction was such that _discharge_ across the space _o_, _o_ (fig. 104.)
occurred. This discharge was proportionate to the rarefaction; but having
taken place, and lowered the tension to a certain degree, that degree was
not at all affected by restoring the pressure and density of the air to
their first quantities.

                    inches of mercury.
Thus at a pressure of 30   the charge was        88°
Again                 30   the charge was        88
Again                 30   the charge was        87
Reduced to            11   the charge was        87
Raised again to       30   the charge was        86
Being now reduced to   3.4 the charge fell to    81
Raised again to       30   the charge was still  81

1285. The charges were low in these experiments, first that they might not
pass off at low pressure, and next that little loss by dissipation might
occur. I now reduced them still lower, that I might rarefy further, and for
this purpose in the following experiment used a measuring interval in the
electrometer of only 15° (1185.). The pressure of air within the apparatus
being reduced to 1.9 inches of mercury, the charge was found to be 29°;
then letting in air till the pressure was 30 inches, the charge was still
29°.

1286. These experiments were repeated with pure oxygen with the same
consequences.

1287. This result of _no variation_ in the electric tension being produced
by variation in the density or pressure of the air, agrees perfectly with
those obtained by Mr. Harris[A], and described in his beautiful and
important investigations contained in the Philosophical Transactions;
namely that induction is the same in rare and dense air, and that the
divergence of an electrometer under such variations of the air continues
the same, provided no electricity pass away from it. The effect is one
entirely independent of that power which dense air has of causing a higher
charge to be _retained_ upon the surface of conductors in it than can be
retained by the same conductors in rare air; a point I propose considering
hereafter.

  [A] Philosophical Transactions, 1834, pp. 223, 224, 237, 244.

1288. I then compared _hot and cold air_ together, by raising the
temperature of one of the inductive apparatus as high as it could be
without injury, and then dividing charges between it and the other
apparatus containing cold air. The temperatures were about 50° and 200°,
Still the power or capacity appeared to be unchanged; and when I
endeavoured to vary the experiment, by charging a cold apparatus and then
warming it by a spirit lamp, I could obtain no proof that the inductive
capacity underwent any alteration.

1289. I compared _damp and dry air_ together, but could find no difference
in the results.

       *       *       *       *       *

1290. _Gases._--A very long series of experiments was then undertaken for
the purpose of comparing _different gases_ one with another. They were all
found to insulate well, except such as acted on the shell-lac of the
supporting stem; these were chlorine, ammonia, and muriatic acid. They were
all dried by appropriate means before being introduced into the apparatus.
It would have been sufficient to have compared each with air; but, in
consequence of the striking result which came out, namely, that _all had
the same power of_ or _capacity for_, sustaining induction through them,
(which perhaps might have been expected after it was found that no
variation of density or pressure produced any effect,) I was induced to
compare them, experimentally, two and two in various ways, that no
difference might escape me, and that the sameness of result might stand in
full opposition to the contrast of property, composition, and condition
which the gases themselves presented.

1291. The experiments were made upon the following pairs of gases.

 1.  Nitrogen and       Oxygen.
 2.  Oxygen             Air.
 3.  Hydrogen           Air.
 4.  Muriatic acid gas  Air.
 5.  Oxygen             Hydrogen.
 5.  Oxygen             Carbonic acid.
 7.  Oxygen             Olefiant gas.
 8.  Oxygen             Nitrous gas.
 9.  Oxygen             Sulphurous acid.
10. Oxygen              Ammonia.
11. Hydrogen            Carbonic acid.
12  Hydrogen            Olefiant gas.
13. Hydrogen            Sulphurous acid.
14. Hydrogen            Fluo-silicic acid.
15. Hydrogen            Ammonia.
16, Hydrogen            Arseniuretted hydrogen.
17. Hydrogen            Sulphuretted hydrogen.
18, Nitrogen            Olefiant gas.
19. Nitrogen            Nitrous gas.
20. Nitrogen            Nitrous oxide.
21. Nitrogen            Ammonia.
22. Carbonic oxide      Carbonic acid.
23. Carbonic oxide      Olefiant gas.
24. Nitrous oxide       Nitrous gas.
25. Ammonia             Sulphurous acid.

1292. Notwithstanding the striking contrasts of all kinds which these gases
present of property, of density, whether simple or compound, anions or
cations (665.), of high or low pressure (1284. 1286.), hot or cold (1288.),
not the least difference in their capacity to favour or admit electrical
induction through them could be perceived. Considering the point
established, that in all these gases induction takes place by an action of
contiguous particles, this is the more important, and adds one to the many
striking relations which hold between bodies having the gaseous condition
and form. Another equally important electrical relation, which will be
examined in the next paper[A], is that which the different gases have to
each other at the _same pressure_ of causing the retention of the _same or
different degrees of charge_ upon conductors in them. These two results
appear to bear importantly upon the subject of electrochemical excitation
and decomposition; for as _all_ these phenomena, different as they seem to
be, must depend upon the electrical forces of the particles of matter, the
very distance at which they seem to stand from each other will do much, if
properly considered, to illustrate the principle by which they are held in
one common bond, and subject, as they must be, to one common law.

  [A] See in relation to this point 1382. &c.--_Dec. 1838._

1293. It is just possible that the gases may differ from each other in
their specific inductive capacity, and yet by quantities so small as not to
be distinguished in the apparatus I have used. It must be remembered,
however, that in the gaseous experiments the gases occupy all the space _o,
o_, (fig. 104.) between the inner and the outer ball, except the small
portion filled by the stem; and the results, therefore, are twice as
delicate as those with solid dielectrics.

1294. The insulation was good in all the experiments recorded, except Nos.
10, 15, 21, and 25, being those in which ammonia was compared with other
gases. When shell-lac is put into ammoniacal gas its surface gradually
acquires conducting power, and in this way the lac part of the stem within
was so altered, that the ammonia apparatus could not retain a charge with
sufficient steadiness to allow of division. In these experiments,
therefore, the other apparatus was charged; its charge measured and divided
with the ammonia apparatus by a quick contact, and what remained untaken
away by the division again measured (1281.). It was so nearly one-half of
the original charge, as to authorize, with this reservation, the insertion
of ammoniacal gas amongst the other gases, as having equal power with them.


¶ vi. _General results as to induction._

1295. Thus _induction_ appears to be essentially an action of contiguous
particles, through the intermediation of which the electric force,
originating or appearing at a certain place, is propagated to or sustained
at a distance, appearing there as a force of the same kind exactly equal in
amount, but opposite in its direction and tendencies (1164.). Induction
requires no sensible thickness in the conductors which may be used to limit
its extent; an uninsulated leaf of gold may be made very highly positive on
one surface, and as highly negative on the other, without the least
interference of the two states whilst the inductions continue. Nor is it
affected by the nature of the limiting conductors, provided time be
allowed, in the case of those which conduct slowly, for them to assume
their final state (1170.).

1296. But with regard to the _dielectrics_ or insulating media, matters are
very different (1167.). Their thickness has an immediate and important
influence on the degree of induction. As to their quality, though all gases
and vapours are alike, whatever their state; yet amongst solid bodies, and
between them and gases, there are differences which prove the existence of
_specific inductive capacities_, these differences being in some cases very
great.

1297. The direct inductive force, which may be conceived to be exerted in
lines between the two limiting and charged conducting surfaces, is
accompanied by a lateral or transverse force equivalent to a dilatation or
repulsion of these representative lines (1224.); or the attractive force
which exists amongst the particles of the dielectric in the direction of
the induction is accompanied by a repulsive or a diverging force in the
transverse direction (1304.).

1298. Induction appears to consist in a certain polarized state of the
particles, into which they are thrown by the electrified body sustaining
the action, the particles assuming positive and negative points or parts,
which are symmetrically arranged with respect to each other and the
inducting surfaces or particles[A]. The state must be a forced one, for it
is originated and sustained only by force, and sinks to the normal or
quiescent state when that force is removed. It can be _continued_ only in
insulators by the same portion of electricity, because they only can retain
this state of the particles (1304).

  [A] The theory of induction which I am stating does not pretend to
  decide whether electricity be a fluid or fluids, or a mere power or
  condition of recognized matter. That is a question which I may be
  induced to consider in the next or following series of these
  researches.

1299. The principle of induction is of the utmost generality in electric
action. It constitutes charge in every ordinary case, and probably in every
case; it appears to be the cause of all excitement, and to precede every
current. The degree to which the particles are affected in this their
forced state, before discharge of one kind or another supervenes, appears
to constitute what we call _intensity_.

1300. When a Leyden jar is _charged_, the particles of the glass are forced
into this polarized and constrained condition by the electricity of the
charging apparatus. _Discharge_ is the return of these particles to their
natural state from their state of tension, whenever the two electric forces
are allowed to be disposed of in some other direction.

1301. All charge of conductors is on their surface, because being
essentially inductive, it is there only that the medium capable of
sustaining the necessary inductive state begins. If the conductors are
hollow and contain air or any other dielectric, still no _charge_ can
appear upon that internal surface, because the dielectric there cannot
assume the polarized state throughout, in consequence of the opposing
actions in different directions.

1302. The known influence of _form_ is perfectly consistent with the
corpuscular view of induction set forth. An electrified cylinder is more
affected by the influence of the surrounding conductors (which complete the
condition of charge) at the ends than at the middle, because the ends are
exposed to a greater sum of inductive forces than the middle; and a point
is brought to a higher condition than a ball, because by relation to the
conductors around, more inductive force terminates on its surface than on
an equal surface of the ball with which it is compared. Here too,
especially, can be perceived the influence of the lateral or transverse
force (1297.), which, being a power of the nature of or equivalent to
repulsion, causes such a disposition of the lines of inductive force in
their course across the dielectric, that they must accumulate upon the
point, the end of the cylinder, or any projecting part.

1303. The influence of _distance_ is also in harmony with the same view.
There is perhaps no distance so great that induction cannot take place
through it[A]; but with the same constraining force (1298.) it takes place
the more easily, according as the extent of dielectric through which it is
exerted is lessened. And as it is assumed by the theory that the particles
of the dielectric, though tending to remain in a normal state, are thrown
into a forced condition during the induction; so it would seem to follow
that the fewer there are of these intervening particles opposing their
tendency to the assumption of the new state, the greater degree of change
will they suffer, i.e. the higher will be the condition they assume, and
the larger the amount of inductive action exerted through them.

  [A] I have traced it experimentally from a ball placed in the middle
  of the large cube formerly described (1173.) to the sides of the cube
  six feet distant, and also from the same ball placed in the middle of
  our large lecture-room to the walls of the room at twenty-six feet
  distance, the charge sustained upon the ball in these cases being
  solely due to induction through these distances.

1304. I have used the phrases _lines of inductive force_ and _curved lines_
of force (1231. 1297. 1298. 1302.) in a general sense only, just as we
speak of the lines of magnetic force. The lines are imaginary, and the
force in any part of them is of course the resultant of compound forces,
every molecule being related to every other molecule in _all_ directions by
the tension and reaction of those which are contiguous. The transverse
force is merely this relation considered in a direction oblique to the
lines of inductive force, and at present I mean no more than that by the
phrase. With respect to the term _polarity_ also, I mean at present only a
disposition of force by which the same molecule acquires opposite powers on
different parts. The particular way in which this disposition is made will
come into consideration hereafter, and probably varies in different bodies,
and so produces variety of electrical relation[A]. All I am anxious about
at present is, that a more particular meaning should not be attached to the
expressions used than I contemplate. Further inquiry, I trust, will enable
us by degrees to restrict the sense more and more, and so render the
explanation of electrical phenomena day by day more and more definite.

  [A] See now 1685. &c.--_Dec. 1838._

1305. As a test of the probable accuracy of my views, I have throughout
this experimental examination compared them with the conclusions drawn by
M. Poisson from his beautiful mathematical inquiries[A]. I am quite unfit
to form a judgment of these admirable papers; but as far as I can perceive,
the theory I have set forth and the results I have obtained are not in
opposition to such of those conclusions as represent the final disposition
and state of the forces in the limited number of cases be has considered.
His theory assumes a very different mode of action in induction to that
which I have ventured to support, and would probably find its mathematical
test in the endeavour to apply it to cases of induction in curved lines. To
my feeling it is insufficient in accounting for the retention of
electricity upon the surface of conductors by the pressure of the air, an
effect which I hope to show is simple and consistent according to the
present view[B]; and it does not touch voltaic electricity, or in any way
associate it and what is called ordinary electricity under one common
principle.

  [A] Mémoires de L'Institut, 1811, tom. xii. the first page 1, and the
second paging 163.

  [B] Refer to 1377, 1378, 1379, 1398.--_Dec. 1838._

I have also looked with some anxiety to the results which that
indefatigable philosopher Harris has obtained in his investigation of the
laws of induction[A], knowing that they were experimental, and having a
full conviction of their exactness; but I am happy in perceiving no
collision at present between them and the views I have taken.

  [A] Philosophical Transactions, 1834, p. 213.

1306. Finally, I beg to say that I put forth my particular view with doubt
and fear, lest it should not bear the test of general examination, for
unless true it will only embarrass the progress of electrical science. It
has long been on my mind, but I hesitated to publish it until the
increasing persuasion of its accordance with all known facts, and the
manner in which it linked together effects apparently very different in
kind, urged me to write the present paper. I as yet see no inconsistency
between it and nature, but, on the contrary, think I perceive much new
light thrown by it on her operations; and my next papers will be devoted to
a review of the phenomena of conduction, electrolyzation, current,
magnetism, retention, discharge, and some other points, with an application
of the theory to these effects, and an examination of it by them.


_Royal Institution,
November 16, 1837._

       *       *       *       *       *

_Supplementary Note to Experimental Researches in Electricity._

_Eleventh Series._

Received March 29, 1838.


1307. I have recently put into an experimental form that general statement
of the question of _specific inductive capacity_ which is given at No. 1252
of Series XI., and the result is such as to lead me to hope the Council of
the Royal Society will authorize its addition to the paper in the form of a
supplementary note. Three circular brass plates, about five inches in
diameter, were mounted side by side upon insulating pillars; the middle
one, A, was a fixture, but the outer plates B and C were moveable on
slides, so that all three could be brought with their sides almost into
contact, or separated to any required distance. Two gold leaves were
suspended in a glass jar from insulated wires; one of the outer plates B
was connected with one of the gold leaves, and the other outer plate with
the other leaf. The outer plates B and C were adjusted at the distance of
an inch and a quarter from the middle plate A, and the gold leaves were
fixed at two inches apart; A was then slightly charged with electricity,
and the plates B and C, with their gold leaves, thrown out of insulation
_at the same time_, and then left insulated. In this state of things A was
charged positive inductrically, and B and C negative inducteously; the same
dielectric, air, being in the two intervals, and the gold leaves hanging,
of course, parallel to each other in a relatively unelectrified state.

1308. A plate of shell-lac three-quarters of an inch in thickness, and four
inches square, suspended by clean white silk thread, was very carefully
deprived of all charge (1203.) (so that it produced no effect on the gold
leaves if A were uncharged) and then introduced between plates A and B; the
electric relation of the three plates was immediately altered, and the gold
leaves attracted each other. On removing the shell-lac this attraction
ceased; on introducing it between A and C it was renewed; on removing it
the attraction again ceased; and the shell-lac when examined by a delicate
Coulomb electrometer was still without charge.

1309. As A was positive, B and C were of course negative; but as the
specific inductive capacity of shell-lac is about twice that of air
(1270.), it was expected that when the lac was introduced between A and B,
A would induce more towards B than towards C; that therefore B would become
more negative than before towards A, and consequently, because of its
insulated condition, be positive externally, as at its back or at the gold
leaves; whilst C would be less negative towards A, and therefore negative
outwards or at the gold leaves. This was found to be the case; for on
whichever side of A the shell-lac was introduced the external plate at that
side was positive, and the external plate on the other side negative
towards each other, and also to uninsulated external bodies.

1310. On employing a plate of sulphur instead of shell-lac, the same
results were obtained; consistent with the conclusions drawn regarding the
high specific inductive capacity of that body already given (1276.).

1311. These effects of specific inductive capacity can be exalted in
various ways, and it is this capability which makes the great value of the
apparatus. Thus I introduced the shell-lac between A and B, and then for a
moment connected B and C, uninsulated them, and finally left them in the
insulated state; the gold leaves were of course hanging parallel to each
other. On removing the shell-lac the gold leaves attracted each other; on
introducing the shell-lac between A and C this attraction was _increased_,
(as had been anticipated from theory,) and the leaves came together, though
not more than four inches long, and hanging three inches apart.

1312. By simply bringing the gold leaves nearer to each other I was able to
show the difference of specific inductive capacity when only thin plates of
shell-lac were used, the rest of the dielectric space being filled with
air. By bringing B and C nearer to A another great increase of sensibility
was made. By enlarging the size of the plates still further power was
gained. By diminishing the extent of the wires, &c. connected with the gold
leaves, another improvement resulted. So that in fact the gold leaves
became, in this manner, as delicate a test of _specific inductive action_
as they are, in Bennet's and Singer's electrometers, of ordinary electrical
charge.

1313. It is evident that by making the three plates the sides of cells,
with proper precautions as regards insulation, &c., this apparatus may be
used in the examination of gases, with far more effect than the former
apparatus (1187. 1290), and may, perhaps, bring out differences which have
as yet escaped me (1292. 1293.)

1314. It is also evident that two metal plates are quite sufficient to form
the instrument; the state of the single inducteous plate when the
dielectric is changed, being examined either by bringing a body excited in
a known manner towards its gold leaves, or, what I think will be better,
employing a carrier ball in place of the leaf, and examining that ball by
the Coulomb electrometer (1180.). The inductive and inducteous surfaces may
even be balls; the latter being itself the carrier ball of the Coulomb's
electrometer (1181. 1229.).

1315. To increase the effect, a small condenser may be used with great
advantage. Thus if, when two inducteous plates are used, a little condenser
were put in the place of the gold leaves, I have no doubt the three
principal plates might be reduced to an inch or even half an inch in
diameter. Even the gold leaves act to each other for the time as the plates
of a condenser. If only two plates were used, by the proper application of
the condenser the same reduction might take place. This expectation is
fully justified by an effect already observed and described (1229.).

1316. In that case the application of the instrument to very extensive
research is evident. Comparatively small masses of dielectrics could be
examined, as diamonds and crystals. An expectation, that the specific
inductive capacity of crystals will vary in different directions, according
as the lines of inductive force (1304.) are parallel to, or in other
positions in relation to the axes of the crystals, can be tested[A]: I
purpose that these and many other thoughts which arise respecting specific
inductive action and the polarity of the particles of dielectric matter,
shall be put to the proof as soon as I can find time.

  [A] Refer for this investigation to 1680-1698.--_Dec. 1838._

1317. Hoping that this apparatus will form an instrument of considerable
use, I beg to propose for it (at the suggestion of a friend) the name of
_Differential Inductometer_.

_Royal Institution,
March 29, 1838._




TWELFTH SERIES.

§ 18. _On Induction (continued)._ ¶ vii. _Conduction, or conductive
discharge._ ¶ viii. _Electrolytic discharge._ ¶ ix. _Disruptive
discharge--Insulation--Spark--Brush--Difference of discharge at the
positive and negative surfaces of conductors._

Received January 11,--Read February 8, 1838.


1318. I Proceed now, according to my promise, to examine, by the great
facts of electrical science, that theory of induction which I have ventured
to put forth (1165. 1295. &c.). The principle of induction is so universal
that it pervades all electrical phenomena; but the general case which I
purpose at present to go into consists of insulation traced into and
terminating with discharge, with the accompanying effects. This case
includes the various _modes_ of discharge, and also the condition and
characters of a current; the elements of magnetic action being amongst the
latter. I shall necessarily have occasion to speak theoretically, and even
hypothetically; and though these papers profess to be experimental
researches, I hope that, considering the facts and investigations contained
in the last series in support of the particular view advanced, I shall not
be considered as taking too much liberty on the present occasion, or as
departing too far from the character which they ought to have, especially
as I shall use every opportunity which presents itself of returning to that
strong test of truth, experiment.

1319. Induction has as yet been considered in these papers only in cases of
insulation; opposed to insulation is _discharge_. The action or effect
which may be expressed by the general term _discharge_, may take place, as
far as we are aware at present, in several modes. Thus, that which is
called simply _conduction_ involves no chemical action, and apparently no
displacement of the particles concerned. A second mode may be called
_electrolytic discharge_; in it chemical action does occur, and particles
must, to a certain degree, be displaced. A third mode, namely, that by
sparks or brushes, may, because of its violent displacement of the
particles of the _dielectric_ in its course, be called the _disruptive
discharge_; and a fourth may, perhaps, be conveniently distinguished for a
time by the words _convection_, or _carrying discharge_, being that in
which discharge is effected either by the carrying power of solid
particles, or those of gases and liquids. Hereafter, perhaps, all these
modes may appear as the result of one common principle, but at present they
require to be considered apart; and I will now speak of the _first_ mode,
for amongst all the forms of discharge, that which we express by the term
conduction appears the most simple and the most directly in contrast with
insulation.


¶ vii. _Conduction, or conductive discharge._

1320. Though assumed to be essentially different, yet neither Cavendish nor
Poisson attempt to explain by, or even state in, their theories, what the
essential difference between insulation and conduction is. Nor have I
anything, perhaps, to offer in this respect, _except_ that, according to my
view of induction, insulation and conduction depend upon the same molecular
action of the dielectrics concerned; are only extreme degrees of _one
common condition_ or effect; and in any sufficient mathematical theory of
electricity must be taken as cases of the same kind. Hence the importance
of the endeavour to show the connection between them under my theory of the
electrical relations of contiguous particles.

1321. Though the action of the insulating dielectric in the charged Leyden
jar, and that of the wire in discharging it, may seem very different, they
may be associated by numerous intermediate links, which carry us on from
one to the other, leaving, I think, no necessary connection unsupplied. We
may observe some of these in succession for information respecting the
whole case.

1322. Spermnceti has been examined and found to be a dielectric, through
which induction can take place (1240. 1246.), its specific inductive
capacity being about or above 1.8 (1279.), and the inductive action has
been considered in it, as in all other substances, an action of contiguous
particles.

1323. But spermaceti is also a _conductor_, though in so low a degree that
we can trace the process of conduction, as it were, step by step through
the mass (1247.); and even when the electric force has travelled through it
to a certain distance, we can, by removing the coercitive (which is at the
same time the inductive) force, cause it to return upon its path and
reappear in its first place (1245. 1246.). Here induction appears to be a
necessary preliminary to conduction. It of itself brings the contiguous
particles of the dielectric into a certain condition, which, if retained by
them, constitutes _insulation_, but if lowered by the communication of
power from one particle to another, constitutes _conduction_.

1324. If _glass_ or _shell-lac_ be the substances under consideration, the
same capabilities of suffering either induction or conduction through them
appear (1233. 1239. 1247.), but not in the same degree. The conduction
almost disappears (1239. 1242.); the induction therefore is sustained, i.e.
the polarized state into which the inductive force has brought the
contiguous particles is retained, there being little discharge action
between them, and therefore the _insulation_ continues. But, what discharge
there is, appears to be consequent upon that condition of the particles
into which the induction throws them; and thus it is that ordinary
insulation and conduction are closely associated together or rather are
extreme cases of one common condition.

1325. In ice or water we have a better conductor than spermaceti, and the
phenomena of induction and insulation therefore rapidly disappear, because
conduction quickly follows upon the assumption of the inductive state. But
let a plate of cold ice have metallic coatings on its sides, and connect
one of these with a good electrical machine in work, and the other with the
ground, and it then becomes easy to observe the phenomena of induction
through the ice, by the electrical tension which can be obtained and
continued on both the coatings (419. 426.). For although that portion of
power which at one moment gave the inductive condition to the particles is
at the next lowered by the consequent discharge due to the conductive act,
it is succeeded by another portion of force from the machine to restore the
inductive state. If the ice be converted into water the same succession of
actions can be just as easily proved, provided the water be distilled, and
(if the machine be not powerful enough) a voltaic battery be employed.

1326. All these considerations impress my mind strongly with the
conviction, that insulation and ordinary conduction cannot be properly
separated when we are examining into their nature; that is, into the
general law or laws under which their phenomena are produced. They appear
to me to consist in an action of contiguous particles dependent on the
forces developed in electrical excitement; these forces bring the particles
into a state of tension or polarity, which constitutes both _induction_ and
_insulation_; and being in this state, the continuous particles have a
power or capability of communicating their forces one to the other, by
which they are lowered, and discharge occurs. Every body appears to
discharge (444. 987.); but the possession of this capability in a _greater
or smaller degree_ in different bodies, makes them better or worse
conductors, worse or better insulators; and both _induction_ and
_conduction_ appear to be the same in their principle and action (1320.),
except that in the latter an effect common to both is raised to the highest
degree, whereas in the former it occurs in the best cases, in only an
almost insensible quantity.

1327. That in our attempts to penetrate into the nature of electrical
action, and to deduce laws more general than those we are at present
acquainted with, we should endeavour to bring apparently opposite effects
to stand side by side in harmonious arrangement, is an opinion of long
standing, and sanctioned by the ablest philosophers. I hope, therefore, I
may be excused the attempt to look at the highest cases of conduction as
analogous to, or even the same in kind with, those of induction and
insulation.

1328. If we consider the slight penetration of sulphur (1241. 1242.) or
shell-lac (1234.) by electricity, or the feebler insulation sustained by
spermaceti (1279. 1240.), as essential consequences and indications of
their _conducting_ power, then may we look on the resistance of metallic
wires to the passage of electricity through them as _insulating_ power. Of
the numerous well-known cases fitted to show this resistance in what are
called the perfect conductors, the experiments of Professor Wheatstone best
serve my present purpose, since they were carried to such an extent as to
show that _time_ entered as an element into the conditions of conduction[A]
even in metals. When discharge was made through a copper wire 2640 feet in
length, and 1/15th of an inch in diameter, so that the luminous sparks at
each end of the wire, and at the middle, could be observed in the same
place, the latter was found to be sensibly behind the two former in time,
they being by the conditions of the experiment simultaneous. Hence a proof
of retardation; and what reason can be given why this retardation should
not be of the same kind as that in spermaceti, or in lac, or sulphur? But
as, in them, retardation is insulation, and insulation is induction, why
should we refuse the same relation to the same exhibitions of force in the
metals?

  [A] Philosophical Transactions, 1834, p. 583.

1329. We learn from the experiment, that if _time_ be allowed the
retardation is gradually overcome; and the same thing obtains for the
spermaceti, the lac, and glass (1248.); give but time in proportion to the
retardation, and the latter is at last vanquished. But if that be the case,
and all the results are alike in kind, the only difference being in the
length of time, why should we refuse to metals the previous inductive
action, which is admitted to occur in the other bodies? The diminution of
_time_ is no negation of the action; nor is the lower degree of tension
requisite to cause the forces to traverse the metal, as compared to that
necessary in the cases of water, spermaceti, or lac. These differences
would only point to the conclusion, that in metals the particles under
induction can transfer their forces when at a lower degree of tension or
polarity, and with greater facility than in the instances of the other
bodies.

1330. Let us look at Mr. Wheatstone's beautiful experiment in another point
of view, If, leaving the arrangement at the middle and two ends of the long
copper wire unaltered, we remove the two intervening portions and replace
them by wires of iron or platina, we shall have a much greater retardation
of the middle spark than before. If, removing the iron, we were to
substitute for it only five or six feet of water in a cylinder of the same
diameter as the metal, we should have still greater retardation. If from
water we passed to spermaceti, either directly or by gradual steps through
other bodies, (even though we might vastly enlarge the bulk, for the
purpose of evading the occurrence of a spark elsewhere (1331.) than at the
three proper intervals,) we should have still greater retardation, until at
last we might arrive, by degrees so small as to be inseparable from each
other, at actual and permanent insulation. What, then, is to separate the
principle of these two extremes, perfect conduction and perfect insulation,
from each other; since the moment we leave in the smallest degree
perfection at either extremity, we involve the element of perfection at the
opposite end? Especially too, as we have not in nature the case of
perfection either at one extremity or the other, either of insulation or
conduction.

1331. Again, to return to this beautiful experiment in the various forms
which may be given to it: the forces are not all in the wire (after they
have left the Leyden jar) during the whole time (1328.) occupied by the
discharge; they are disposed in part through the surrounding dielectric
under the well-known form of induction; and if that dielectric be air,
induction takes place from the wire through the air to surrounding
conductors, until the ends of the wire are electrically related through its
length, and discharge has occurred, i.e. for the _time_ during which the
middle spark is retarded beyond the others. This is well shown by the old
experiment, in which a long wire is so bent that two parts (Plate VIII.
fig. 115.), _a, b_, near its extremities shall approach within a short
distance, as a quarter of an inch, of each other in the air. If the
discharge of a Leyden jar, charged to a sufficient degree, be sent through
such a wire, by far the largest portion of the electricity will pass as a
spark across the air at the interval, and not by the metal. Does not the
middle part of the wire, therefore, act here as an insulating medium,
though it be of metal? and is not the spark through the air an indication
of the tension (simultaneous with _induction_) of the electricity in the
ends of this single wire? Why should not the wire and the air both be
regarded as dielectrics; and the action at its commencement, and whilst
there is tension, as an inductive action? If it acts through the contorted
lines of the wire, so it also does in curved and contorted lines through
air (1219, 1224, 1231.), and other insulating dielectrics (1228); and we
can apparently go so far in the analogy, whilst limiting the case to the
inductive action only, as to show that amongst insulating dielectrics some
lead away the lines of force from others (1229.), as the wire will do from
worse conductors, though in it the principal effect is no doubt due to the
ready discharge between the particles whilst in a low state of tension. The
retardation is for the time insulation; and it seems to me we may just as
fairly compare the air at the interval _a, b_ (fig. 115.) and the wire in
the circuit, as two bodies of the same kind and acting upon the same
principles, as far as the first inductive phenomena are concerned,
notwithstanding the different forms of discharge which ultimately
follow[A], as we may compare, according to Coulomb's investigations[B]
_different lengths_ of different insulating bodies required to produce the
same amount of insulating effect.

  [A] These will be examined hereafter (1348. &c.).

  [B] Mémoires de l'Académie, 1785, p. 612. or Ency. Britann. First
  Supp. vol. i. p. 614.

1332. This comparison is still more striking when we take into
consideration the experiment of Mr. Harris, in which he stretched a fine
wire across a glass globe, the air within being rarefied[A]. On sending a
charge through the joint arrangement of metal and rare air, as much, if not
more, electricity passed by the latter as by the former. In the air,
rarefied as it was, there can be no doubt the discharge was preceded by
induction (1284.); and to my mind all the circumstances indicate that the
same was the case with the metal; that, in fact, both substances are
dielectrics, exhibiting the same effects in consequence of the action of
the same causes, the only variation being one of degree in the different
substances employed.

  [A] Philosophical Transactions, 1834, p, 212.

1333. Judging on these principles, velocity of discharge through the _same
wire_ may be varied greatly by attending to the circumstances which cause
variations of discharge through spermaceti or sulphur. Thus, for instance,
it must vary with the tension or intensity of the first urging force (1234.
1240.), which tension is charge and induction. So if the two ends of the
wire, in Professor Wheatstone's experiment, were immediately connected with
two large insulated metallic surfaces exposed to the air, so that the
primary act of induction, after making the contact for discharge, might be
in part removed from the internal portion of the wire at the first instant,
and disposed for the moment on its surface jointly with the air and
surrounding conductors, then I venture to anticipate that the middle spark
would be more retarded than before; and if these two plates were the inner
and outer coating of a large jar or a Leyden battery, then the retardation
of that spark would be still greater.

1334. Cavendish was perhaps the first to show distinctly that discharge was
not always by one channel[A], but, if several are present, by many at once.
We may make these different channels of different bodies, and by
proportioning their thicknesses and lengths, may include such substances as
air, lac, spermaceti, water, protoxide of iron, iron and silver, and by
_one_ discharge make each convey its proportion of the electric force.
Perhaps the air ought to be excepted, as its discharge by conduction is
questionable at present (1336.); but the others may all be limited in their
mode of discharge to pure conduction. Yet several of them suffer previous
induction, precisely like the induction through the air, it being a
necessary preliminary to their discharging action. How can we therefore
separate any one of these bodies from the others, as to the _principles and
mode_ of insulating and conducting, except by mere degree? All seem to me
to be dielectrics acting alike, and under the same common laws.

  [A] _Philosophical Transactions_, 1776, p. 197.

1335. I might draw another argument in favour of the general sameness, in
nature and action, of good and bad conductors (and all the bodies I refer
to are conductors more or less), from the perfect equipoise in action of
very different bodies when opposed to each other in magneto-electric
inductive action, as formerly described (213.), but am anxious to be as
brief as is consistent with the clear examination of the probable truth of
my views.

1336. With regard to the possession by the gases of any conducting power
of the simple kind now under consideration, the question is a very
difficult one to determine at present. Experiments seem to indicate that
they do insulate certain low degrees of tension perfectly, and that the
effects which may have appeared to be occasioned by _conduction_ have been
the result of the carrying power of the charged particles, either of the
air or of dust, in it. It is equally certain, however, that with higher
degrees of tension or charge the particles discharge to one another, and
that is conduction. If the gases possess the power of insulating a certain
low degree of tension continuously and perfectly, such a result may be due
to their peculiar physical state, and the condition of separation under
which their particles are placed. But in that, or in any case, we must not
forget the fine experiments of Cagniard de la Tour[A], in which he has
shown that liquids and their vapours can be made to pass gradually into
each other, to the entire removal of any marked distinction of the two
states. Thus, hot dry steam and cold water pass by insensible gradations
into each other; yet the one is amongst the gases as an insulator, and the
other a comparatively good conductor. As to conducting power, therefore,
the transition from metals even up to gases is gradual; substances make but
one series in this respect, and the various cases must come under one
condition and law (444.). The specific differences of bodies as to
conducting power only serves to strengthen the general argument, that
conduction, like insulation, is a result of induction, and is an action of
contiguous particles.

  [A] Annales de Chimie, xxi. pp. 127, 178, or Quarterly Journal of
  Science, xv. 145.

1337. I might go on now to consider induction and its concomitant,
_conduction_, through mixed dielectrics, as, for instance, when a charged
body, instead of acting across air to a distant uninsulated conductor, acts
jointly through it and an interposed insulated conductor. In such a case,
the air and the conducting body are the mixed dielectrics; and the latter
assumes a polarized condition as a mass, like that which my theory assumes
_each particle_ of the air to possess at the same time (1679). But I fear
to be tedious in the present condition of the subject, and hasten to the
consideration of other matter.

1338. To sum up, in some degree, what has been said, I look upon the first
effect of an excited body upon neighbouring matters to be the production of
a polarized state of their particles, which constitutes _induction_; and
this arises from its action upon the particles in immediate contact with
it, which again act upon those contiguous to them, and thus the forces are
transferred to a distance. If the induction remain undiminished, then
perfect insulation is the consequence; and the higher the polarized
condition which the particles can acquire or maintain, the higher is the
intensity which may be given to the acting forces. If, on the contrary, the
contiguous particles, upon acquiring the polarized state, have the power to
communicate their forces, then conduction occurs, and the tension is
lowered, conduction being a distinct act of discharge between neighbouring
particles. The lower the state of tension at which this discharge between
the particles of a body takes place, the better conductor is that body. In
this view, insulators may be said to be bodies whose particles can retain
the polarized state; whilst conductors are those whose particles cannot be
permanently polarized. If I be right in my view of induction, then I
consider the reduction of these two effects (which have been so long held
distinct) to an action of contiguous particles obedient to one common law,
as a very important result; and, on the other hand, the identity of
character which the two acquire when viewed by the theory (1326.), is
additional presumptive proof in favour of the correctness of the latter.

       *       *       *       *       *

1339. That heat has great influence over simple conduction is well known
(445.), its effect being, in some cases, almost an entire change of the
characters of the body (432. 1340.). Harris has, however, shown that it in
no respect affects gaseous bodies, or at least air[A]; and Davy has taught
us that, as a class, metals have their conducting power _diminished_ by
it[B].

  [A] _Philosophical Transactions_, 1834, p. 230

  [B] Ibid. 1821, p. 431.

1340. I formerly described a substance, sulphuret of silver, whose
conducting power was increased by heat (433. 437. 438.); and I have since
then met with another as strongly affected in the same way: this is
fluoride of lead. When a piece of that substance, which had been fused and
cooled, was introduced into the circuit of a voltaic battery, it stopped
the current. Being heated, it acquired conducting powers before it was
visibly red-hot in daylight; and even sparks could be taken against it
whilst still solid. The current alone then raised its temperature (as in
the case of sulphuret of silver) until it fused, after which it seemed to
conduct as well as the metallic vessel containing it; for whether the wire
used to complete the circuit touched the fused fluoride only, or was in
contact with the platina on which it was supported, no sensible difference
in the force of the current was observed. During all the time there was
scarcely a trace of decomposing action of the fluoride, and what did occur,
seemed referable to the air and moisture of the atmosphere, and not to
electrolytic action.

1341. I have now very little doubt that periodide of mercury (414. 448.
691.) is a case of the same kind, and also corrosive sublimate (692.). I am
also inclined to think, since making the above experiments, that the
anomalous action of the protoxide of antimony, formerly observed and
described (693. 801.), may be referred in part to the same cause.

1342. I have no intention at present of going into the particular relation
of heat and electricity, but we may hope hereafter to discover by
experiment the law which probably holds together all the above effects with
those of the _evolution_ and the _disappearance_ of heat by the current,
and the striking and beautiful results of thermo-electricity, in one common
bond.


¶ viii. _Electrolytic discharge._

1343. I have already expressed in a former paper (1164.), the view by which
I hope to associate ordinary induction and electrolyzation. Under that
view, the discharge of electric forces by electrolyzation is rather an
effect superadded, in a certain class of bodies, to those already described
as constituting induction and insulation, than one independent of and
distinct from these phenomena.

1344. Electrolytes, as respects their insulating and conducting forces,
belong to the general category of bodies (1320. 1334.); and if they are in
the solid state (as nearly all can assume that state), they retain their
place, presenting then no new phenomenon (426. &c.); or if one occur, being
in so small a proportion as to be almost unimportant. When liquefied, they
also belong to the same list whilst the electric intensity is below a
certain degree; but at a given intensity (910. 912. 1007.), fixed for each,
and very low in all known cases, they play a new part, causing discharge in
proportion (783.) to the development of certain chemical effects of
combination and decomposition; and at this point, move out from the general
class of insulators and conductors, to form a distinct one by themselves.
The former phenomena have been considered (1320. 1338.); it is the latter
which have now to be revised, and used as a test of the proposed theory of
induction.

1345. The theory assumes, that the particles of the dielectric (now an
electrolyte) are in the first instance brought, by ordinary inductive
action, into a polarized state, and raised to a certain degree of tension
or intensity before discharge commences; the inductive state being, in
fact, a _necessary preliminary_ to discharge. By taking advantage of those
circumstances which bear upon the point, it is not difficult to increase
the tension indicative of this state of induction, and so make the state
itself more evident. Thus, if distilled water be employed, and a long
narrow portion of it placed between the electrodes of a powerful voltaic
battery, we have at once indications of the intensity which can be
sustained at these electrodes by the inductive action through the water as
a dielectric, for sparks may be obtained, gold leaves diverged, and Leyden
bottles charged at their wires. The water is in the condition of the
spermaceti (1322. 1323.) a bad conductor and a bad insulator; but what it
does insulate is by virtue of inductive action, and that induction is the
preparation for and precursor of discharge (1338.).

1346. The induction and tension which appear at the limits of the portion
of water in the direction of the current, are only the sums of the
induction and tension of the contiguous particles between those limits; and
the limitation of the inductive tension, to a certain degree shows (time
entering in each case as an important element of the result), that when the
particles have acquired a certain relative state, _discharge_, or a
transfer of forces equivalent to ordinary conduction, takes place.

1347. In the inductive condition assumed by water before discharge comes
on, the particles polarized are the particles of the _water_ that being the
dielectric used[A]; but the discharge between particle and particle is not,
as before, a mere interchange of their powers or forces at the polar parts,
but an actual separation of them into their two elementary particles, the
oxygen travelling in one direction, and carrying with it its amount of the
force it had acquired during the polarization, and the hydrogen doing the
same thing in the other direction, until they each meet the next
approaching particle, which is in the same electrical state with that they
have left, and by association of their forces with it, produce what
constitutes discharge. This part of the action may be regarded as a
carrying one (1319. 1572. 1622.), performed by the constituent particles of
the dielectric. The latter is always a compound body (664. 823.); and by
those who have considered the subject and are acquainted with the
philosophical view of transfer which was first put forth by Grotthuss[B],
its particles may easily be compared to a series of metallic conductors
under inductive action, which, whilst in that state, are divisible into
these elementary moveable halves.

  [A] See 1699-1708.--_Dec. 1838_

  [B] Annales de Chimie, lviii. 60. and lxiii, 20.

1348. Electrolytic discharge depends, of necessity, upon the non-conduction
of the dielectric as a whole, and there are two steps or acts in the
process: first a polarization of the molecules of the substance and then a
lowering of the forces by the separation, advance in opposite directions,
and recombination of the elements of the molecules, these being, as it
were, the halves of the originally polarized conductors or particles.

1349. These views of the decomposition of electrolytes and the consequent
effect of discharge, which, as to the particular case, are the same with
those of Grotthuss (481.) and Davy (482.), though they differ from those of
Biot (487.), De la Rive (490.), and others, seem to me to be fully in
accordance not merely with the theory I have given of induction generally
(1165.), but with all the known _facts_ of common induction, conduction,
and electrolytic discharge; and in that respect help to confirm in my mind
the truth of the theory set forth. The new mode of discharge which
electrolyzation presents must surely be an evidence of the _action of
contiguous particles_; and as this appears to depend directly upon a
previous inductive state, which is the same with common induction, it
greatly strengthens the argument which refers induction in all cases to an
action of contiguous particles also (1295, &c.).

1350. As an illustration of the condition of the polarized particles in a
dielectric under induction, I may describe an experiment. Put into a glass
vessel some clear rectified oil of turpentine, and introduce two wires
passing through glass tubes where they coincide with the surface of the
fluid, and terminating either in balls or points. Cut some very clean dry
white silk into small particles, and put these also into the liquid: then
electrify one of the wires by an ordinary machine and discharge by the
other. The silk will immediately gather from all parts of the liquid, and
form a band of particles reaching from wire to wire, and if touched by a
glass rod will show considerable tenacity; yet the moment the supply of
electricity ceases, the band will fall away and disappear by the dispersion
of its parts. The _conduction_ by the silk is in this case very small; and
after the best examination I could give to the effects, the impression on
my mind is, that the adhesion of the whole is due to the polarity which
each filament acquires, exactly as the particles of iron between the poles
of a horse-shoe magnet are held together in one mass by a similar
disposition of forces. The particles of silk therefore represent to me the
condition of the molecules of the dielectric itself, which I assume to be
polar, just as that of the silk is. In all cases of conductive discharge
the contiguous polarized particles of the body are able to effect a
neutralization of their forces with greater or less facility, as the silk
does also in a very slight degree. Further we are not able to carry the
parallel, except in imagination; but if we could divide each particle of
silk into two halves, and let each half travel until it met and united with
the next half in an opposite state, it would then exert its carrying power
(1347.), and so far represent electrolytic discharge.

1351. Admitting that electrolytic discharge is a consequence of previous
induction, then how evidently do its numerous cases point to induction in
curved lines (521. 1216.), and to the divergence or lateral action of the
lines of inductive force (1231.), and so strengthen that part of the
general argument in the former paper! If two balls of platina, forming the
electrodes of a voltaic battery, are put into a large vessel of dilute
sulphuric acid, the whole of the surfaces are covered with the respective
gases in beautifully regulated proportions, and the mind has no difficulty
in conceiving the direction of the curved lines of discharge, and even the
intensity of force of the different lines, by the quantity of gas evolved
upon the different parts of the surface. From this condition of the lines
of inductive force arise the general effects of diffusion; the appearance
of the anions or cathions round the edges and on the further side of the
electrodes when in the form of plates; and the manner in which the current
or discharge will follow all the forms of the electrolyte, however
contorted. Hence, also, the effects which Nobili has so well examined and
described[A] in his papers on the distribution of currents in conducting
masses. All these effects indicate the curved direction of the currents or
discharges which occur in and through the dielectrics, and these are in
every case _preceded_ by equivalent inductive actions of the contiguous
particles.

  [A] Bibliothèque Universelle, 1835, lix. 263. 416.

1352. Hence also the advantage, when the exciting forces are weak or
require assistance, of enlarging the mass of the electrolyte; of increasing
the size of the electrodes; of making the coppers surround the zincs:--all
is in harmony with the view of induction which I am endeavouring to
examine; I do not perceive as yet one fact against it.

1353. There are many points of _electrolytic discharge_ which ultimately
will require to be very closely considered, though I can but slightly touch
upon them. It is not that, as far as I have investigated them, they present
any contradiction to the view taken (for I have carefully, though
unsuccessfully, sought for such cases), but simply want of time as yet to
pursue the inquiry, which prevents me from entering upon them here.

1354. One point is, that different electrolytes or dielectrics require
different initial intensities for their decomposition (912.). This may
depend upon the degree of polarization which the particles require before
electrolytic discharge commences. It is in direct relation to the chemical
affinity of the substances concerned; and will probably be found to have a
relation or analogy to the specific inductive capacity of different bodies
(1252. 1296.). It thus promises to assist in causing the great truths of
those extensive sciences, which are occupied in considering the forces of
the particles of matter, to fall into much closer order and arrangement
than they have heretofore presented.

1355. Another point is the facilitation of electrolytic conducting power or
discharge by the addition of substances to the dielectric employed. This
effect is strikingly shown where water is the body whose qualities are
improved, but, as yet, no general law governing all the phenomena has been
detected. Thus some acids, as the sulphuric, phosphoric, oxalic, and
nitric, increase the power of water enormously; whilst others, as the
tartaric and citric acids, give but little power; and others, again, as the
acetic and boracic acids, do not produce a change sensible to the
voltameter (739.). Ammonia produces no effect, but its carbonate does. The
caustic alkalies and their carbonates produce a fair effect. Sulphate of
soda, nitre (753.), and many soluble salts produce much effect. Percyanide
of mercury and corrosive sublimate produce no effect; nor does iodine, gum,
or sugar, the test being a voltameter. In many cases the added substance is
acted on either directly or indirectly, and then the phenomena are more
complicated; such substances are muriatic acid (758.), the soluble
protochlorides (766.), and iodides (769.), nitric acid (752.), &c. In other
cases the substance added is not, when alone, subject to or a conductor of
the powers of the voltaic battery, and yet both gives and receives power
when associated with water. M. de la Rive has pointed this result out in
sulphurous acid[A], iodine and bromine[B]; the chloride of arsenic produces
the same effect. A far more striking case, however, is presented by that
very influential body sulphuric acid (681.): and probably phosphoric acid
also is in the same peculiar relation.

  [A] Quarterly Journal, xxvii. 407. or Bibliothèque Universelle, xl.
  205. Kemp says sulphurous acid is a very good conductor, Quarterly
  Journal, 1831, p. 613.

  [B] Quarterly Journal, xxiv, 465. or Annales de Chimie, xxxv. 161.

1356. It would seem in the cases of those bodies which suffer no change
themselves, as sulphuric acid (and perhaps in all), that they affect water
in its conducting power only as an electrolyte; for whether little or much
improved, the decomposition is proportionate to the quantity of electricity
passing (727. 730.), and the transfer is therefore due to electrolytic
discharge. This is in accordance with the fact already stated as regards
water (984.), that the conducting power is not improved for electricity of
force below the electrolytic intensity of the substance acting as the
dielectric; but both facts (and some others) are against the opinion which
I formerly gave, that the power of salts, &c. might depend upon their
assumption of the liquid state by solution in the water employed (410.). It
occurs to me that the effect may perhaps be related to, and have its
explanation in differences of specific inductive capacities.

1357. I have described in the last paper, cases, where shell-lac was
rendered a conductor by absorption of ammonia (1294.). The same effect
happens with muriatic acid; yet both these substances, when gaseous, are
non-conductors; and the ammonia, also when in strong solution (718.). Mr.
Harris has mentioned instances[A] in which the conducting power of metals
is seriously altered by a very little alloy. These may have no relation to
the former cases, but nevertheless should not be overlooked in the general
investigation which the whole question requires.

  [A] Philosophical Transactions, 1827, p. 22.

1358. Nothing is perhaps more striking in that class of dielectrics which
we call electrolytes, than the extraordinary and almost complete suspension
of their peculiar mode of effecting discharge when they are rendered
_solid_ (380, &c.), even though the intensity of the induction acting
through them may be increased a hundredfold or more (419.). It not only
establishes a very general relation between the physical properties of
these bodies and electricity acting by induction through them, but draws
both their physical and chemical relations so near together, as to make us
hope we shall shortly arrive at the full comprehension of the influence
they mutually possess over each other.


¶ ix. _Disruptive discharge and insulation._

1359. The next form of discharge has been distinguished by the adjective
_disruptive_ (1319.), as it in every case displaces more or less the
particles amongst and across which it suddenly breaks. I include under it,
discharge in the form of sparks, brushes, and glow (1405.), but exclude the
cases of currents of air, fluids, &c., which, though frequently
accompanying the former, are essentially distinct in their nature.

1360. The conditions requisite for the production of an electric spark in
its simplest form are well-known. An insulating dielectric must be
interposed between two conducting surfaces in opposite states of
electricity, and then if the actions be continually increased in strength,
or otherwise favoured, either by exalting the electric state of the two
conductors, or bringing them nearer to each other, or diminishing the
density of the dielectric, a _spark_ at last appears, and the two forces
are for the time annihilated, for _discharge_ has occurred.

1361. The conductors (which may be considered as the termini of the
inductive action) are in ordinary cases most generally metals, whilst the
dielectrics usually employed are common air and glass. In my view of
induction, however, every dielectric becomes of importance, for as the
results are considered essentially dependent on these bodies, it was to be
expected that differences of action never before suspected would be evident
upon close examination, and so at once give fresh confirmation of the
theory, and open new doors of discovery into the extensive and varied
fields of our science. This hope was especially entertained with respect to
the gases, because of their high degree of insulation, their uniformity in
physical condition, and great difference in chemical properties.

1362. All the effects prior to the discharge are inductive; and the degree
of tension which it is necessary to attain before the spark passes is
therefore, in the examination I am now making of the new view of induction,
a very important point. It is the limit of the influence which the
dielectric exerts in resisting discharge; it is a measure, consequently, of
the conservative power of the dielectric, which in its turn may be
considered as becoming a measure, and therefore a representative of the
intensity of the electric forces in activity.

1363. Many philosophers have examined the circumstances of this limiting
action in air, but, as far as I know, none have come near Mr. Harris as to
the accuracy with, and the extent to, which he has carried on his
investigations[A]. Some of his results I must very briefly notice,
premising that they are all obtained with the use of air as the
_dielectric_ between the conducting surfaces.

  [A] Philosophical Transactions, 1834, p. 225.

1364. First as to the _distance_ between the two balls used, or in other
words, the _thickness_ of the dielectric across which the induction was
sustained. The quantity of electricity, measured by a unit jar, or
otherwise on the same principle with the unit jar, in the charged or
inductive ball, necessary to produce spark discharge, was found to vary
exactly with the distance between the balls, or between the discharging
points, and that under very varied and exact forms of experiment[A].

  [A] Philosophical Transactions, 1834, p. 225.

1365. Then with respect to variation in the _pressure_ or _density_ of the
air. The quantities of electricity required to produce discharge across a
_constant_ interval varied exactly with variations of the density; the
quantity of electricity and density of the air being in the same simple
ratio. Or, if the quantity was retained the same, whilst the interval and
density of the air were varied, then these were found in the inverse simple
ratio of each other, the same quantity passing across twice the distance
with air rarefied to one-half[A].

  [A] Philosophical Transactions, 1834, p.229.

1366. It must be remembered that these effects take place without any
variation of the _inductive_ force by condensation or rarefaction of the
air. That force remains the same in air[A], and in all gases (1284. 1292.),
whatever their rarefaction may be.

  [A] Philosophical Transactions, 1834, p. 237, 244.

1367. Variation of the _temperature_ of the air produced no variation of
the quantity of electricity required to cause discharge across a given
interval[A].

  [A] Philosophical Transactions, 1834, p. 230

Such are the general results, which I have occasion for at present,
obtained by Mr. Harris, and they appear to me to be unexceptionable.

1368. In the theory of induction founded upon a molecular action of the
dielectric, we have to look to the state of that body principally for the
cause and determination of the above effects. Whilst the induction
continues, it is assumed that the particles of the dielectric are in a
certain polarized state, the tension of this state rising higher in each
particle as the induction is raised to a higher degree, either by
approximation of the inducing surfaces, variation of form, increase of the
original force, or other means; until at last, the tension of the particles
having reached the utmost degree which they can sustain without subversion
of the whole arrangement, discharge immediately after takes place.

1369. The theory does not assume, however, that _all_ the particles of the
dielectric subject to the inductive action are affected to the same amount,
or acquire the same tension. What has been called the lateral action of the
lines of inductive force (1231. 1297.), and the diverging and occasionally
curved form of these lines, is against such a notion. The idea is, that any
section taken through the dielectric across the lines of inductive force,
and including _all of them,_ would be equal, in the sum of the forces, to
the sum of the forces in any other section; and that, therefore, the whole
amount of tension for each such section would be the same.

1370. Discharge probably occurs, not when all the particles have attained
to a certain degree of tension, but when that particle which is most
affected has been exalted to the subverting or turning point (1410.). For
though _all_ the particles in the line of induction resist charge, and are
associated in their actions so as to give a sum of resisting force, yet
when any one is brought up to the overturning point, _all_ must give way in
the case of a spark between ball and ball. The breaking down of that one
must of necessity cause the whole barrier to be overturned, for it was at
its utmost degree of resistance when it possessed the aiding power of that
one particle, in addition to the power of the rest, and the power of that
one is now lost. Hence _tension_ or _intensity_[A] may, according to the
theory, be considered as represented by the particular condition of the
particles, or the amount in them of forced variation from their normal
state (1298. 1368.).

  [A] See Harris on proposed particular meaning of these terms,
  Philosophical Transactions, 1834, p. 222.

1371. The whole effect produced by a charged conductor on a distant
conductor, insulated or not, is by my theory assumed to be due to an action
propagated from particle to particle of the intervening and insulating
dielectric, all the particles being considered as thrown for the time into
a forced condition, from which they endeavour to return to their normal or
natural state. The theory, therefore, seems to supply an easy explanation
of the influence of _distance_ in affecting induction (1303. 1364.). As the
distance is diminished induction increases; for there are then fewer
particles in the line of inductive force to oppose their united resistance
to the assumption of the forced or polarized state, and _vice versa._
Again, as the distance diminishes, discharge across happens with a lower
charge of electricity; for if, as in Harris's experiments (1364), the
interval be diminished to one-half, then half the electricity required to
discharge across the first interval is sufficient to strike across the
second; and it is evident, also, that at that time there are only half the
number of interposed molecules uniting their forces to resist the
discharge.

1372. The effect of enlarging the conducting surfaces which are opposed to
each other in the act of induction, is, if the electricity be limited in
its supply, to lower the intensity of action; and this follows as a very
natural consequence from the increased area of the dielectric across which
the induction is effected. For by diffusing the inductive action, which at
first was exerted through one square inch of sectional area of the
dielectric, over two or three square inches of such area, twice or three
times the number of molecules of the dielectric are brought into the
polarized condition, and employed in sustaining the inductive action, and
consequently the tension belonging to the smaller number on which the
limited force was originally accumulated, must fall in a proportionate
degree.

1373. For the same reason diminishing these opposing surfaces must increase
the intensity, and the effect will increase until the surfaces become
points. But in this case, the tension of the particles of the dielectric
next the points is higher than that of particles midway, because of the
lateral action and consequent bulging, as it were, of the lines of
inductive force at the middle distance (1369.).

1374. The more exalted effects of induction on a point _p_, or any small
surface, as the rounded end of a rod, when it is opposed to a large
surface, as that of a ball or plate, rather than to another point or end,
the distance being in both cases the same, fall into harmonious relation
with my theory (1302.). For in the latter case, the small surface _p_ is
affected only by those particles which are brought into the inductive
condition by the equally small surface of the opposed conductor, whereas
when that is a ball or plate the lines of inductive force from the latter
are concentrated, as it were, upon the end _p_. Now though the molecules of
the dielectric against the large surface may have a much lower state of
tension than those against the corresponding smaller surface, yet they are
also far more numerous, and, as the lines of inductive force converge
towards a point, are able to communicate to the particles contained in any
cross section (1369.) nearer the small surface an amount of tension equal
to their own, and consequently much higher for each individual particle; so
that, at the surface of the smaller conductor, the tension of a particle
rises much, and if that conductor were to terminate in a point, the tension
would rise to an infinite degree, except that it is limited, as before
(1368.), by discharge. The nature of the discharge from small surfaces and
points under induction will be resumed hereafter (1425. &c.)

1375. _Rarefaction_ of the air does not alter the _intensity_ of inductive
action (1284. 1287.); nor is there any reason, as far as I can perceive,
why it should. If the quantity of electricity and the distance remain the
same, and the air be rarefied one-half, then, though one-half of the
particles of the dielectric are removed, the other half assume a double
degree of tension in their polarity, and therefore the inductive forces are
balanced, and the result remains unaltered as long as the induction and
insulation are sustained. But the case of _discharge_ is very different;
for as there are only half the number of dielectric particles in the
rarefied atmosphere, so these are brought up to the discharging intensity
by half the former quantity of electricity; discharge, therefore, ensues,
and such a consequence of the theory is in perfect accordance with Mr.
Harris's results (1365.).

1376. The _increase_ of electricity required to cause discharge over the
same distance, when the pressure of the air or its density is increased,
flows in a similar manner, and on the same principle (1375.), from the
molecular theory.

1377. Here I think my view of induction has a decided advantage over
others, especially over that which refers the retention of electricity on
the surface of conductors in air to the _pressure of the atmosphere_
(1305.). The latter is the view which, being adopted by Poisson and
Biot[A], is also, I believe, that generally received; and it associates two
such dissimilar things, as the ponderous air and the subtile and even
hypothetical fluid or fluids of electricity, by gross mechanical relations;
by the bonds of mere static pressure. My theory, on the contrary, sets out
at once by connecting the electric forces with the particles of matter; it
derives all its proofs, and even its origin in the first instance, from
experiment; and then, without any further assumption, seems to offer at
once a full explanation of these and many other singular, peculiar, and, I
think, heretofore unconnected effects.

  [A] Encyclopædia Britannica, Supplement, vol. iv. Article Electricity,
  pp. 76, 81. &c.

1378. An important assisting experimental argument may here be adduced,
derived from the difference of specific inductive capacity of different
dielectrics (1269. 1274. 1278.). Consider an insulated sphere electrified
positively and placed in the centre of another and larger sphere
uninsulated, a uniform dielectric, as air, intervening. The case is really
that of my apparatus (1187.), and also, in effect, that of any ball
electrified in a room and removed to some distance from irregularly-formed
conductors. Whilst things remain in this state the electricity is
distributed (so to speak) uniformly over the surface of the electrified
sphere. But introduce such a dielectric as sulphur or lac, into the space
between the two conductors on one side only, or opposite one part of the
inner sphere, and immediately the electricity on the latter is diffused
unequally (1229. 1270. 1309.), although the form of the conducting
surfaces, their distances, and the _pressure_ of the atmosphere remain
perfectly unchanged.

1379. Fusinieri took a different view from that of Poisson, Biot, and
others, of the reason why rarefaction of air caused easy diffusion of
electricity. He considered the effect as due to the removal of the
_obstacle_ which the air presented to the expansion of the substances from
which the electricity passed[A]. But platina balls show the phenomena _in
vacuo_ as well as volatile metals and other substances; besides which, when
the rarefaction is very considerable, the electricity passes with scarcely
any resistance, and the production of no sensible heat; so that I think
Fusinieri's view of the matter is likely to gain but few assents.

  [A] Bib. Univ. 1831, xlviii. 375.

1380. I have no need to remark upon the discharging or collecting power of
flame or hot air. I believe, with Harris, that the mere heat does nothing
(1367.), the rarefaction only being influential. The effect of rarefaction
has been already considered generally (1375.); and that caused by the heat
of a burning light, with the pointed form of the wick, and the carrying
power of the carbonaceous particles which for the time are associated with
it, are fully sufficient to account for all the effects.

1381. We have now arrived at the important question, how will the inductive
tension requisite for insulation and disruptive discharge be sustained in
gases, which, having the same physical state and also the _same
pressure_ and the _same temperature_ as _air_, differ from it in specific
gravity, in chemical qualities, and it may be in peculiar relations, which
not being as yet recognized, are purely electrical (1361.)?

1382. Into this question I can enter now only as far as is essential for
the present argument, namely, that insulation and inductive tension do not
depend merely upon the charged conductors employed, but also, and
essentially, upon the interposed dielectric, in consequence of the
molecular action of its particles (1292.).

1383. A glass vessel _a_ (fig. 127.)[A] was ground at the top and bottom so
as to be closed by two ground brass plates, _b_ and _c_; _b_ carried a
stuffing-box, with a sliding rod _d_ terminated by a brass ball _s_ below,
and a ring above. The lower plate was connected with a foot, stop-cock, and
socket, _e_, _f_ and _g_; and also with a brass ball _l_, which by means of
a stem attached to it and entering the socket _g_, could be fixed at
various heights. The metallic parts of this apparatus were not varnished,
but the glass was well-covered with a coat of shell-lac previously
dissolved in alcohol. On exhausting the vessel at the air-pump it could be
filled with any other gas than air, and, in such cases, the gas so passed
in was dried whilst entering by fused chloride of calcium.

  [A] The drawing is to a scale of 1/6.

1384. The other part of the apparatus consisted of two insulating pillars,
_h_ and _i_, to which were fixed two brass balls, and through these passed
two sliding rods, _k_ and _m_, terminated at each end by brass balls; _n_
is the end of an insulated conductor, which could be rendered either
positive or negative from an electrical machine; _o_ and _p_ are wires
connecting it with the two parts previously described, and _q_ is a wire
which, connecting the two opposite sides of the collateral arrangements,
also communicates with a good discharging train _r_ (292.).

1385. It is evident that the discharge from the machine electricity may
pass either between _s_ and _l_, or S and L. The regulation adopted in the
first experiments was to keep _s_ and _l_ with their distance _unchanged_,
but to introduce first one gas and then another into the vessel _a_, and
then balance the discharge at the one place against that at the other; for
by making the interval at _a_ sufficiently small, all the discharge would
pass there, or making it sufficiently large it would all occur at the
interval _v_ in the receiver. On principle it seemed evident, that in this
way the varying interval _u_ might be taken as a measure, or rather
indication of the resistance to discharge through the gas at the constant
interval _v_. The following are the constant dimensions.

  Ball _s_        0.93 of an inch.
  Ball S          0.96 of an inch.
  Ball _l_        2.02 of an inch.
  Ball L          0.62 of an inch.
  Interval _v_    0.62 of an inch.

1386. On proceeding to experiment it was found that when air or any gas was
in the receiver _a_, the interval _u_ was not a fixed one; it might be
altered through a certain range of distance, and yet sparks pass either
there or at _v_ in the receiver. The extremes were therefore noted, i.e.
the greatest distance short of that at which the discharge _always_ took
place at _v_ in the gas, and the least distance short of that at which it
_always_ took place at _u_ in the air. Thus, with air in the receiver, the
extremes at _u_ were 0.56 and 0.79 of an inch, the range of 0.23 between
these distances including intervals at which sparks passed occasionally
either at one place or the other.

1387. The small balls _s_ and S could be rendered either positive or
negative from the machine, and as gases were expected and were found to
differ from each other in relation to this change (1399.), the results
obtained under these differences of charge were also noted.

1388. The following is a Table of results; the gas named is that in the
vessel _a_. The smallest, greatest, and mean interval at _u_ in air is
expressed in parts of an inch, the interval _v_ being constantly 0.62 of an
inch.

                                  Smallest.   Greatest.    Mean.
 _
| Air, _s_ and S, pos.               0.60       0.79       0.695
|_Air, _s_ and S, neg.               0.59       0.68       0.635
 _
| Oxygen, _s_ and S, pos.            0.41       0.60       0.505
|_Oxygen, _s_ and S, neg.            0.50       0.52       0.510
 _
| Nitrogen, _s_ and S, pos.          0.55       0.68       0.615
|_Nitrogen, _s_ and S, neg.          0.59       0.70       0.645
 _
| Hydrogen, _s_ and S, pos.          0.30       0.44       0.370
|_Hydrogen, _s_ and S, neg.          0.25       0.30       0.275
 _
| Carbonic acid, _s_ and S, pos.     0.56       0.72       0.640
|_Carbonic acid, _s_ and S, neg.     0.58       0.60       0.590
 _
| Olefiant gas, _s_ and S, pos.      0.64       0.86       0.750
|_Olefiant gas, _s_ and S, neg.      0.69       0.77       0.730
 _
| Coal gas, _s_ and S, pos.          0.37       0.61       0.490
|_Coal gas, _s_ and S, neg.          0.47       0.58       0.525
 _
| Muriatic acid gas, _s_ and S, pos. 0.89       1.32       1.105
|_Muriatic acid gas, _s_ and S, neg. 0.67       0.75       0.710

1389. The above results were all obtained at one time. On other occasions
other experiments were made, which gave generally the same results as to
order, though not as to numbers. Thus:

  Hydrogen, _s_ and S, pos.          0.23       0.57       0.400
  Carbonic acid, _s_ and S, pos.     0.51       1.05       0.780
  Olefiant gas, _s_ and S, pos.      0.66       1.27       0.965

I did not notice the difference of the barometer on the days of
experiment[A].

  [A] Similar experiments in different gases are described at 1507.
  1508.--_Dec. 1838._

1390. One would have expected only two distances, one for each interval,
for which the discharge might happen either at one or the other; and that
the least alteration of either would immediately cause one to predominate
constantly over the other. But that under common circumstances is not the
case. With air in the receiver, the variation amounted to 0.2 of an inch
nearly on the smaller interval of 0.6, and with muriatic acid gas, the
variation was above 0.4 on the smaller interval of 0.9. Why is it that when
a fixed interval (the one in the receiver) will pass a spark that cannot go
across 0.6 of air at one time, it will immediately after, and apparently
under exactly similar circumstances, not pass a spark that can go across
0.8 of air?

1391. It is probable that part of this variation will be traced to
particles of dust in the air drawn into and about the circuit (1568.). I
believe also that part depends upon a variable charged condition of the
surface of the glass vessel _a_. That the whole of the effect is not
traceable to the influence of circumstances in the vessel _a_, may be
deduced from the fact, that when sparks occur between balls in free air
they frequently are not straight, and often pass otherwise than by the
shortest distance. These variations in air itself, and at different parts
of the very same balls, show the presence and influence of circumstances
which are calculated to produce effects of the kind now under
consideration.

1392. When a spark had passed at either interval, then, generally, more
tended to appear at the _same_ interval, as if a preparation had been made
for the passing of the latter sparks. So also on continuing to work the
machine quickly the sparks generally followed at the same place. This
effect is probably due in part to the warmth of the air heated by the
preceding spark, in part to dust, and I suspect in part, to something
unperceived as yet in the circumstances of discharge.

1393. A very remarkable difference, which is _constant_ in its direction,
occurs when the electricity communicated to the balls _s_ and S is changed
from positive to negative, or in the contrary direction. It is that the
range of variation is always greater when the small bulls are positive than
when they are negative. This is exhibited in the following Table, drawn
from the former experiments.

                           Pos.      Neg.
In Air the range was       0.19      0.09
   Oxygen                  0.19      0.02
   Nitrogen                0.18      0.11
   Hydrogen                0.14      0.05
   Carbonic acid           0.16      0.02
   Olefiant gas            0.22      0.08
   Coal gas                0.24      0.12
   Muriatic acid           0.43      0.08

I have no doubt these numbers require considerable correction, but the
general result is striking, and the differences in several cases very
great.

       *       *       *       *       *

1394. Though, in consequence of the variation of the striking distance
(1386.), the interval in air fails to be a measure, as yet, of the
insulating or resisting power of the gas in the vessel, yet we may for
present purposes take the mean interval as representing in some degree that
power. On examining these mean intervals as they are given in the third
column (1388.), it will be very evident, that gases, when employed as
dielectrics, have peculiar electrical relations to insulation, and
therefore to induction, very distinct from such as might be supposed to
depend upon their mere physical qualities of specific gravity or pressure.

1395. First, it is clear that at the _same pressure_ they are not alike,
the difference being as great as 37 and 110. When the small balls are
charged positively, and with the same surfaces and the same pressure,
muriatic acid gas has three times the insulating or restraining power
(1362.) of hydrogen gas, and nearly twice that of oxygen, nitrogen, or air.

1396. Yet it is evident that the difference is not due to specific gravity,
for though hydrogen is the lowest, and therefore lower than oxygen, oxygen
is much beneath nitrogen, or olefiant gas; and carbonic acid gas, though
considerably heavier than olefiant gas or muriatic acid gas, is lower than
either. Oxygen as a heavy, and olefiant as a light gas, are in strong
contrast with each other; and if we may reason of olefiant gas from
Harris's results with air (1365.), then it might be rarefied to two-thirds
its usual density, or to a specific gravity of 9.3 (hydrogen being 1), and
having neither the same density nor pressure as oxygen, would have equal
insulating powers with it, or equal tendency to resist discharge.

1397. Experiments have already been described (1291. 1292.) which show that
the gases are sensibly alike in their inductive capacity. This result is
not in contradiction with the existence of great differences in their
restraining power. The same point has been observed already in regard to
dense and rare air (1375.).

1398. Hence arises a new argument proving that it cannot be mere pressure
of the atmosphere which prevents or governs discharge (1377. 1378.), but a
specific electric quality or relation of the gaseous medium. Hence also
additional argument for the theory of molecular inductive action.

1399. Other specific differences amongst the gases may be drawn from the
preceding series of experiments, rough and hasty as they are. Thus the
positive and negative series of mean intervals do not give the same
differences. It has been already noticed that the negative numbers are
lower than the positive (1393.), but, besides that, the _order_ of the
positive and negative results is not the same. Thus, on comparing the mean
numbers (which represent for the present insulating tension,) it appears
that in air, hydrogen, carbonic acid, olefiant gas and muriatic acid, the
tension rose higher when the smaller ball was made positive than when
rendered negative, whilst in oxygen, nitrogen, and coal gas, the reverse
was the case. Now though the numbers cannot be trusted as exact, and though
air, oxygen, and nitrogen should probably be on the same side, yet some of
the results, as, for instance, those with muriatic acid, fully show a
peculiar relation and difference amongst gases in this respect. This was
further proved by making the interval in air 0.8 of an inch whilst muriatic
acid gas was in the vessel _a_; for on charging the small balls _s_ and S
positively, _all_ the discharge took place through the _air_; but on
charging them negatively, _all_ the discharge took place through the
_muriatic acid gas_.

1400. So also, when the conductor _n_ was connected _only_ with the
muriatic acid gas apparatus, it was found that the discharge was more
facile when the small ball _s_ was negative than when positive; for in the
latter case, much of the electricity passed off as brush discharge through
the air from the connecting wire _p_ but in the former case, it all seemed
to go through the muriatic acid.

1401. The consideration, however, of positive and negative discharge across
air and other gases will be resumed in the further part of this, or in the
next paper (1465. 1525.).

1402. Here for the present I must leave this part of the subject, which had
for its object only to observe how far gases agreed or differed as to their
power of retaining a charge on bodies acting by induction through them. All
the results conspire to show that Induction is an action of contiguous
molecules (1295. &c.); but besides confirming this, the first principle
placed for proof in the present inquiry, they greatly assist in developing
the specific properties of each gaseous dielectric, at the same time
showing that further and extensive experimental investigation is necessary,
and holding out the promise of new discovery as the reward of the labour
required.

       *       *       *       *       *

1403. When we pass from the consideration of dielectrics like the gases to
that of bodies having the liquid and solid condition, then our reasonings
in the present state of the subject assume much more of the character of
mere supposition. Still I do not perceive anything adverse to the theory,
in the phenomena which such bodies present. If we take three insulating
dielectrics, as air, oil of turpentine, and shell-lac, and use the same
balls or conductors at the same intervals in these three substances,
increasing the intensity of the induction until discharge take place, we
shall find that it must be raised much higher in the fluid than for the
gas, and higher still in the solid than for the fluid. Nor is this
inconsistent with the theory; for with the liquid, though its molecules are
free to move almost as easily as those of the gas, there are many more
particles introduced into the given interval; and such is also the case
when the solid body is employed. Besides that with the solid, the cohesive
force of the body used will produce some effect; for though the production
of the polarized states in the particle of a solid may not be obstructed,
but, on the contrary, may in some cases be even favoured (1164. 1344.) by
its solidity or other circumstances, yet solidity may well exert an
influence on the point of final subversion, (just as it prevents discharge
in an electrolyte,) and so enable inductive intensity to rise to a much
higher degree.

1404. In the cases of solids and liquids too, bodies may, and most probably
do, possess specific differences as to their ability of assuming the
polarized state, and also as to the extent to which that polarity must rise
before discharge occurs. An analogous difference exists in the specific
inductive capacities already pointed out in a few substances (1278.) in the
last paper. Such a difference might even account for the various degrees of
insulating and conducting power possessed by different bodies, and, if it
should be found to exist, would add further strength to the argument in
favour of the molecular theory of inductive action.

       *       *       *       *       *

1405. Having considered these various cases of sustained insulation in
non-conducting dielectrics up to the highest point which they can attain,
we find that they terminate at last in _disruptive discharge_; the peculiar
condition of the molecules of the dielectric which was necessary to the
continuous induction, being equally essential to the occurrence of that
effect which closes all the phenomena. This discharge is not only in its
appearance and condition different to the former modes by which the
lowering of the powers was effected (1320. 1343.), but, whilst really the
same in principle, varies much from itself in certain characters, and thus
presents us with the forms of _spark_, _brush_, and _glow_ (1359.). I will
first consider _the spark_, limiting it for the present to the case of
discharge between two oppositely electrified conducting surfaces.

_The electric spark or flash._

1406. The _spark_ is consequent upon a discharge or lowering of the
polarized inductive state of many dielectric particles, by a particular
action of a few of the particles occupying a very small and limited space;
all the previously polarized particles returning to their first or normal
condition in the inverse order in which they left it, and uniting their
powers meanwhile to produce, or rather to continue, (1417.--1436.) the
discharge effect in the place where the subversion of force first occurred.
My impression is, that the few particles situated where discharge occurs
are not merely pushed apart, but assume a peculiar state, a highly exulted
condition for the time, i.e. have thrown upon them all the surrounding
forces in succession, and rising up to a proportionate intensity of
condition, perhaps equal to that of chemically combining atoms, discharge
the powers, possibly in the same manner as they do theirs, by some
operation at present unknown to us; and so the end of the whole. The
ultimate effect is exactly as if a metallic wire had been put into the
place of the discharging particles; and it does not seem impossible that
the principles of action in both cases, may, hereafter, prove to be the
same.

1407. The _path of the spark_, or of the discharge, depends on the degree
of tension acquired by the particles in the line of discharge,
circumstances, which in every common case are very evident and by the
theory easy to understand, rendering it higher in them than in their
neighbours, and, by exalting them first to the requisite condition, causing
them to determine the course of the discharge. Hence the selection of the
path, and the solution of the wonder which Harris has so well described[A]
as existing under the old theory. All is prepared amongst the molecules
beforehand, by the prior induction, for the path either of the electric
spark or of lightning itself.

  [A] Nautical Magazine, 1834, p 229.

1408. The same difficulty is expressed as a principle by Nobili for voltaic
electricity, almost in Mr. Harris's words, namely[A], "electricity directs
itself towards the point where it can most easily discharge itself," and
the results of this as a principle he has well wrought out for the case of
voltaic currents. But the _solution_ of the difficulty, or the proximate
cause of the effects, is the same; induction brings the particles up to or
towards a certain degree of tension (1370.); and by those which first
attain it, is the discharge first and most efficiently performed.

  [A] Bibliothèque Universelle, 1835, lix. 275.

1409. The _moment_ of discharge is probably determined by that molecule of
the dielectric which, from the circumstances, has its tension most quickly
raised up to the maximum intensity. In all cases where the discharge passes
from conductor to conductor this molecule must be on the surface of one of
them; but when it passes between a conductor and a nonconductor, it is,
perhaps, not always so (1453.). When this particle has acquired its maximum
tension, then the whole barrier of resistance is broken down in the line or
lines of inductive action originating at it, and disruptive discharge
occurs (1370.): and such an inference, drawn as it is from the theory,
seems to me in accordance with Mr. Harris's facts and conclusions
respecting the resistance of the atmosphere, namely, that it is not really
greater at any one discharging distance than another[A].

  [A] Philosophical Transactions, 1834, pp. 227, 229.

1410. It seems probable, that the tension of a particle of the same
dielectric, as air, which is requisite to produce discharge, is a _constant
quantity_, whatever the shape of the part of the conductor with which it is
in contact, whether ball or point; whatever the thickness or depth of
dielectric throughout which induction is exerted; perhaps, even, whatever
the state, as to rarefaction or condensation of the dielectric; and
whatever the nature of the conductor, good or bad, with which the particle
is for the moment associated. In saying so much, I do not mean to exclude
small differences which may be caused by the reaction of neighbouring
particles on the deciding particle, and indeed, it is evident that the
intensity required in a particle must be related to the condition of those
which are contiguous. But if the expectation should be found to approximate
to truth, what a generality of character it presents! and, in the
definiteness of the power possessed by a particular molecule, may we not
hope to find an immediate relation to the force which, being electrical, is
equally definite and constitutes chemical affinity?

1411. Theoretically it would seem that, at the moment of discharge by the
spark in one line of inductive force, not merely would all the other lines
throw their forces into this one (1406.), but the lateral effect,
equivalent to a repulsion of these lines (1224. 1297.), would be relieved
and, perhaps, followed by a contrary action, amounting to a collapse or
attraction of these parts. Having long sought for some transverse force in
statical electricity, which should be the equivalent to magnetism or the
transverse force of current electricity, and conceiving that it might be
connected with the transverse action of the lines of inductive force,
already described (1297.), I was desirous, by various experiments, of
bringing out the effect of such a force, and making it tell upon the
phenomena of electro-magnetism and magneto-electricity[A].

  [A] See further investigations of this subject, 1658-1666.
  1709-1735.--_Dec. 1838._

1412. Amongst other results, I expected and sought for the mutual
affection, or even the lateral coalition of two similar sparks, if they
could be obtained simultaneously side by side, and sufficiently near to
each other. For this purpose, two similar Leyden jars were supplied with
rods of copper projecting from their balls in a horizontal direction, the
rods being about 0.2 of an inch thick, and rounded at the ends. The jars
were placed upon a sheet of tinfoil, and so adjusted that their rods, _a_
and _b_, were near together, in the position represented in plan at fig.
116: _c_ and _d_ were two brass balls connected by a brass rod and
insulated: _e_ was also a brass ball connected, by a wire, with the ground
and with the tinfoil upon which the Leyden jars were placed. By laying an
insulated metal rod across from _a_ to _b_, charging the jars, and removing
the rod, both the jars could be brought up to the same intensity of charge
(1370.). Then, making the ball _e_ approach the ball _d_, at the moment the
spark passed there, two sparks passed between the rods _n_, _o_, and the
ball _c_; and as far as the eye could judge, or the conditions determine,
they were simultaneous.

1413. Under these circumstances two modes of discharge took place; either
each end had its own particular spark to the ball, or else one end only was
associated by a spark with the ball, but was at the same time related to
the other end by a spark between the two.

1414. When the ball _c_ was about an inch in diameter, the ends _n_ and
_o_, about half an inch from it, and about 0.4 of an inch from each other,
the two sparks to the ball could be obtained. When for the purpose of
bringing the sparks nearer together, the ends, _n_ and _o_, were brought
closer to each other, then, unless very carefully adjusted, only one end
had a spark with the ball, the other having a spark to it; and the least
variation of position would cause either _n_ or _o_ to be the end which,
giving the direct spark to the ball, was also the one through, or by means
of which, the other discharged its electricity.

1415. On making the ball _c_ smaller, I found that then it was needful to
make the interval between the ends _n_ and _o_ larger in proportion to the
distance between them and the ball _c_. On making _c_ larger, I found I
could diminish the interval, and so bring the two simultaneous separate
sparks closer together, until, at last, the distance between them was not
more at the widest part than 0.6 of their whole length.

1416. Numerous sparks were then passed and carefully observed. They were
very rarely straight, but either curved or bent irregularly. In the average
of cases they were, I think, decidedly convex towards each other; perhaps
two-thirds presented more or less of this effect, the rest bulging more or
less outwards. I was never able, however, to obtain sparks which,
separately leaving the ends of the wires _n_ and _o_, conjoined into one
spark before they reached or communicated with the ball _c_. At present,
therefore, though I think I saw a tendency in the sparks to unite, I cannot
assert it as a fact.

1417. But there is one very interesting effect here, analogous to, and it
may be in part the same with, that I was searching for: I mean the
increased facility of discharge where the spark passes. For instance, in
the cases where one end, as _n_, discharged the electricity of both ends to
the ball _c_, fig. 116, the electricity of the other end _o_, had to pass
through an interval of air 1.5 times as great as that which it might have
taken, by its direct passage between the end and the ball itself. In such
cases, the eye could not distinguish, even by the use of Wheatstone's
means[A], that the spark from the end _n_, which contained both portions of
electricity, was a double spark. It could not have consisted of two sparks
taking separate courses, for such an effect would have been visible to the
eye; but it is just possible, that the spark of the first end _n_ and its
jar, passing at the smallest interval of time before that of the other _o_
had heated and expanded the air in its course, and made it so much more
favourable to discharge, that the electricity of the end _o_ preferred
leaping across to it and taking a very circuitous route, rather than the
more direct one to the ball. It must, however, be remarked, in answer to
this supposition, that the one spark between _d_ and _e_ would, by its
influence, tend to produce simultaneous discharges at _n_ and _o_, and
certainly did so, when no preponderance was given to one wire over the
other, as to the previous inductive effect (1414.).

  [A] Philosophical Transactions, 1834, pp. 584, 585.

1418. The fact, however, is, that disruptive discharge is favourable to
itself. It is at the outset a case of tottering equilibrium: and if _time_
be an element in discharge, in however minute a proportion (1436.), then
the commencement of the act at any point favours its continuance and
increase there, and portions of power will be discharged by a course which
they would not otherwise have taken.

1419. The mere heating and expansion of the air itself by the first portion
of electricity which passes, must have a great influence in producing this
result.

1420. As to the result itself, we see its effect in every electric spark;
for it is not the whole quantity which passes that determines the
discharge, but merely that small portion of force which brings the deciding
molecule (1370.) up to its maximum tension; then, when its forces are
subverted and discharge begins, all the rest passes by the same course,
from the influence of the favouring circumstances just referred to; and
whether it be the electricity on a square inch, or a thousand square inches
of charged glass, the discharge is complete. Hereafter we shall find the
influence of this effect in the formation of brushes (1435.); and it is not
impossible that we may trace it producing the jagged spark and the forked
lightning.

       *       *       *       *       *

1421. The characters of the electric spark in _different gases_ vary, and
the variation _may_ be due simply to the effect of the heat evolved at the
moment. But it may also be due to that specific relation of the particles
and the electric forces which I have assumed as the basis of a theory of
induction; the facts do not oppose such a view; and in that view the
variation strengthens the argument for molecular action, as it would seem
to show the influence of the latter in every part of the electrical effect
(1423. 1454.).

1422. The appearances of the sparks in different gases have often been
observed and recorded[A], but I think it not out of place to notice briefly
the following results; they were obtained with balls of brass, (platina
surfaces would have been better,) and at common pressures. In _air_, the
sparks have that intense light and bluish colour which are so well known,
and often have faint or dark parts in their course, when the quantity of
electricity passing is not great. In _nitrogen_, they are very beautiful,
having the same general appearance as in air, but have decidedly more
colour of a bluish or purple character, and I thought were remarkably
sonorous. In _oxygen_, the sparks were whiter than in air or nitrogen, and
I think not so brilliant. In _hydrogen_, they had a very fine crimson
colour, not due to its rarity, for the character passed away as the
atmosphere was rarefied (1459.)[B]. Very little sound was produced in this
gas; but that is a consequence of its physical condition[C]. In _carbonic
acid gas_, the colour was similar to that of the spark in air, but with a
little green in it: the sparks were remarkably irregular in form, more so
than in common air: they could also, under similar circumstances as to size
of ball, &c., be obtained much longer than in air, the gas showing a
singular readiness to cause the discharge in the form of spark. In
_muriatic acid gas_, the spark was nearly white: it was always bright
throughout, never presenting those dark parts which happen in air,
nitrogen, and some other gases. The gas was dry, and during the whole
experiment the surface of the glass globe within remained quite dry and
bright. In _coal gas_, the spark was sometimes green, sometimes red, and
occasionally one part was green and another red: black parts also occur
very suddenly in the line of the spark, i.e. they are not connected by any
dull part with bright portions, but the two seem to join directly one with
the other.

  [A] See Van Marum's description of the Teylerian machine, vol. i. p.
  112, and vol. ii. p. 196; also Ency. Britan., vol. vi., Article
  Electricity, pp. 505, 507.

  [B] Van Marum says they are about four times as large in hydrogen as
  in air. vol. i. p. 122.

  [C] Leslie. Cambridge Phil. Transactions, 267.

1423. These varieties of character impress my mind with a feeling, that
they are due to a direct relation of the electric powers to the particles
of the dielectric through which the discharge occurs, and are not the mere
results of a casual ignition or a secondary kind of action of the
electricity, upon the particles which it finds in its course and thrusts
aside in its passage (1454.).

1424. The spark may be obtained in media which are far denser than air, as
in oil of turpentine, olive oil, resin, glass, &c.: it may also be obtained
in bodies which being denser likewise approximate to the condition of
conductors, as spermaceti, water, &c. But in these cases, nothing occurs
which, as far as I can perceive, is at all hostile to the general views I
have endeavoured to advocate.

_The electrical brush._

1425. The _brush_ is the next form of disruptive discharge which I shall
consider. There are many ways of obtaining it, or rather of exalting its
characters; and all these ways illustrate the principles upon which it is
produced. If an insulated conductor, connected with the positive conductor
of an electrical machine, have a metal rod 0.3 of an inch in diameter
projecting from it outwards from the machine, and terminating by a rounded
end or a small ball, it will generally give good brushes; or, if the
machine be not in good action, then many ways of assisting the formation of
the brush can be resorted to; thus, the hand or any _large_ conducting
surface may be approached towards the termination to increase inductive
force (1374.): or the termination may be smaller and of badly conducting
matter, as wood: or sparks may be taken between the prime conductor of the
machine and the secondary conductor to which the termination giving brushes
belongs: or, which gives to the brushes exceedingly fine characters and
great magnitude, the air around the termination may be rarefied more or
less, either by heat or the air-pump; the former favourable circumstances
being also continued.

1426. The brush when obtained by a powerful machine on a ball about 0.7 of
an inch in diameter, at the end of a long brass rod attached to the
positive prime conductor, had the general appearance as to form represented
in fig. 117: a short conical bright part or root appeared at the middle
part of the ball projecting directly from it, which, at a little distance
from the ball, broke out suddenly into a wide brush of pale ramifications
having a quivering motion, and being accompanied at the same time with a
low dull chattering sound.

1427. At first the brush seems continuous, but Professor Wheatstone has
shown that the whole phenomenon consists of successive intermitting
discharges[A]. If the eye be passed rapidly, not by a motion of the head,
but of the eyeball itself, across the direction of the brush, by first
looking steadfastly about 10° or 15° above, and then instantly as much
below it, the general brush will be resolved into a number of individual
brushes, standing in a row upon the line which the eye passed over; each
elementary brush being the result of a single discharge, and the space
between them representing both the time during which the eye was passing
over that space, and that which elapsed between one discharge and another.

  [A] Philosophical Transactions, 1834, p. 586.

1428. The single brushes could easily be separated to eight or ten times
their own width, but were not at the same time extended, i.e. they did not
become more indefinite in shape, but, on the contrary, less so, each being
more distinct in form, ramification, and character, because of its
separation from the others, in its effects upon the eye. Each, therefore,
was instantaneous in its existence (1436.). Each had the conical root
complete (1426.).

1429. On using a smaller ball, the general brush was smaller, and the
sound, though weaker, more continuous. On resolving the brush into its
elementary parts, as before, these were found to occur at much shorter
intervals of time than in the former case, but still the discharge was
intermitting.

1430. Employing a wire with a round end, the brush was still smaller, but,
as before, separable into successive discharges. The sound, though feebler,
was higher in pitch, being a distinct musical note.

1431. The sound is, in fact, due to the recurrence of the noise of each
separate discharge, and these, happening at intervals nearly equal under
ordinary circumstances, cause a definite note to be heard, which, rising in
pitch with the increased rapidity and regularity of the intermitting
discharges, gives a ready and accurate measure of the intervals, and so may
be used in any case when the discharge is heard, even though the
appearances may not be seen, to determine the element of _time_. So when,
by bringing the hand towards a projecting rod or ball, the pitch of the
tone produced by a brushy discharge increases, the effect informs us that
we have increased the induction (1374.), and by that means increased the
rapidity of the alternations of charge and discharge.

1432. By using wires with finer terminations, smaller brushes were
obtained, until they could hardly be distinguished as brushes; but as long
as _sound_ was heard, the discharge could be ascertained by the eye to be
intermitting; and when the sound ceased, the light became _continuous_ as a
glow (1359. 1405. 1526-1543.).

1433. To those not accustomed to use the eye in the manner I have
described, or, in cases where the recurrence is too quick for any
unassisted eye, the beautiful revolving mirror of Professor Wheatstone[A]
will be useful for such developments of condition as those mentioned above.
Another excellent process is to produce the brush or other luminous
phenomenon on the end of a rod held in the hand opposite to a charged
positive or negative conductor, and then move the rod rapidly from side to
side whilst the eye remains still. The successive discharges occur of
course in different places, and the state of things before, at, and after a
single coruscation or brush can be exceedingly well separated.

  [A] Philosophical Transactions, 1834, pp. 581, 585.

1434. The _brush_ is in reality a discharge between a bad or a
non-conductor and either a conductor or another non-conductor. Under common
circumstances, the brush is a discharge between a conductor and air, and I
conceive it to take place in something like the following manner. When the
end of an electrified rod projects into the middle of a room, induction
takes place between it and the walls of the room, across the dielectric,
air; and the lines of inductive force accumulate upon the end in greater
quantity than elsewhere, or the particles of air at the end of the rod are
more highly polarized than those at any other part of the rod, for the
reasons already given (1374.). The particles of air situated in sections
across these lines of force are least polarized in the sections towards the
walls and most polarized in those nearer to the end of the wires (1369.):
thus, it may well happen, that a particle at the end of the wire is at a
tension that will immediately terminate in discharge, whilst in those even
only a few inches off, the tension is still beneath that point. But suppose
the rod to be charged positively, a particle of air A, fig. 118, next it,
being polarized, and having of course its negative force directed towards
the rod and its positive force outwards; the instant that discharge takes
place between the positive force of the particle of the rod opposite the
air and the negative force of the particle of air towards the rod, the
whole particle of air becomes positively electrified; and when, the next
instant, the discharged part of the rod resumes its positive state by
conduction from the surface of metal behind, it not only acts on the
particles beyond A, by throwing A into a polarized state again, but A
itself, because of its charged state, exerts a distinct inductive act
towards these further particles, and the tension is consequently so much
exalted between A and B, that discharge takes place there also, as well as
again between the metal and A.

1435. In addition to this effect, it has been shown, that, the act of
discharge having once commenced, the whole operation, like a case of
unstable equilibrium, is hastened to a conclusion (1370. 1418.), the rest
of the act being facilitated in its occurrence, and other electricity than
that which caused the first necessary tension hurrying to the spot. When,
therefore, disruptive discharge has once commenced at the root of a brush,
the electric force which has been accumulating in the conductor attached to
the rod, finds a more ready discharge there than elsewhere, and will at
once follow the course marked out as it were for it, thus leaving the
conductor in a partially discharged state, and the air about the end of the
wire in a charged condition; and the time necessary for restoring the full
charge of the conductor, and the dispersion of the charged air in a greater
or smaller degree, by the joint forces of repulsion from the conductor and
attraction towards the walls of the room, to which its inductive action is
directed, is just that time which forms the interval between brush and
brush (1420. 1427. 1431. 1447.).

1436. The words of this description are long, but there is nothing in the
act or the forces on which it depends to prevent the discharge being
_instantaneous_, as far as we can estimate and measure it. The
consideration of _time_ is, however, important in several points of view
(1418.), and in reference to disruptive discharge, it seemed from theory
far more probable that it might be detected in a brush than in a spark; for
in a brush, the particles in the line through which the discharge passes
are in very different states as to intensity, and the discharge is already
complete in its act at the root of the brush, before the particles at the
extremity of the ramifications have yet attained their maximum intensity.

1437. I consider _brush_ discharge as probably a successive effect in this
way. Discharge begins at the root (1426. 1553.), and, extending itself in
succession to all parts of the single brush, continues to go on at the root
and the previously formed parts until the whole brush is complete; then, by
the fall in intensity and power at the conductor, it ceases at once in all
parts, to be renewed, when that power has risen again to a sufficient
degree. But in a _spark_, the particles in the line of discharge being,
from the circumstances, nearly alike in their intensity of polarization,
suffer discharge so nearly at the same moment as to make the time quite
insensible to us.

1438. Mr. Wheatstone has already made experiments which fully illustrate
this point. He found that the brush generally had a sensible duration, but
that with his highest capabilities he could not detect any such effect in
the spark[A]. I repeated his experiment on the brush, though with more
imperfect means, to ascertain whether I could distinguish a longer duration
in the stem or root of the brush than in the extremities, and the
appearances were such as to make me think an effect of this kind was
produced.

  [A] Philosophical Transactions, 1836, pp. 586, 590.

1439. That the discharge breaks into several ramifications, and by them
passes through portions of air alike, or nearly alike, as to polarization
and the degree of tension the particles there have acquired, is a very
natural result of the previous state of things, and rather to be expected
than that the discharge should continue to go straight out into space in a
single line amongst those particles which, being at a distance from the end
of the rod, are in a lower state of tension than those which are near: and
whilst we cannot but conclude, that those parts where the branches of a
single brush appear, are more favourably circumstanced for discharge than
the darker parts between the ramifications, we may also conclude, that in
those parts where the light of concomitant discharge is equal, there the
circumstances are nearly equal also. The single successive brushes are by
no means of the same particular shape even when they are observed without
displacement of the rod or surrounding objects (1427. 1433.), and the
successive discharges may be considered as taking place into the mass of
air around, through different roads at each brush, according as minute
circumstances, such as dust, &c. (1391. 1392.), may have favoured the
course by one set of particles rather than another.

1440. Brush discharge does not essentially require any current of the
medium in which the brush appears: the current almost always occurs, but is
a consequence of the brush, and will be considered hereafter (1562-1610.).
On holding a blunt point positively charged towards uninsulated water, a
star or glow appeared on the point, a current of air passed from it, and
the surface of the water was depressed; but on bringing the point so near
that sonorous brushes passed, then the current of air instantly ceased, and
the surface of the water became level.

1441. The discharge by a brush is not to all the particles of air that are
near the electrified conductor from which the brush issues; only those
parts where the ramifications pass are electrified: the air in the central
dark parts between them receives no charge, and, in fact, at the time of
discharge, has its electric and inductive tension considerably lowered. For
consider fig. 128 to represent a single positive brush;--the induction
before the discharge is from the end of the rod outwards, in diverging
lines towards the distant conductors, as the walls of the room, &c., and a
particle at _a_ has polarity of a certain degree of tension, and tends with
a certain force to become charged; but at the moment of discharge, the air
in the ramifications _b_ and _d_, acquiring also a positive state, opposes
its influence to that of the positive conductor on _a_, and the tension of
the particle at _a_ is therefore diminished rather than increased. The
charged particles at _b_ and _d_ are now inductive bodies, but their lines
of inductive action are still outwards towards the walls of the room; the
direction of the polarity and the tendency of other particles to charge
from these, being governed by, or in conformity with, these lines of force.

1442. The particles that are charged are probably very highly charged, but,
the medium being a non-conductor, they cannot communicate that state to
their neighbours. They travel, therefore, under the influence of the
repulsive and attractive forces, from the charged conductor towards the
nearest uninsulated conductor, or the nearest body in a different state to
themselves, just as charged particles of dust would travel, and are then
discharged; each particle acting, in its course, as a centre of inductive
force upon any bodies near which it may come. The travelling of these
charged particles when they are numerous, causes wind and currents, but
these will come into consideration under _carrying discharge_ (1319. 1562.
&c.).

1443. When air is said to be electrified, and it frequently assumes this
state near electrical machines, it consists, according to my view, of a
mixture of electrified and unelectrified particles, the latter being in
very large proportion to the former. When we gather electricity from air,
by a flame or by wires, it is either by the actual discharge of these
particles, or by effects dependent on their inductive action, a case of
either kind being produceable at pleasure. That the law of equality between
the two forces or forms of force in inductive action is as strictly
preserved in these as in other cases, is fully shown by the fact, formerly
stated (1173. 1174.), that, however strongly air in a vessel might be
charged positively, there was an exactly equal amount of negative force on
the inner surface of the vessel itself, for no residual portion of either
the one or the other electricity could be obtained.

1444. I have nowhere said, nor does it follow, that the air is charged only
where the luminous brush appears. The charging may extend beyond those
parts which are visible, i.e. particles to the right or left of the lines
of light may receive electricity, the parts which are luminous being so
only because much electricity is passing by them to other parts (1437.);
just as in a spark discharge the light is greater as more electricity
passes, though it has no necessary relation to the quantity required to
commence discharge (1370. 1420.). Hence the form we see in a brush may by
no means represent the whole quantity of air electrified; for an invisible
portion, clothing the visible form to a certain depth, may, at the same
time, receive its charge (1552.).

1445. Several effects which I have met with in muriatic acid gas tend to
make me believe, that that gaseous body allows of a dark discharge. At the
same time, it is quite clear from theory, that in some gases, the reverse
of this may occur, i.e. that the charging of the air may not extend even so
far as the light. We do not know as yet enough of the electric light to be
able to state on what it depends, and it is very possible that, when
electricity bursts forth into air, all the particles of which are in a
state of tension, light may be evolved by such as, being very near to, are
not of, those which actually receive a charge at the time.

1446. The further a brush extends in a gas, the further no doubt is the
charge or discharge carried forward; but this may vary between different
gases, and yet the intensity required for the first moment of discharge not
vary in the same, but in some other proportion. Thus with respect to
nitrogen and muriatic acid gases, the former, as far as my experiments have
proceeded, produces far finer and larger brushes than the latter (1458.
1462.), but the intensity required to commence discharge is much higher for
the muriatic acid than the nitrogen (1395.). Here again, therefore, as in
many other qualities, specific differences are presented by different
gaseous dielectrics, and so prove the special relation of the latter to the
act and the phenomena of induction.

1447. To sum up these considerations respecting the character and condition
of the brush, I may state that it is a spark to air; a diffusion of
electric force to matter, not by conduction, but disruptive discharge, a
dilute spark which, passing to very badly conducting matter, frequently
discharges but a small portion of the power stored up in the conductor; for
as the air charged reacts on the conductor, whilst the conductor, by loss
of electricity, sinks in its force (1435.), the discharge quickly ceases,
until by the dispersion of the charged air and the renewal of the excited
conditions of the conductor, circumstances have risen up to their first
effective condition, again to cause discharge, and again to fall and rise,

1448. The brush and spark gradually pass into one another, Making a small
ball positive by a good electrical machine with a large prime conductor,
and approaching a large uninsulated discharging ball towards it, very
beautiful variations from the spark to the brush may be obtained. The
drawings of long and powerful sparks, given by Van Marum[A], Harris[B], and
others, also indicate the same phenomena. As far as I have observed,
whenever the spark has been brushy in air of common pressures, the whole of
the electricity has not been discharged, but only portions of it, more or
less according to circumstances; whereas, whenever the effect has been a
distinct spark throughout the whole of its course, the discharge has been
perfect, provided no interruption had been made to it elsewhere, in the
discharging circuit, than where the spark occurred.

  [A] Description of the Teylerian machine, vol. i. pp. 28. 32.; vol.
  ii. p. 226, &c.

  [B] Philosophical Transactions, 1834, p. 213.

1449. When an electrical brush from an inch to six inches in length or more
is issuing into free air, it has the form given, fig. 117. But if the hand,
a ball, of any knobbed conductor be brought near, the extremities of the
coruscations turn towards it and each other, and the whole assumes various
forms according to circumstances, as in figs. 119, 120, and 121. The
influence of the circumstances in each case is easily traced, and I might
describe it here, but that I should be ashamed to occupy the time of the
Society in things so evident. But how beautifully does the curvature of the
ramifications illustrate the curved form of the lines of inductive force
existing previous to the discharge! for the former are consequences of the
latter, and take their course, in each discharge, where the previous
inductive tension had been raised to the proper degree. They represent
these curves just as well as iron filings represent magnetic curves, the
visible effects in both cases being the consequences of the action of the
forces in _the places where_ the effects appear. The phenomena, therefore,
constitute additional and powerful testimony (1216. 1230.) to that already
given in favour both of induction through dielectrics in curved lines
(1231.), and of the lateral relation of these lines, by an effect
equivalent to a repulsion producing divergence, or, as in the cases
figured, the bulging form.

1450. In reference to the theory of molecular inductive action, I may also
add, the proof deducible from the long brushy ramifying spark which, may be
obtained between a small ball on the positive conductor of an electrical
machine, and a larger one at a distance (1448. 1504.). What a fine
illustration that spark affords of the previous condition of _all_ the
particles of the dielectric between the surfaces of discharge, and how
unlike the appearances are to any which would be deduced from the theory
which assumes inductive action to be action at a distance, in straight
lines only; and charge, as being electricity retained upon the surface of
conductors by the mere pressure of the atmosphere!

       *       *       *       *       *

1451. When the brush is obtained in rarefied air, the appearances vary
greatly, according to circumstances, and are exceedingly beautiful.
Sometimes a brush may be formed of only six or seven branches, these being
broad and highly luminous, of a purple colour, and in some parts an inch or
more apart: by a spark discharge at the prime conductor (1455.) single
brushes may be obtained at pleasure. Discharge in the form of a brush is
favoured by rarefaction of the air, in the same manner and for the same
reason as discharge in the form of a spark (1375.); but in every case there
is previous induction and charge through the dielectric, and polarity of
its particles (1437.), the induction being, as in any other instance,
alternately raised by the machine and lowered by the discharge. In certain
experiments the rarefaction was increased to the utmost degree, and the
opposed conducting surfaces brought as near together as possible without
producing glow (1529.): the brushes then contracted in their lateral
dimensions, and recurred so rapidly as to form an apparently continuous arc
of light from metal to metal. Still the discharge could be observed to
intermit (1427.), so that even under these high conditions, induction
preceded each single brush, and the tense polarized condition of the
contiguous particles was a necessary preparation for the discharge itself.

1452. The brush form of disruptive discharge may be obtained not only in
air and gases, but also in much denser media. I procured it in _oil of
turpentine_ from the end of a wire going through a glass tube into the
fluid contained in a metal vessel. The brush was small and very difficult
to obtain; the ramifications were simple, and stretched out from each
other, diverging very much. The light was exceedingly feeble, a perfectly
dark room being required for its observation. When a few solid particles,
as of dust or silk, were in the liquid, the brush was produced with much
greater facility.

1453. The running together or coalescence of different lines of discharge
(1412.) is very beautifully shown in the brush in air. This point may
present a little difficulty to those who are not accustomed to see in every
discharge an equal exertion of power in opposite directions, a positive
brush being considered by such (perhaps in consequence of the common phrase
_direction of a current_) as indicating a breaking forth in different
directions of the original force, rather than a tendency to convergence and
union in one line of passage. But the ordinary case of the brush may be
compared, for its illustration, with that in which, by holding the knuckle
opposite to highly excited glass, a discharge occurs, the ramifications of
a brush then leading from the glass and converging into a spark on the
knuckle. Though a difficult experiment to make, it is possible to obtain
discharge between highly excited shell-lac and the excited glass of a
machine: when the discharge passes, it is, from the nature of the charged
bodies, brush at each end and spark in the middle, beautifully illustrating
that tendency of discharge to facilitate like action, which I have
described in a former page (1418.).

1454. The brush has _specific characters_ in different gases, indicating a
relation to the particles of these bodies even in a stronger degree than
the spark (1422. 1423.). This effect is in strong contrast with the
non-variation caused by the use of different substances as _conductors_
from which the brushes are to originate. Thus, using such bodies as wood,
card, charcoal, nitre, citric acid, oxalic acid, oxide of lead, chloride of
lead, carbonate of potassa, potassa fusa, strong solution of potash, oil of
vitriol, sulphur, sulphuret of antimony, and hæmatite, no variation in the
character of the brushes was obtained, except that (dependent upon their
effect as better or worse conductors) of causing discharge with more or
less readiness and quickness from the machine[A].

  [A] Exception must, of course, be made of those cases where the root
  of the brush, becoming a spark, causes a little diffusion or even
  decomposition of the matter there, and so gains more or less of a
  particular colour at that part.

1455. The following are a few of the effects I observed in different gasses
at the positively charged surfaces, and with atmospheres varying in their
pressure. The general effect of rarefaction was the same for all the gases:
at first, sparks passed; these gradually were converted into brushes, which
became larger and more distinct in their ramifications, until, upon further
rarefaction, the latter began to collapse and draw in upon each other, till
they formed a stream across from conductor to conductor: then a few lateral
streams shot out towards the glass of the vessel from the conductors; these
became thick and soft in appearance, and were succeeded by the full
constant glow which covered the discharging wire. The phenomena varied with
the size of the vessel (1477.), the degree of rarefaction, and the
discharge of electricity from the machine. When the latter was in
successive sparks, they were most beautiful, the effect of a spark from a
small machine being equal to, and often surpassing, that produced by the
_constant_ discharge of a far more powerful one.

1456. _Air._--Fine positive brushes are easily obtained in air at common
pressures, and possess the well-known purplish light. When the air is
rarefied, the ramifications are very long, filling the globe (1477.); the
light is greatly increased, and is of a beautiful purple colour, with an
occasional rose tint in it.

1457. _Oxygen._--At common pressures, the brush is very close and
compressed, and of a dull whitish colour. In rarefied oxygen, the form and
appearance are better, the colour somewhat purplish, but all the characters
very poor compared to those in air.

1458. _Nitrogen_ gives brushes with great facility at the positive surface,
far beyond any other gas I have tried: they are almost always fine in form,
light, and colour, and in rarefied nitrogen, are magnificent. They surpass
the discharges in any other gas as to the quantity of light evolved.

1459. _Hydrogen_, at common pressures, gave a better brush than oxygen, but
did not equal nitrogen; the colour was greenish gray. In rarefied hydrogen,
the ramifications were very fine in form and distinctness, but pale in
colour, with a soft and velvety appearance, and not at all equal to those
in nitrogen. In the rarest state of the gas, the colour of the light was a
pale gray green.

1460. _Coal gas._--The brushes were rather difficult to produce, the
contrast with nitrogen being great in this respect. They were short and
strong, generally of a greenish colour, and possessing much of the spark
character: for, occurring on both the positive and negative terminations,
often when there was a dark interval of some length between the two
brushes, still the quick, sharp sound of the spark was produced, as if the
discharge had been sudden through this gas, and partaking, in that respect,
of the character of a spark. In rare coal gas, the brush forms were better,
but the light very poor and the colour gray.

1461. _Carbonic acid gas_ produces a very poor brush at common pressures,
as regards either size, light, or colour; and this is probably connected
with the tendency which this gas has to discharge the electricity as a
spark (1422.). In rarefied carbonic acid, the brush is better in form, but
weak as to light, being of a dull greenish or purplish line, varying with
the pressure and other circumstances.

1462. _Muriatic acid gas._--It is very difficult to obtain the brush in
this gas at common pressures. On gradually increasing the distance of the
rounded ends, the sparks suddenly ceased when the interval was about an
inch, and the discharge, which was still through the gas in the globe, was
silent and dark. Occasionally a very short brush could for a few moments be
obtained, but it quickly disappeared. Even when the intermitting spark
current (1455.) from the machine was used, still I could only with
difficulty obtain a brush, and that very short, though I used rods with
rounded terminations (about 0.25 of an inch in diameter) which had before
given them most freely in air and nitrogen. During the time of this
difficulty with the muriatic gas, magnificent brushes were passing off from
different parts of the machine into the surrounding air. On rarefying the
gas, the formation of the brush was facilitated, but it was generally of a
low squat form, very poor in light, and very similar on both the positive
and negative surfaces. On rarefying the gas still more, a few large
ramifications were obtained of a pale bluish colour, utterly unlike those
in nitrogen.

       *       *       *       *       *

1463. In all the gases, the different forms of disruptive discharge may be
linked together and gradually traced from one extreme to the other, i.e.
from the spark to the glow (1405. 1526.), or, it may be, to a still further
condition to be called dark discharge (1544-1560.); but it is,
nevertheless, very surprising to see what a specific character each keeps
whilst under the predominance of the general law. Thus, in muriatic acid,
the brush is very difficult to obtain, and there comes in its place almost
a dark discharge, partaking of the readiness of the spark action. Moreover,
in muriatic acid, I have _never_ observed the spark with any dark interval
in it. In nitrogen, the spark readily changes its character into that of
brush. In carbonic acid gas, there seems to be a facility to occasion spark
discharge, whilst yet that gas is unlike nitrogen in the facility of the
latter to form brushes, and unlike muriatic acid in its own facility to
continue the spark. These differences add further force, first to the
observations already made respecting the spark in various gases (1422.
1423.), and then, to the proofs deducible from it, of the relation of the
electrical forces to the particles of matter.

1464. The peculiar characters of nitrogen in relation to the electric
discharge (1422. 1458.) must, evidently, have an important influence over
the form and even the occurrence of lightning. Being that gas which most
readily produces coruscations, and, by them, extends discharge to a greater
distance than any other gas tried, it is also that which constitutes
four-fifths of our atmosphere; and as, in atmospheric electrical phenomena,
one, and sometimes both the inductive forces are resident on the particles
of the air, which, though probably affected as to conducting power by the
aqueous particles in it, cannot be considered as a good conductor; so the
peculiar power possessed by nitrogen, to originate and effect discharge in
the form of a brush or of ramifications, has, probably, an important
relation to its electrical service in nature, as it most seriously affects
the character and condition of the discharge when made. The whole subject
of discharge from and through gases is of great interest, and, if only in
reference to atmospheric electricity, deserves extensive and close
experimental investigation.

_Difference of discharge at the positive and negative conducting surfaces._

1465. I have avoided speaking of this well-known phenomenon more than was
quite necessary, that I might bring together here what I have to say on the
subject. When the brush discharge is observed in air at the positive and
negative surfaces, there is a very remarkable difference, the true and full
comprehension of which would, no doubt, be of the utmost importance to the
physics of electricity; it would throw great light on our present subject,
i.e. the molecular action of dielectrics under induction, and its
consequences; and seems very open to, and accessible by, experimental
inquiry.

1466. The difference in question used to be expressed in former times by
saying, that a point charged positively gave brushes into the air, whilst
the same point charged negatively gave a star. This is true only of bad
conductors, or of metallic conductors charged intermittingly, or otherwise
controlled by collateral induction. If metallic points project _freely_
into the air, the positive and negative light upon them differ very little
in appearance, and the difference can be observed only upon close
examination.

1467. The effect varies exceedingly under different circumstances, but, as
we must set out from some position, may perhaps be stated thus: if a
metallic wire with a rounded termination in free air be used to produce the
brushy discharge, then the brushes obtained when the wire is charged
negatively are very poor and small, by comparison with those produced when
the charge is positive. Or if a large metal ball connected with the
electrical machine be charged _positively_, and a fine uninsulated point be
gradually brought towards it, a star appears on the point when at a
considerable distance, which, though it becomes brighter, does not change
its form of a star until it is close up to the ball: whereas, if the ball
be charged negatively, the point at a considerable distance has a star on
it as before; but when brought nearer, (in my case to the distance of 1-1/2
inch,) a brush formed on it, extending to the negative ball; and when still
nearer, (at 1/8 of an inch distance,) the brush ceased, and bright sparks
passed. These variations, I believe, include the whole series of
differences, and they seem to show at once, that the negative surface tends
to retain its discharging character unchanged, whilst the positive surface,
under similar circumstances, permits of great variation.

1468. There are several points in the character of the negative discharge
to air which it is important to observe. A metal rod, 0.3 of an inch in
diameter, with a rounded end projecting into the air, was charged
negatively, and gave a short noisy brush (fig. 122.). It was ascertained
both by sight (1427. 1433.) and sound (1431.), that the successive
discharges were very rapid in their recurrence, being seven or eight times
more numerous in the same period, than those produced when the rod was
charged positively to an equal degree. When the rod was positive, it was
easy, by working the machine a little quicker, to replace the brush by a
glow (1405. 1463.), but when it was negative no efforts could produce this
change. Even by bringing the hand opposite the wire, the only effect was to
increase the number of brush discharges in a given period, raising at the
same time the sound to a higher pitch.

1469. A point opposite the negative brush exhibited a star, and as it was
approximated caused the size and sound of the negative brush to diminish,
and, at last, to cease, leaving the negative end silent and dark, yet
effective as to discharge.

1470. When the round end of a smaller wire (fig. 123.) was advanced towards
the negative brush, it (becoming positive by induction) exhibited the quiet
glow at 8 inches distance, the negative brush continuing. When nearer, the
pitch of the sound of the negative brush rose, indicating quicker
intermittences (1431.); still nearer, the positive end threw off
ramifications and distinct brushes; at the same time, the negative brush
contracted in its lateral directions and collected together, giving a
peculiar narrow longish brush, in shape like a hair pencil, the two brushes
existing at once, but very different in their form and appearance, and
especially in the more rapid recurrence of the negative discharges than of
the positive. On using a smaller positive wire for the same experiment, the
glow first appeared on it, and then the brush, the negative brush being
affected at the same time; and the two at one distance became exceedingly
alike in appearance, and the sounds, I thought, were in unison; at all
events they were in harmony, so that the intermissions of discharge were
either isochronous, or a simple ratio existed between the intervals. With a
higher action of the machine, the wires being retained unaltered, the
negative surface became dark and silent, and a glow appeared on the
positive one. A still higher action changed the latter into a spark. Finer
positive wires gave other variations of these effects, the description of
which I must not allow myself to go into here.

1471. A thinner rod was now connected with the negative conductor in place
of the larger one (1468.), its termination being gradually diminished to a
blunt point, as in fig. 124; and it was beautiful to observe that,
notwithstanding the variation of the brush, the same general order of
effects was produced. The end gave a small sonorous negative brush, which
the approach of the hand or a large conducting surface did not alter, until
it was so near as to produce a spark. A fine point opposite to it was
luminous at a distance; being nearer it did not destroy the light and sound
of the negative brush, but only tended to have a brush produced on itself,
which, at a still less distance, passed into a spark joining the two
surfaces.

1472. When the distinct negative and positive brushes are produced
simultaneously in relation to each other in air, the former almost always
has a contracted form, as in fig. 125, very much indeed resembling the
figure which the positive brush itself has when influenced by the lateral
vicinity of positive parts acting by induction. Thus a brush issuing from a
point in the re-entering angle of a positive conductor has the same
compressed form (fig. 126.).

1473. The character of the negative brush is not affected by the chemical
nature of the substances of the conductors (1454.), but only by their
possession of the conducting power in a greater or smaller degree.

1474. Rarefaction of common air about a negative ball or blunt point
facilitated the development of the negative brush, the effect being, I
think, greater than on a positive brush, though great on both. Extensive
ramifications could be obtained from a ball or end electrified negatively
to the plate of the air-pump on which the jar containing it stood.

1475. A very important variation of the relative forms and conditions of
the positive and negative brush takes place on varying the dielectric in
which they are produced. The difference is so very great that it points to
a specific relation of this form of discharge to the particular gas in
which it takes place, and opposes the idea that gases are but obstructions
to the discharge, acting one like another and merely in proportion to their
pressure (1377.).

1476. In _air_, the superiority of the positive brush is well known (1467.
1472.). In _nitrogen_, it is as great or even greater than in air (1458.).
In _hydrogen_, the positive brush loses a part of its superiority, not
being so good as in nitrogen or air; whilst the negative brush does not
seem injured (1459.). In _oxygen_, the positive brush is compressed and
poor (1457); whilst the negative did not become less: the two were so alike
that the eye frequently could not tell one from the other, and this
similarity continued when the oxygen was gradually rarefied. In _coal gas_,
the brushes are difficult of production as compared to nitrogen (1460.),
and the positive not much superior to the negative in its character, either
at common or low pressures. In _carbonic acid gas_, this approximation of
character also occurred. In _muriatic acid gas_, the positive brush was
very little better than the negative, and both difficult to produce (1462.)
as compared with the facility in nitrogen or air.

1477. These experiments were made with rods of brass about a quarter of an
inch thick having rounded ends, these being opposed in a glass globe 7
inches in diameter, containing the gas to be experimented with. The
electric machine was used to communicate directly, sometimes the positive,
and sometimes the negative state, to the rod in connection with it.

1478. Thus we see that, notwithstanding there is a general difference in
favour of the superiority of the positive brush over the negative, that
difference is at its maximum in nitrogen and air; whilst in carbonic acid,
muriatic acid, coal gas, and oxygen, it diminishes, and at last almost
disappears. So that in this particular effect, as in all others yet
examined, the evidence is in favour of that view which refers the results
to a direct relation of the electric forces with the molecules of the
matter concerned in the action (1421. 1423. 1463.). Even when special
phenomena arise under the operation of the general law, the theory adopted
seems fully competent to meet the case.

1479. Before I proceed further in tracing the probable cause of the
difference between the positive and negative brush discharge, I wish to
know the results of a few experiments which are in course of preparation:
and thinking this Series of Researches long enough, I shall here close it
with the expectation of being able in a few weeks to renew the inquiry, and
entirely redeem my pledge (1306.).

_Royal Institution,
Dec. 23rd, 1837._




THIRTEENTH SERIES.


§ 18. _On Induction (continued)._ ¶ ix. _Disruptive discharge
(continued)--Peculiarities of positive and negative discharge either as
spark or brush--Glow discharge--Dark discharge._ ¶ x. _Convection, or
carrying discharge._ ¶ xi. _Relation of a vacuum to electrical phenomena._
§ 19. _Nature of the electrical current._

Received February 22,--Read March 15, 1838.


¶ ix. _Disruptive discharge (continued)._


1480. Let us now direct our attention to the general difference of the
positive and negative disruptive discharge, with the object of tracing, as
far as possible, the cause of that difference, and whether it depends on
the charged conductors principally, or on the interposed dielectric; and as
it appears to be great in air and nitrogen (1476.), let us observe the
phenomena in air first.

1481. The general case is best understood by a reference to surfaces of
considerable size rather than to points, which involve (as a secondary
effect) the formation of currents (1562). My investigation, therefore, was
carried on with balls and terminations of different diameters, and the
following are some of the principal results.

1482. If two balls of very different dimensions, as for instance one-half
an inch, and the other three inches in diameter, be arranged at the ends of
rods so that either can be electrified by a machine and made to discharge
by sparks to the other, which is at the same time uninsulated; then, as is
well known, far longer sparks are obtained when the small ball is positive
and the large ball negative, than when the small ball is negative and the
large ball positive. In the former case, the sparks are 10 or 12 inches in
length; in the latter, an inch or an inch and a half only.

       *       *       *       *       *

1483. But previous to the description of further experiments, I will
mention two words, for which with many others I am indebted to a friend,
and which I think it would be expedient to introduce and use. It is
important in ordinary inductive action, to distinguish at which charged
surface the induction originates and is sustained: i.e. if two or more
metallic balls, or other masses of matter, are in inductive relation, to
express which are charged originally, and which are brought by them into
the opposite electrical condition. I propose to call those bodies which are
originally charged, _inductric_ bodies; and those which assume the opposite
state, in consequence of the induction, _inducteous_ bodies. This
distinction is not needful because there is any difference between the sums
of the _inductric_ and the _inducteous_ forces; but principally because,
when a ball A is inductric, it not merely brings a ball B, which is
opposite to it, into an inducteous state, but also many other surrounding
conductors, though some of them may be a considerable distance off, and the
consequence is, that the balls do not bear the same precise relation to
each other when, first one, and then the other, is made the inductric ball;
though, in each case, the _same ball_ be made to assume the _same state._

1484, Another liberty which I may also occasionally take in language I will
explain and limit. It is that of calling a particular spark or brush,
_positive_ or _negative_, according as it may be considered as
_originating_ at a positive or a negative surface. We speak of the brush as
positive or negative when it shoots out from surfaces previously in those
states; and the experiments of Mr. Wheatstone go to prove that it _really
begins_ at the charged surface, and from thence extends into the air (1437.
1438.) or other dielectric. According to my view, _sparks_ also originate
or are determined at one particular spot (1370.), namely, that where the
tension first rises up to the maximum degree; and when this can be
determined, as in the simultaneous use of large and small balls, in which
case the discharge begins or is determined by the latter, I would call that
discharge which passes _at once_, a positive spark, if it was at the
positive surface that the maximum intensity was first obtained; or a
negative spark, if that necessary intensity was first obtained at the
negative surface.

      *       *       *       *       *

1485. An apparatus was arranged, as in fig. 129. (Plate VIII.): A and B
were brass balls of very different diameters attached to metal rods, moving
through sockets on insulating pillars, so that the distance between the
balls could be varied at pleasure. The large ball A, 2 inches in diameter,
was connected with an insulated brass conductor, which could be rendered
positive or negative directly from a cylinder machine: the small ball B,
0.25 of an inch in diameter, was connected with a discharging train (292.)
and perfectly uninsulated. The brass rods sustaining the balls were 0.2 of
an inch in thickness.

1486. When the large ball was _positive_ and inductric (1483.), negative
sparks occurred until the interval was 0.49 of an inch; then mixed brush
and spark between that and 0.51; and from 0.52 and upwards, negative brush
alone. When the large ball was made _negative_ and inductric, then positive
spark alone occurred until the interval was as great as 1.15 inches; spark
and brush from that up to 1.55; and to have the positive brush alone, it
required an interval of at least 1.65 inches.

1487. The balls A and B were now changed for each other. Then making the
small ball B inductric _positively_, the positive sparks alone continued
only up to 0.67; spark and brush occurred from 0.68 up to 0.72; and
positive brush alone from 0.74 and upwards. Rendering the small ball B
inductric and _negative_, negative sparks alone occurred up to 0.40; then
spark and brush at 0.42; whilst from 0.44 and upwards the noisy negative
brush alone took place.

1488. We thus find a great difference as the balls are rendered inductric
or inducteous; the small ball rendered _positive_ inducteously giving a
spark nearly twice as long as that produced when it was charged positive
inductrically, and a corresponding difference, though not, under the
circumstances, to the same extent, was manifest, when it was rendered
_negative_[A].

  [A] For similar experiments on different gases, see 1518.--_Dec. 1838._

1489. Other results are, that the small ball rendered positive gives a much
longer spark than when it is rendered negative, and that the small ball
rendered negative gives a brush more readily than when positive, in
relation to the effect produced by increasing the distance between the two
balls.

1490. When the interval was below 0.4 of an inch, so that the small ball
should give sparks, whether positive or negative, I could not observe that
there was any constant difference, either in their ready occurrence or the
number which passed in a given time. But when the interval was such that
the small ball when negative gave a brush, then the discharges from it, as
separate negative brushes, were far more numerous than the corresponding
discharges from it when rendered positive, whether those positive
discharges were as sparks or brushes.

1491. It is, therefore, evident that, when a ball is discharging
electricity in the form of brushes, the brushes are far more numerous, and
each contains or carries off far less electric force when the electricity
so discharged is negative, than when it is positive.

1492. In all such experiments as those described, the point of change from
spark to brush is very much governed by the working state of the electrical
machine and the size of the conductor connected with the discharging ball.
If the machine be in strong action and the conductor large, so that much
power is accumulated quickly for each discharge, then the interval is
greater at which the sparks are replaced by brushes; but the general effect
is the same[A].

  [A] For similar experiments in different gases, see 1510-1517.--_Dec.
  1838._

1493. These results, though indicative of very striking and peculiar
relations of the electric force or forces, do not show the relative degrees
of charge which the small ball acquires before discharge occurs, i.e. they
do not tell whether it acquires a higher condition in the negative, or in
the positive state, immediately preceding that discharge. To illustrate
this important point I arranged two places of discharge as represented, fig
130. A and D are brass balls 2 inches diameter, B and C are smaller brass
balls 0.25 of an inch in diameter; the forks L and R supporting them were
of brass wire 0.2 of an inch in diameter; the space between the large and
small ball on the same fork was 5 inches, that the two places of discharge
_n_ and _o_ might be sufficiently removed from each other's influence. The
fork L was connected with a projecting cylindrical conductor, which could
be rendered positive or negative at pleasure, by an electrical machine, and
the fork R was attached to another conductor, but thrown into an
uninsulated state by connection with a discharging train (292.). The two
intervals or places of discharge _n_ and _o_ could be varied at pleasure,
their extent being measured by the occasional introduction of a diagonal
scale. It is evident, that, as the balls A and B connected with the same
conductor are always charged at once, and that discharge may take place to
either of the balls connected with the discharging train, the intervals of
discharge _n_ and _o_ may be properly compared to each other, as respects
the influence of large and small balls when charged positively and
negatively in air.

1494. When the intervals _n_ and _o_ were each made = 0.9 of an inch, and
the balls A and B inductric _positively_, the discharge was all at _n_ from
the small ball of the conductor to the large ball of the discharging train,
and mostly by positive brush, though once by a spark. When the balls A and
B were made inductric _negatively_, the discharge was still from the same
small ball, at _n_, by a constant negative brush.

1495. I diminished the intervals _n_ and _o_ to 0.6 of an inch. When A and
B were inductric _positively_, all the discharge was at _n_ as a positive
brush: when A and B were inductric _negatively_, still all the discharge
was at _n_, as a negative brush.

1496. The facility of discharge at the positive and negative small balls,
therefore, did not appear to be very different. If a difference had
existed, there were always two small balls, one in each state, that the
discharge might happen at that most favourable to the effect. The only
difference was, that one was in the inductric, and the other in the
inducteous state, but whichsoever happened for the time to be in that
state, whether positive or negative, had the advantage.

1497. To counteract this interfering influence, I made the interval _n_ =
0.79 and interval _o_ = 0.58 of an inch. Then, when the balls A and B were
_inductric positive_, the discharge was about equal at both intervals.
When, on the other hand, the balls A and B were inductric _negative_, there
was discharge, still at both, but most at _n_, as if the small ball
_negative_ could discharge a little easier than the same ball _positive_.

1498. The small balls and terminations used in these and similar
experiments may very correctly be compared, in their action, to the same
balls and ends when electrified in free air at a much greater distance from
conductors, than they were in those cases from each other. In the first
place, the discharge, even when as a spark, is, according to my view,
determined, and, so to speak, begins at a spot on the surface of the small
ball (1374.), occurring when the intensity there has risen up to a certain
maximum degree (1370.); this determination of discharge at a particular
spot first, being easily traced from the spark into the brush, by
increasing the distance, so as, at last, even to render the time evident
which is necessary for the production of the effect (1436. 1438.). In the
next place, the large balls which I have used might be replaced by larger
balls at a still greater distance, and so, by successive degrees, may be
considered as passing into the sides of the rooms; these being under
general circumstances the inducteous bodies, whilst the small ball rendered
either positive or negative is the inductric body.

1499. But, as has long been recognised, the small ball is only a blunt end,
and, electrically speaking, a point only a small ball; so that when a point
or blunt end is throwing out its brushes into the air, it is acting exactly
as the small balls have acted in the experiments already described, and by
virtue of the same properties and relations.

1500. It may very properly be said with respect to the experiments, that
the large negative ball is as essential to the discharge as the small
positive ball, and also that the large negative ball shows as much
superiority over the large positive ball (which is inefficient in causing a
spark from its opposed small negative ball) as the small positive ball does
over the small negative ball; and probably when we understand the real
cause of the difference, and refer it rather to the condition of the
particles of the dielectric than to the sizes of the conducting balls, we
may find much importance in such an observation. But for the present, and
whilst engaged in investigating the point, we may admit, what is the fact,
that the forces are of higher intensity at the surfaces of the smaller
balls than at those of the larger (1372. 1374.); that the former,
therefore, determine the discharge, by first rising up to that exalted
condition which is necessary for it; and that, whether brought to this
condition by induction towards the walls of a room or the large balls I
have used, these may fairly be compared one with the other in their
influence and actions.

1501. The conclusions I arrive at are: first, that when two equal small
conducting surfaces equally placed in air are electrified, one positively
and the other negatively, that which is negative can discharge to the air
at a tension a little lower than that required for the positive ball:
second, that when discharge does take place, much more passes at each time
from the positive than from the negative surface (1491.). The last
conclusion is very abundantly proved by the optical analysis of the
positive and negative brushes already described (1468.), the latter set of
discharges being found to recur five or six times oftener than the
former[A].

  [A] A very excellent mode of examining the relation of small positive
  and negative surfaces would be by the use of drops of gum water,
  solutions, or other liquids. See onwards (1581. 1593.).

1502. If, now, a small ball be made to give brushes or brushy sparks by a
powerful machine, we can, in some measure, understand and relate the
difference perceived when it is rendered positive or negative. It is known
to give when positive a much larger and more powerful spark than when
negative, and with greater facility (1482.): in fact, the spark, although
it takes away so much more electricity at once, commences at a tension
higher only in a small degree, if at all. On the other hand, if rendered
negative, though discharge may commence at a lower degree, it continues but
for a very short period, very little electricity passing away each time.
These circumstances are directly related; for the extent to which the
positive spark can reach, and the size and extent of the positive brush,
are consequences of the capability which exists of much electricity passing
off at one discharge from the positive surface (1468. 1501.).

1503. But to refer these effects only to the form and size of the
conductor, would, according to my notion of induction, be a very imperfect
mode of viewing the whole question (1523. 1600.). I apprehend that the
effects are due altogether to the mode in which the particles of the
interposed dielectric polarize, and I have already given some experimental
indications of the differences presented by different electrics in this
respect (1475. 1476.). The modes of polarization, as I shall have occasion
hereafter to show, may be very diverse in different dielectrics. With
respect to common air, what seems to be the consequence of a superiority in
the positive force at the surface of the small ball, may be due to the more
exalted condition of the negative polarity of the particles of air, or of
the nitrogen in it (the negative part being, perhaps, more compressed,
whilst the positive part is more diffuse, or _vice versa_ (1687. &c.)); for
such a condition could determine certain effects at the positive ball which
would not take place to the same degree at the negative ball, just as well
as if the positive ball had possessed some special and independent power of
its own.

1504. The opinion, that the effects are more likely to be dependent upon
the dielectric than the ball, is supported by the character of the two
discharges. If a small positive ball be throwing off brushes with
ramifications ten inches long, how can the ball affect that part of a
ramification which is five inches from it? Yet the portion beyond that
place has the same character as that preceding it, and no doubt has that
character impressed by the same general principle and law. Looking upon the
action of the contiguous particles of a dielectric as fully proved, I see,
in such a ramification, a propagation of discharge from particle to
particle, each doing for the one next it what was done for it by the
preceding particle, and what was done for the first particle by the charged
metal against which it was situated.

1505. With respect to the general condition and relations of the positive
and negative brushes in dense or rare air, or in other media and gases, if
they are produced at different times and places they are of course
independent of each other. But when they are produced from opposed ends or
balls at the same time, in the same vessel of gas (1470. 1477.), they are
frequently related; and circumstances may be so arranged that they shall be
isochronous, occurring in equal numbers in equal times; or shall occur in
multiples, i.e. with two or three negatives to one positive; or shall
alternate, or be quite irregular. All these variations I have witnessed;
and when it is considered that the air in the vessel, and also the glass of
the vessel, can take a momentary charge, it is easy to comprehend their
general nature and cause.

       *       *       *       *       *

1506. Similar experiments to those in air (1485. 1493.) were made in
different gases, the results of which I will describe as briefly as
possible. The apparatus is represented fig. 131, consisting of a bell-glass
eleven inches in diameter at the widest part, and ten and a half inches
high up to the bottom of the neck. The balls are lettered, as in fig. 130,
and are in the same relation to each other; but A and B were on separate
sliding wires, which, however, were generally joined by a cross wire, _w_,
above, and that connected with the brass conductor, which received its
positive or negative charge from the machine. The rods of A and B were
graduated at the part moving through the stuffing-box, so that the
application of a diagonal scale applied there, told what was the distance
between these balls and those beneath them. As to the position of the balls
in the jar, and their relation to each other, C and D were three and a
quarter inches apart, their height above the pump plate five inches, and
the distance between any of the balls and the glass of the jar one inch and
three quarters at least, and generally more. The balls A and D were two
inches in diameter, as before (1493.); the balls B and C only 0.15 of an
inch in diameter.

Another apparatus was occasionally used in connection with that just
described, being an open discharger (fig. 132.), by which a comparison of
the discharge in air and that in gases could be obtained. The balls E and
F, each 0.6 of an inch in diameter, were connected with sliding rods and
other balls, and were insulated. When used for comparison, the brass
conductor was associated at the same time with the balls A and B of figure
131 and ball E of this apparatus (fig. 132.); whilst the balls C, D and F
were connected with the discharging train.

1507. I will first tabulate the results as to the _restraining power_ of
the gases over discharge. The balls A and C (fig. 131.) were thrown out of
action by distance, and the effects at B and D, or the interval _n_ in the
gas, compared with those at the interval _p_ in the air, between E and F
(fig. 132.). The Table sufficiently explains itself. It will be understood
that all discharge was in the air, when the interval there was less than
that expressed in the first or third columns of figures; and all the
discharge in the gas, when the interval in air was greater than that in the
second or fourth column of figures. At intermediate distances the discharge
was occasionally at both places, i.e. sometimes in the air, sometimes in
the gas.

 _____________________________________________________________________
|                 |                                                   |
|                 |      Interval _p_ in parts of an inch             |
|_________________|___________________________________________________|
|                 |                         |                         |
|                 | When the small ball B   | When the small ball B   |
| Constant inter- |   was inductric and     |   was inductric and     |
| val _n_ between |    _positive_ the       |    _negative_ the       |
| B and D = 1     |   discharge was all     |   discharge was all     |
| inch            | at _p_ in    at _n_ in  | at _p_ in    at _n_ in  |
|                 | air before    the gas   | air before    the gas   |
|                 |                after    |                after    |
|_________________|_________________________|_________________________|
|                 |    _p_ =   |    _p_ =   |    _p_ =   |    _p_ =   |
|In Air           |    0.10    |    0.50    |    0.28    |    0.33    |
|In Nitrogen      |    0.30    |    0.65    |    0.31    |    0.40    |
|In Oxygen        |    0.33    |    0.52    |    0.27    |    0.30    |
|In Hydrogen      |    0.20    |    0.10    |    0.22    |    0.24    |
|In Coal Gas      |    0.20    |    0.90    |    0.20    |    0.27    |
|In Carbonic Acid |    0.61    |    1.30    |    0.30    |    0.15    |
|_________________|____________|____________|____________|____________|

1508. These results are the same generally, as far as they go, as those of
the like nature in the last series (1388.), and confirm the conclusion that
different gases restrain discharge in very different proportions. They are
probably not so good as the former ones, for the glass jar not being
varnished, acted irregularly, sometimes taking a certain degree of charge
as a non-conductor, and at other times acting as a conductor in the
conveyance and derangement of that charge. Another cause of difference in
the ratios is, no doubt, the relative sizes of the discharge balls in air;
in the former case they were of very different size, here they were alike.

1509. In future experiments intended to have the character of accuracy, the
influence of these circumstances ought to be ascertained, and, above all
things, the gases themselves ought to be contained in vessels of metal, and
not of glass.

       *       *       *       *       *

1510. The next set of results are those obtained when the intervals _n_ and
_o_ (fig. 131.) were made equal to each other, and relate to the greater
facility of discharge at the small ball, when rendered positive or negative
(1493.).

1511. In _air_, with the intervals = 0.4 of an inch, A and B being
inductric and positive, discharge was nearly equal at _n_ and _o_; when A
and B were inductric and negative, the discharge was mostly at _n_ by
negative brush. When the intervals were = 0.8 of an inch, with A and B
inductric positively, all discharge was at _n_ by positive brush; with A
and B inductric negatively, all the discharge was at _n_ by a negative
brush. It is doubtful, therefore, from these results, whether the negative
ball has any greater facility than the positive.

1512. _Nitrogen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric
positive, discharge at both intervals, most at _n_, by positive sparks; A,
B inductric negative, discharge equal at _n_ and _o_. The intervals made =
0.8 of an inch: A, B inductric positive, discharge all at _n_ by positive
brush; A, B inductric negative, discharge most at _o_ by positive brush. In
this gas, therefore, though the difference is not decisive, it would seem
that the positive small ball caused the most ready discharge.

1513. _Oxygen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric
positive, discharge nearly equal; inductric negative, discharge mostly at
_n_ by negative brush. Made the intervals = 0.8 of an inch: A, B inductric
positive, discharge both at _n_ and _o_; inductric negative, discharge all
at _o_ by negative brush. So here the negative small ball seems to give the
most ready discharge.

1514. _Hydrogen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric
positive, discharge nearly equal: inductric negative, discharge mostly at
_o_. Intervals = 0.8 of an inch: A and B inductric positive, discharge
mostly at _n_, as positive brush; inductric negative, discharge mostly at
_o_, as positive brush. Here the positive discharge seems most facile.

1515. _Coal gas._--_n_ and _o_ = 0.4 of an inch: A, B inductric positive,
discharge nearly all at _o_ by negative spark: A, B inductric negative,
discharge nearly all at _n_ by negative spark. Intervals = 0.8 of an inch,
and A, B inductric positive, discharge mostly at _o_ by negative brush: A,
B inductric negative, discharge all at _n_ by negative brush. Here the
negative discharge most facile.

1516. _Carbonic acid gas._--_n_ and _o_ = 0.1 of an inch: A, B inductric
positive, discharge nearly all at _o_, or negative: A, B inductric
negative, discharge nearly all at _n_, or negative. Intervals = 0.8 of an
inch: A, B inductric positive, discharge mostly at _o_, or negative. A, B
inductric negative, discharge all at _n_, or negative. In this case the
negative had a decided advantage in facility of discharge.

1517. Thus, if we may trust this form of experiment, the negative small
ball has a decided advantage in facilitating disruptive discharge over the
positive small ball in some gases, as in carbonic acid gas and coal gas
(1399.), whilst in others that conclusion seems more doubtful; and in
others, again, there seems a probability that the positive small ball may
be superior. All these results were obtained at very nearly the same
pressure of the atmosphere.

       *       *       *       *       *

1518. I made some experiments in these gases whilst in the air jar (fig.
131.), as to the change from spark to brush, analogous to those in the open
air already described (1486. 1487.). I will give, in a Table, the results
as to when brush began to appear mingled with the spark; but the after
results were so varied, and the nature of the discharge in different gases
so different, that to insert the results obtained without further
investigation, would be of little use. At intervals less than those
expressed the discharge was always by spark.

 _______________________________________________________________________
|               |                           |                           |
|               |     Discharge between     |    Discharge between      |
|               |       balls B and D.      |      balls A and C.       |
|               |___________________________|___________________________|
|               |             |             |             |             |
|               | Small ball  | Small ball  | Large ball  | Large ball  |
|               | B inductric | B inductric | A inductric | A inductric |
|               |   _pos_.    |   _neg_.    |   _pos_.    |   _neg_.    |
|_______________|_____________|_____________|_____________|_____________|
|               |             |             |             |             |
| Air           |     0.55    |    0.30     |    0.40     |     0.75    |
| Nitrogen      |     0.30    |    0.40     |    0.52     |     0.41    |
| Oxygen        |     0.70    |    0.30     |    0.45     |     0.82    |
| Hydrogen      |     0.20    |    0.10     |             |             |
| Coal gas      |     0.13    |    0.30     |    0.30     |     0.44    |
| Carbonic acid |     0.82    |    0.43     |    1.60     | {above 1.80;|
|               |             |             |             |   had not   |
|               |             |             |             |   space.)   |
|_______________|_____________|_____________|_____________|_____________|

1519. It is to be understood that sparks occurred at much higher intervals
than these; the table only expresses that distance beneath which all
discharge was as spark. Some curious relations of the different gases to
discharge are already discernible, but it would be useless to consider them
until illustrated by further experiments.

       *       *       *       *       *

1520. I ought not to omit noticing here, that Professor Belli of Milan has
published a very valuable set of experiments on the relative dissipation of
positive and negative electricity in the air[A]; he finds the latter far
more ready, in this respect, than the former.

  [A] Bibliothèque Universelle, 1836, September, p. 152.

1521. I made some experiments of a similar kind, but with sustained high
charges; the results were less striking than those of Signore Belli, and I
did not consider them as satisfactory. I may be allowed to mention, in
connexion with the subject, an interfering effect which embarrassed me for
a long time. When I threw positive electricity from a given point into the
air, a certain intensity was indicated by an electrometer on the conductor
connected with the point, but as the operation continued this intensity
rose several degrees; then making the conductor negative with the same
point attached to it, and all other things remaining the same, a certain
degree of tension was observed in the first instance, which also gradually
rose as the operation proceeded. Returning the conductor to the positive
state, the tension was at first low, but rose as before; and so also when
again made negative.

1522. This result appeared to indicate that the point which had been giving
off one electricity, was, by that, more fitted for a short time to give off
the other. But on closer examination I found the whole depended upon the
inductive reaction of that air, which being charged by the point, and
gradually increasing in quantity before it, as the positive or negative
issue was continued, diverted and removed a part of the inductive action of
the surrounding wall, and thus apparently affected the powers of the point,
whilst really it was the dielectric itself that was causing the change of
tension.

       *       *       *       *       *

1523. The results connected with the different conditions of positive and
negative discharge will have a far greater influence on the philosophy of
electrical science than we at present imagine, especially if, as I believe,
they depend on the peculiarity and degree of polarized condition which the
molecules of the dielectrics concerned acquire (1503. 1600.). Thus, for
instance, the relation of our atmosphere and the earth within it, to the
occurrence of spark or brush, must be especial and not accidental (1464.).
It would not else consist with other meteorological phenomena, also of
course dependent on the special properties of the air, and which being
themselves in harmony the most perfect with the functions of animal and
vegetable life, are yet restricted in their actions, not by loose
regulations, but by laws the most precise.

1524. Even in the passage through air of the voltaic current we see the
peculiarities of positive and negative discharge at the two charcoal
points; and if these discharges are made to take place simultaneously to
mercury, the distinction is still more remarkable, both as to the sound and
the quantity of vapour produced.

1525. It seems very possible that the remarkable difference recently
observed and described by my friend Professor Daniell[A], namely, that when
a zinc and a copper ball, the same in size, were placed respectively in
copper and zinc spheres, also the same in size, and excited by electrolytes
or dielectrics of the same strength and nature, the zinc ball far surpassed
the zinc sphere in action, may also be connected with these phenomena; for
it is not difficult to conceive how the polarity of the particles shall be
affected by the circumstance of the positive surface, namely the zinc,
being the larger or the smaller of the two inclosing the electrolyte. It is
even possible, that with different electrolytes or dielectrics the ratio
may be considerably varied, or in some cases even inverted.

  [A] Philosophical Transactions, 1838, p. 47.

       *       *       *       *       *

_Glow discharge._

1526. That form of disruptive discharge which appears as a _glow_ (1359.
1405.), is very peculiar and beautiful: it seems to depend on a quick and
almost continuous charging of the air close to, and in contact with, the
conductor.

1527. _Diminution of the charging surface_ will produce it. Thus, when a
rod 0.3 of an inch in diameter, with a rounded termination, was rendered
positive in free air, it gave fine brushes from the extremity, but
occasionally these disappeared, and a quiet phosphorescent continuous glow
took their place, covering the whole of the end of the wire, and extending
a very small distance from the metal into the air. With a rod 0.2 of an
inch in diameter the glow was more readily produced. With still smaller
rods, and also with blunt conical points, it occurred still more readily;
and with a fine point I could not obtain the brush in free air, but only
this glow. The positive glow and the positive star are, in fact, the same.

1528. _Increase of power in the machine_ tends to produce the glow; for
rounded terminations which will give only brushes when the machine is in
weak action, will readily give the glow when it is in good order.

1529. _Rarefaction of the air_ wonderfully favours the glow phenomena. A
brass ball, two and a half inches in diameter, being made positively
inductric in an air-pump receiver, became covered with glow over an area of
two inches in diameter, when the pressure was reduced to 4.4 inches of
mercury. By a little adjustment the ball could be covered all over with
this light. Using a brass ball 1.25 inches in diameter, and making it
inducteously positive by an inductric negative point, the phenomena, at
high degrees of rarefaction, were exceedingly beautiful. The glow came over
the positive ball, and gradually increased in brightness, until it was at
last very luminous; and it also stood up like a low flame, half an inch or
more in height. On touching the sides of the glass jar this lambent flame
was affected, assumed a ring form, like a crown on the top of the ball,
appeared flexible, and revolved with a comparatively slow motion, i.e.
about four or five times in a second. This ring-shape and revolution are
beautifully connected with the mechanical currents (1576.) taking place
within the receiver. These glows in rarefied air are often highly exalted
in beauty by a spark discharge at the conductor (1551. _Note_.).

1530. To obtain a _negative glow_ in air at common pressures is difficult.
I did not procure it on the rod 0.3 of an inch in diameter by my machine,
nor on much smaller rods; and it is questionable as yet, whether, even on
fine points, what is called the negative star is a very reduced and minute,
but still intermitting brush, or a glow similar to that obtained on a
positive point.

1531. In rarefied air the negative glow can easily be obtained. If the
rounded ends of two metal rods, about O.2 of an inch in diameter, are
introduced into a globe or jar (the air within being rarefied), and being
opposite to each other, are about four inches apart, the glow can be
obtained on both rods, covering not only the ends, but an inch or two of
the part behind. On using _balls_ in the air-pump jar, and adjusting the
distance and exhaustion, the negative ball could be covered with glow,
whether it were the inductric or the inducteous surface.

1532. When rods are used it is necessary to be aware that, if placed
concentrically in the jar or globe, the light on one rod is often reflected
by the sides of the vessel on to the other rod, and makes it apparently
luminous, when really it is not so. This effect may be detected by shifting
the eye at the time of observation, or avoided by using blackened rods.

1533. It is curious to observe the relation _of glow, brush_, and _spark_
to each other, as produced by positive or negative surfaces; thus,
beginning with spark discharge, it passes into brush much sooner when the
surface at which the discharge commences (1484.) is negative, than it does
when positive; but proceeding onwards in the order of change, we find that
the positive brush passes into _glow_ long before the negative brush does.
So that, though each presents the three conditions in the same general
order, the series are not precisely the same. It is probable, that, when
these points are minutely examined, as they must be shortly, we shall find
that each different gas or dielectric presents its own peculiar results,
dependent upon the mode in which its particles assume polar electric
condition.

1534. The glow occurs in all gases in which I have looked for it. These are
air, nitrogen, oxygen, hydrogen, coal gas, carbonic acid, muriatic acid,
sulphurous acid and ammonia. I thought also that I obtained it in oil of
turpentine, but if so it was very dull and small.

1535. The glow is always accompanied by a wind proceeding either directly
out from the glowing part, or directly towards it; the former being the
most general case. This takes place even when the glow occurs upon a ball
of considerable size: and if matters be so arranged that the ready and
regular access of air to a part exhibiting the glow be interfered with or
prevented, the glow then disappears.

1536. I have never been able to analyse or separate the glow into visible
elementary intermitting discharges (1427. 1433.), nor to obtain the other
evidence of intermitting action, namely an audible sound (1431.). The want
of success, as respects trials made by ocular means, may depend upon the
large size of the glow preventing the separation of the visible images:
and, indeed, if it does intermit, it is not likely that all parts intermit
at once with a simultaneous regularity.

1537. All the effects tend to show, that _glow_ is due to a continuous
charge or discharge of air; in the former case being accompanied by a
current from, and in the latter by one to, the place of the glow. As the
surrounding air comes up to the charged conductor, on attaining that spot
at which the tension of the particles is raised to the sufficient degree
(1370. 1410.), it becomes charged, and then moves off, by the joint action
of the forces to which it is subject; and, at the same time that it makes
way for other particles to come and be charged in turn, actually helps to
form that current by which they are brought into the necessary position.
Thus, through the regularity of the forces, a constant and quiet result is
produced; and that result is, the charging of successive portions of air,
the production of a current, and of a continuous glow.

1538. I have frequently been able to make the termination of a rod, which,
when left to itself, would produce a brush, produce in preference a glow,
simply by aiding the formation of a current of air at its extremity; and,
on the other hand, it is not at all difficult to convert the glow into
brushes, by affecting the current of air (1574. 1579.) or the inductive
action near it.

1539. The transition from glow, on the one hand, to brush and spark, on the
other, and, therefore, their connexion, may be established in various ways.
Those circumstances which tend to facilitate the charge of the air by the
excited conductor, and also those which tend to keep the tension at the
same degree notwithstanding the discharge, assist in producing the glow;
whereas those which tend to resist the charge of the air or other
dielectric, and those which favour the accumulation of electric force prior
to discharge, which, sinking by that act, has to be exalted before the
tension can again acquire the requisite degree, favour intermitting
discharge, and, therefore, the production of brush or spark. Thus,
rarefaction of the air, the removal of large conducting surfaces from the
neighbourhood of the glowing termination, the presentation of a sharp point
towards it, help to sustain or produce the glow: but the condensation of
the air, the presentation of the hand or other large surface, the gradual
approximation of a discharging ball, tend to convert the glow into brush or
even spark. All these circumstances may be traced and reduced, in a manner
easily comprehensible, to their relative power of assisting to produce,
either a _continuous_ discharge to the air, which gives the glow; or an
_interrupted_ one, which produces the brush, and, in a more exalted
condition, the spark.

1540. The rounded end of a brass rod, 0.3 of an inch in diameter, was
covered with a positive glow by the working of an electrical machine: on
stopping the machine, so that the charge of the connected conductor should
fall, the glow changed for a moment into brushes just before the discharge
ceased altogether, illustrating the necessity for a certain high continuous
charge, for a certain sized termination. Working the machine so that the
intensity should be just low enough to give continual brushes from the end
in free air, the approach of a fine point changed these brushes into a
glow. Working the machine so that the termination presented a continual
glow in free air, the gradual approach of the hand caused the glow to
contract at the very end of the wire, then to throw out a luminous point,
which, becoming a foot stalk (1426.), finally produced brushes with large
ramifications. All these results are in accordance with what is stated
above (1539.).

1541. Greasing the end of a rounded wire will immediately make it produce
brushes instead of glow. A ball having a blunt point which can be made to
project more or less beyond its surface, at pleasure, can be made to
produce every gradation from glow, through brush, to spark.

1542. It is also very interesting and instructive to trace the transition
from spark to glow, through the intermediate condition of stream, between
ends in a vessel containing air more or less rarefied; but I fear to be
prolix.

1543. All the effects show, that the glow is in its nature exactly the same
as the luminous part of a brush or ramification, namely a charging of air;
the only difference being, that the glow has a continuous appearance from
the constant renewal of the same action in the same place, whereas the
ramification is due to a momentary, independent and intermitting action of
the same kind.

       *       *       *       *       *

_Dark discharge._

1544. I will now notice a very remarkable circumstance in the luminous
discharge accompanied by negative glow, which may, perhaps, be correctly
traced hereafter into discharges of much higher intensity. Two brass rods,
0.3 of an inch in diameter, entering a glass globe on opposite sides, had
their ends brought into contact, and the air about them very much rarefied.
A discharge of electricity from the machine was then made through them, and
whilst that was continued the ends were separated from each other. At the
moment of separation a continuous glow came over the end of the negative
rod, the positive termination remaining quite dark. As the distance was
increased, a purple stream or haze appeared on the end of the positive rod,
and proceeded directly outwards towards the negative rod; elongating as the
interval was enlarged, but never joining the negative glow, there being
always a short dark space between. This space, of about 1/16th or 1/20th of
an inch, was apparently invariable in its extent and its position, relative
to the negative rod; nor did the negative glow vary. Whether the negative
end were inductric or inducteous, the same effect was produced. It was
strange to see the positive purple haze diminish or lengthen as the ends
were separated, and yet this dark space and the negative glow remain
unaltered (fig. 133).

1545. Two balls were then used in a large air-pump receiver, and the air
rarefied. The usual transitions in the character of the discharge took
place; but whenever the luminous stream, which appears after the spark and
the brush have ceased, was itself changed into glow at the balls, the dark
space occurred, and that whether the one or the other ball was made
inductric, or positive, or negative.

1546. Sometimes when the negative ball was large, the machine in powerful
action, and the rarefaction high, the ball would be covered over half its
surface with glow, and then, upon a hasty observation, would seem to
exhibit no dark space: but this was a deception, arising from the
overlapping of the convex termination of the negative glow and the concave
termination of the positive stream. More careful observation and experiment
have convinced me, that when the negative glow occurs, it never visibly
touches the luminous part of the positive discharge, but that the dark
space is always there.

1547. This singular separation of the positive and negative discharge, as
far as concerns their luminous character, under circumstances which one
would have thought very favourable to their coalescence, is probably
connected with their differences when in the form of brush, and is perhaps
even dependent on the same cause. Further, there is every likelihood that
the dark parts which occur in feeble sparks are also connected with these
phenomena[A]. To understand them would be very important, for it is quite
clear that in many of the experiments, indeed in all that I have quoted,
discharge is taking place across the dark part of the dielectric to an
extent quite equal to what occurs in the luminous part. This difference in
the result would seem to imply a distinction in the modes by which the two
electric forces are brought into equilibrium in the respective parts; and
looking upon all the phenomena as giving additional proofs, that it is to
the condition of the particles of the dielectric we must refer for the
principles of induction and discharge, so it would be of great importance
if we could know accurately in what the difference of action in the dark
and the luminous parts consisted.

  [A] See Professor Johnson's experiments. Silliman's Journal, xxv. p. 57.

1548. The dark discharge through air (1552.), which in the case mentioned
is very evident (1544.), leads to the inquiry, whether the particles of air
are generally capable of effecting discharge from one to another without
becoming luminous; and the inquiry is important, because it is connected
with that degree of tension which is necessary to originate discharge
(1368. 1370.). Discharge between _air and conductors_ without luminous
appearances are very common; and non-luminous discharges by carrying
currents of air and other fluids (1562. 1595.) are also common enough: but
these are not cases in point, for they are not discharges between
insulating particles.

1549. An arrangement was made for discharge between two balls (1485.) (fig.
129.) but, in place of connecting the inducteous ball directly with the
discharging train, it was put in communication with the inside coating of a
Leyden jar, and the discharging train with the outside coating. Then
working the machine, it was found that whenever sonorous and luminous
discharge occurred at the balls A B, the jar became charged; but that when
these did not occur, the jar acquired no charge: and such was the case when
small rounded terminations were used in place of the balls, and also in
whatever manner they were arranged. Under these circumstances, therefore,
discharge even between the air and conductors was always luminous.

1550. But in other cases, the phenomena are such as to make it almost
certain, that dark discharge can take place across air. If the rounded end
of a metal rod, 0.15 of an inch in diameter, be made to give a good
negative brush, the approach of a smaller end or a blunt point opposite to
it will, at a certain distance, cause a diminution of the brush, and a glow
will appear on the positive inducteous wire, accompanied by a current of
air passing from it. Now, as the air is being charged both at the positive
and negative surfaces, it seems a reasonable conclusion, that the charged
portions meet somewhere in the interval, and there discharge to each other,
without producing any luminous phenomena. It is possible, however, that the
air electrified positively at the glowing end may travel on towards the
negative surface, and actually form that atmosphere into which the visible
negative brushes dart, in which case dark discharge need not, of necessity,
occur. But I incline to the former opinion, and think, that the diminution
in size of the negative brush, as the positive glow comes on to the end of
the opposed wire, is in favour of that view.

1551. Using rarefied air as the dielectric, it is very easy to obtain
luminous phenomena as brushes, or glow, upon both conducting balls or
terminations, whilst the interval is dark, and that, when the action is so
momentary that I think we cannot consider currents as effecting discharge
across the dark part. Thus if two balls, about an inch in diameter, and 4
or more inches apart, have the air rarefied about them, and are then
interposed in the course of discharge, an interrupted or spark current
being produced at the machine[A], each termination may be made to show
luminous phenomena, whilst more or less of the interval is quite dark. The
discharge will pass as suddenly as a retarded spark (295. 334.), i.e. in an
interval of time almost inappreciably small, and in such a case, I think it
must have passed across the dark part as true disruptive discharge, and not
by convection.

  [A] By spark current I mean one passing in a series of spark between
  the conductor of the machine and the apparatus: by a continuous
  current one that passes through metallic conductors, and in that
  respect without interruption at the same place.

1552. Hence I conclude that dark disruptive discharge may occur (1547.
1550.); and also, that, in the luminous brush, the visible ramifications
may not show the full extent of the disruptive discharge (1444. 1452.), but
that each may have a dark outside, enveloping, as it were, every part
through which the discharge extends. It is probable, even, that there are
such things as dark discharges analogous in form to the brush and the
spark, but not luminous in any part (1445.).

1553. The occurrence of dark discharge in any case shows at how low a
tension disruptive discharge may occur (1548,), and indicates that the
light of the ultimate brush or spark is in no relation to the intensity
required (1368. 1370.). So to speak, the discharge begins in darkness, and
the light is a mere consequence of the quantity which, after discharge has
commenced, flows to that spot and there finds its most facile passage
(1418. 1435.). As an illustration of the growth generally of discharge, I
may remark that, in the experiments on the transition in oxygen of the
discharge from spark to brush (1518.), every spark was immediately preceded
by a short brush.

1554. The phenomena relative to dark discharge in other gases, though
differing in certain characters from those in air, confirm the conclusions
drawn above. The two rounded terminations (1544.) (fig. 133.), were placed
in _muriatic acid gas_ (1445. 1463.) at the pressure of 6.5 inches of
mercury, and a continuous machine current of electricity sent through the
apparatus: bright sparks occurred until the interval was about or above an
inch, when they were replaced by squat brushy intermitting glows upon both
terminations, with a dark part between. When the current at the machine was
in spark, then each spark caused a discharge across the muriatic acid gas,
which, with a certain interval, was bright; with a larger interval, was
straight across and flamy, like a very exhausted and sudden, but not a
dense sharp spark; and with a still larger interval, produced a feeble
brush on the inductric positive end, and a glow on the inducteous negative
end, the dark part being between (1544.); and at such times, the spark at
the conductor, instead of being sudden and sonorous, was dull and quiet
(334.).

1555. On introducing more muriatic acid gas, until the pressure was 29.97
inches, the same terminations gave bright sparks within at small distances;
but when they were about an inch or more apart, the discharge was generally
with very small brushes and glow, and frequently with no light at all,
though electricity had passed through the gas. Whenever the bright spark
did pass through the muriatic acid gas at this pressure, it was bright
throughout, presenting no dark or dull space.

1556. In _coal gas_, at common pressures, when the distance was about an
inch, the discharge was accompanied by short brushes on the ends, and a
dark interval of half an inch or more between them, notwithstanding the
discharge had the sharp quick sound of a dull spark, and could not have
depended in the dark part on _convection_ (1562.).

1557. This gas presents several curious points in relation to the bright
and dark parts of spark discharge. When bright sparks passed between the
rod ends 0.3 of an inch in diameter (1544.), very sudden dark parts would
occur next to the brightest portions of the spark. Again with these ends
and also with balls (1422.), the bright sparks would be sometimes red,
sometimes green, and occasionally green and red in different parts of the
same spark. Again, in the experiments described (1518.), at certain
intervals a very peculiar pale, dull, yet sudden discharge would pass,
which, though apparently weak, was very direct in its course, and
accompanied by a sharp snapping noise, as if quick in its occurrence.

1558. _Hydrogen_ frequently gave peculiar sparks, one part being bright
red, whilst the other was a dull pale gray, or else the whole spark was
dull and peculiar.

1559. _Nitrogen_ presents a very remarkable discharge, between two balls of
the respective diameters of 0.15 and 2 inches (1506. 1518.), the smaller
one being rendered negative either directly inducteously. The peculiar
discharge occurs at intervals between 0.42 and 0.68, and even at 1.4 inches
when the large ball was inductric positively; it consisted of a little
brushy part on the small negative ball, then a dark space, and lastly a
dull straight line on the large positive ball (fig. 134.). The position of
the dark space was very constant, and is probably in direct relation to the
dark space described when negative glow was produced (1544.). When by any
circumstance a bright spark was determined, the contrast with the peculiar
spark described was very striking; for it always had a faint purple part,
but the place of this part was constantly near the positive ball.

1560. Thus dark discharge appears to be decidedly established. But its
establishment is accompanied by proofs that it occurs in different degrees
and modes in different gases. Hence then another specific action, added to
the many (1296. 1398. 1399. 1423. 1454. 1503.) by which the electrical
relations of insulating dielectrics are distinguished and established, and
another argument in favour of that molecular theory of induction, which is
at present under examination[A].

  [A] I cannot resist referring here by a note to Biot's philosophical
  view of the nature of the light of the electric discharge, Annales de
  Chimie, liii. p. 321.

       *       *       *       *       *

1561. What I have had to say regarding disruptive discharge has extended to
some length, but I hope will be excused in consequence of the importance of
the subject. Before concluding my remarks, I will again intimate in the
form of a query, whether we have not reason to consider the tension or
retention and after discharge in air or other insulating dielectrics, as
the same thing with retardation and discharge in a metal wire, differing
only, but almost infinitely, in degree (1334. 1336.). In other words, can
we not, by a gradual chain of association, carry up discharge from its
occurrence in air, through spermaceti and water, to solutions, and then on
to chlorides, oxides and metals, without any essential change in its
character; and, at the same time, connecting the insensible conduction of
air, through muriatic acid gas and the dark discharge, with the better
conduction of spermaceti, water, and the all but perfect conduction of the
metals, associate the phenomena at both extremes? and may it not be, that
the retardation and ignition of a wire are effects exactly correspondent in
their nature to the retention of charge and spark in air? If so, here again
the two extremes in property amongst dielectrics will be found to be in
intimate relation, the whole difference probably depending upon the mode
and degree in which their particles polarize under the influence of
inductive actions (1338. 1603. 1610.).

       *       *       *       *       *

¶ x. _Convection, or carrying discharge._

1562. The last kind of discharge which I have to consider is that effected
by the motion of charged particles from place to place. It is apparently
very different in its nature to any of the former modes of discharge
(1319.), but, as the result is the same, may be of great importance in
illustrating, not merely the nature of discharge itself, but also of what
we call the electric current. It often, as before observed, in cases of
brush and glow (1440. 1535.), joins its effect to that of disruptive
discharge, to complete the act of neutralization amongst the electric
forces.

1563. The particles which being charged, then travel, may be either of
insulating or conducting matter, large or small. The consideration in the
first place of a large particle of conducting matter may perhaps help our
conceptions.

1564. A copper boiler 3 feet in diameter was insulated and electrified, but
so feebly, that dissipation by brushes or disruptive discharge did not
occur at its edges or projecting parts in a sensible degree. A brass ball,
2 inches in diameter, suspended by a clean white silk thread, was brought
towards it, and it was found that, if the ball was held for a second or two
near any part of the charged surface of the boiler, at such distance (two
inches more or less) as not to receive any direct charge from it, it became
itself charged, although insulated the whole time; and its electricity was
the _reverse_ of that of the boiler.

1565. This effect was the strongest opposite the edges and projecting parts
of the boiler, and weaker opposite the sides, or those extended portions of
the surface which, according to Coulomb's results, have the weakest charge.
It was very strong opposite a rod projecting a little way from the boiler.
It occurred when the copper was charged negatively as well as positively:
it was produced also with small balls down to 0.2 of an inch and less in
diameter, and also with smaller charged conductors than the copper. It is,
indeed, hardly possible in some cases to carry an insulated ball within an
inch or two of a charged plane or convex surface without its receiving a
charge of the contrary kind to that of the surface.

1566. This effect is one of induction between the bodies, not of
communication. The ball, when related to the positive charged surface by
the intervening dielectric, has its opposite sides brought into contrary
states, that side towards the boiler being negative and the outer side
positive. More inductric action is directed towards it than would have
passed across the same place if the ball had not been there, for several
reasons; amongst others, because, being a conductor, the resistance of the
particles of the dielectric, which otherwise would have been there, is
removed (1298.); and also, because the reacting positive surface of the
ball being projected further out from the boiler than when there is no
introduction of conducting matter, is more free therefore to act through
the rest of the dielectric towards surrounding conductors, and so favours
the exaltation of that inductric polarity which is directed in its course.
It is, as to the exaltation of force upon its outer surface beyond that
upon the inductric surface of the boiler, as if the latter were itself
protuberant in that direction. Thus it acquires a state like, but higher
than, that of the surface of the boiler which causes it; and sufficiently
exalted to discharge at its positive surface to the air, or to affect small
particles, as it is itself affected by the boiler, and they flying to it,
take a charge and pass off; and so the ball, as a whole, is brought into
the contrary inducteous state. The consequence is, that, if free to move,
its tendency, under the influence of all the forces, to approach the boiler
is increased, whilst it at the same time becomes more and more exalted in
its condition, both of polarity and charge, until, at a certain distance,
discharge takes place, it acquires the same state as the boiler, is
repelled, and passing to that conductor most favourably circumstanced to
discharge it, there resumes its first indifferent condition.

1567. It seems to me, that the manner in which inductric bodies affect
uncharged floating or moveable conductors near them, is very frequently of
this nature, and generally so when it ends in a carrying operation (1562.
1602.). The manner in which, whilst the dominant inductric body cannot give
off its electricity to the air, the inducteous body _can_ effect the
discharge of the same kind of force, is curious, and, in the case of
elongated or irregularly shaped conductors, such as filaments or particles
of dust, the effect will often be very ready, and the consequent attraction
immediate.

1568. The effect described is also probably influential in causing those
variations in spark discharge referred to in the last series (1386. 1390.
1391.): for if a particle of dust were drawn towards the axis of induction
between the balls, it would tend, whilst at some distance from that axis,
to commence discharge at itself, in the manner described (1566.), and that
commencement might so far facilitate the act (1417. 1420.) as to make the
complete discharge, as spark, pass through the particle, though it might
not be the shortest course from ball to ball. So also, with equal balls at
equal distances, as in the experiments of comparison already described
(1493. 1506.), a particle being between one pair of balls would cause
discharge there in preference; or even if a particle were between each,
difference of size or shape would give one for the time a predominance over
the other.

1569. The power of particles of dust to carry off electricity in cases of
high tension is well known, and I have already mentioned some instances of
the kind in the use of the inductive apparatus (1201.). The general
operation is very well shown by large light objects, as the toy called the
electrical spider; or, if smaller ones are wanted for philosophical
investigation, by the smoke of a glowing green wax taper, which, presenting
a successive stream of such particles, makes their course visible.

1570. On using oil of turpentine as the dielectric, the action and course
of small conducting carrying particles in it can be well observed. A few
short pieces of thread will supply the place of carriers, and their
progressive action is exceedingly interesting.

1571. A very striking effect was produced on oil of turpentine, which,
whether it was due to the carrying power of the particles in it, or to any
other action of them, is perhaps as yet doubtful. A portion of that fluid
in a glass vessel had a large uninsulated silver dish at the bottom, and an
electrified metal rod with a round termination dipping into it at the top.
The insulation was very good, and the attraction and other phenomena
striking. The rod end, with a drop of gum water attached to it, was then
electrified in the fluid; the gum water soon spun off in fine threads, and
was quickly dissipated through the oil of turpentine. By the time that four
drops had in this way been commingled with a pint of the dielectric, the
latter had lost by far the greatest portion of its insulating power; no
sparks could be obtained in the fluid; and all the phenomena dependent upon
insulation had sunk to a low degree. The fluid was very slightly turbid.
Upon being filtered through paper only, it resumed its first clearness, and
now insulated as well as before. The water, therefore, was merely diffused
through the oil of turpentine, not combined with or dissolved in it: but
whether the minute particles acted as carriers, or whether they were not
rather gathered together in the line of highest inductive tension (1350.),
and there, being drawn into elongated forms by the electric forces,
combined their effects to produce a band of matter having considerable
conducting power, as compared with the oil of turpentine, is as yet
questionable.

1572. The analogy between the action of solid conducting carrying particles
and that of the charged particles of fluid insulating substances, acting as
dielectrics, is very evident and simple; but in the latter case the result
is, necessarily, currents in the mobile media. Particles are brought by
inductric action into a polar state; and the latter, after rising to a
certain tension (1370.), is followed by the communication of a part of the
force originally on the conductor; the particles consequently become
charged, and then, under the joint influence of the repellent and
attractive forces, are urged towards a discharging place, or to that spot
where these inductric forces are most easily compensated by the contrary
inducteous forces.

1573. Why a point should be so exceedingly favourable to the production of
currents in a fluid insulating dielectric, as air, is very evident. It is
at the extremity of the point that the intensity necessary to charge the
air is first acquired (1374.); it is from thence that the charged particle
recedes; and the mechanical force which it impresses on the air to form a
current is in every way favoured by the shape and position of the rod, of
which the point forms the termination. At the same time, the point, having
become the origin of an active mechanical force, does, by the very act of
causing that force, namely, by discharge, prevent any other part of the rod
from acquiring the same necessary condition, and so preserves and sustains
its own predominance.

1574. The very varied and beautiful phenomena produced by sheltering or
enclosing the point, illustrate the production of the current exceedingly
well, and justify the same conclusions; it being remembered that in such
cases the effect upon the discharge is of two kinds. For the current may be
interfered with by stopping the access of fresh uncharged air, or retarding
the removal of that which has been charged, as when a point is electrified
in a tube of insulating matter closed at one extremity; or the _electric
condition_ of the point itself may be altered by the relation of other
parts in its neighbourhood, also rendered electric, as when the point is in
a metal tube, by the metal itself, or when it is in the glass tube, by a
similar action of the charged parts of the glass, or even by the
surrounding air which has been charged, and which cannot escape.

1575. Whenever it is intended to observe such inductive phenomena in a
fluid dielectric as have a direct relation to, and dependence upon, the
fluidity of the medium, such, for instance, as discharge from points, or
attractions and repulsions, &c., then the mass of the fluid should be
great, and in such proportion to the distance between the inductric and
inducteous surfaces as to include all the _lines of inductive force_
(1369.) between them; otherwise, the effects of currents, attraction, &c.,
which are the resultants of all these forces, cannot be obtained. The
phenomena, which occur in the open air, or in the middle of a globe filled
with oil of turpentine, will not take place in the same media if confined
in tubes of glass, shell-lac, sulphur, or other such substances, though
they be excellent insulating dielectrics; nor can they be expected: for in
such cases, the polar forces, instead of being all dispersed amongst fluid
particles, which tend to move under their influence, are now associated in
many parts with particles that, notwithstanding their tendency to motion,
are constrained by their solidity to remain quiescent.

1576. The varied circumstances under which, with conductors differently
formed and constituted, currents can occur, all illustrate the same
simplicity of production. A _ball_, if the intensity be raised sufficiently
on its surface, and that intensity be greatest on a part consistent with
the production of a current of air up to and off from it, will produce the
effect like a point (1537); such is the case whenever the glow occurs upon
a ball, the current being essential to that phenomenon. If as large a
sphere as can well be employed with the production of glow be used, the
glow will appear at the place where the current leaves the ball, and that
will be the part directly opposite to the connection of the ball and rod
which supports it; but by increasing the tension elsewhere, so as to raise
it above the tension upon that spot, which can easily be effected
inductively, then the place of the glow and the direction of the current
will also change, and pass to that spot which for the time is most
favourable for their production (1591.).

1577. For instance, approaching the hand towards the ball will tend to
cause brush (1539.), but by increasing the supply of electricity the
condition of glow may be preserved; then on moving the hand about from side
to side the position of the glow will very evidently move with it.

1578. A point brought towards a glowing ball would at twelve or fourteen
inches distance make the glow break into brush, but when still nearer, glow
was reproduced, probably dependent upon the discharge of wind or air
passing from the point to the ball, and this glow was very obedient to the
motion of the point, following it in every direction.

1579. Even a current of wind could affect the place of the glow; for a
varnished glass tube being directed sideways towards the ball, air was
sometimes blown through it at the ball and sometimes not. In the former
case, the place of the glow was changed a little, as if it were blown away
by the current, and this is just the result which might have been
anticipated. All these effects illustrate beautifully the general causes
and relations, both of the glow and the current of air accompanying it
(1574.).

1580. Flame facilitates the production of a current in the dielectric
surrounding it. Thus, if a ball which would not occasion a current in the
air have a flame, whether large or small, formed on its surface, the
current is produced with the greatest ease; but not the least difficulty
can occur in comprehending the effective action of the flame in this case,
if its relation, as part of the surrounding dielectric, to the electrified
ball, be but for a moment considered (1375. 1380.).

1581. Conducting fluid terminations, instead of rigid points, illustrate in
a very beautiful manner the formation of the currents, with their effects
and influence in exalting the conditions under which they were commenced.
Let the rounded end of a brass rod, 0.3 of an inch or thereabouts in
diameter, point downwards in free air; let it be amalgamated, and have a
drop of mercury suspended from it; and then let it be powerfully
electrized. The mercury will present the phenomenon of _glow_; a current of
air will rush along the rod, and set off from the mercury directly
downwards; and the form of the metallic drop will be slightly affected, the
convexity at a small part near the middle and lower part becoming greater,
whilst it diminishes all round at places a little removed from this spot.
The change is from the form of _a_ (fig. 135.) to that of _b_, and is due
almost, if not entirely, to the mechanical force of the current of air
sweeping over its surface.

1582. As a comparative observation, let it be noticed, that a ball
gradually brought towards it converts the glow into brushes, and ultimately
sparks pass from the most projecting part of the mercury. A point does the
same, but at much smaller distances.

1583. Take next a drop of strong solution of muriate of lime; being
electrified, a part will probably be dissipated, but a considerable
portion, if the electricity be not too powerful, will remain, forming a
conical drop (fig. 136.), accompanied by a strong current. If glow be
produced, the drop will be smooth on the surface. If a short low brush is
formed, a minute tremulous motion of the liquid will be visible; but both
effects coincide with the principal one to be observed, namely, the regular
and successive charge of air, the formation of a wind or current, and the
form given by that current to the fluid drop, if a discharge ball be
gradually brought toward the cone, sparks will at last pass, and these will
be from the apex of the cone to the approached ball, indicating a
considerable degree of conducting power in this fluid.

1584. With a drop of water, the effects were of the same kind, and were
best obtained when a portion of gum water or of syrup hung from a ball
(fig. 137.). When the machine was worked slowly, a fine large quiet conical
drop, with concave lateral outline, and a small rounded end, was produced,
on which the glow appeared, whilst a steady wind issued, in a direction
from the point of the cone, of sufficient force to depress the surface of
uninsulated water held opposite to the termination. When the machine was
worked more rapidly some of the water was driven off; the smaller pointed
portion left was roughish on the surface, and the sound of successive brush
discharges was heard. With still more electricity, more water was
dispersed; that which remained was elongated and contracted, with an
alternating motion; a stronger brush discharge was heard, and the
vibrations of the water and the successive discharges of the individual
brushes were simultaneous. When water from beneath was brought towards the
drop, it did not indicate the same regular strong contracted current of air
as before; and when the distance was such that sparks passed, the water
beneath was _attracted_ rather than driven away, and the current of air
_ceased_.

1585. When the discharging ball was brought near the drop in its first
quiet glowing state (1582.), it converted that glow into brushes, and
caused the vibrating motion of the drop. When still nearer, sparks passed,
but they were always from the metal of the rod, over the surface of the
water, to the point, and then across the air to the ball. This is a natural
consequence of the deficient conducting power of the fluid (1584. 1585.).

1586. Why the drop vibrated, changing its form between the periods of
discharging brushes, so as to be more or less acute at particular instants,
to be most acute when the brush issued forth, and to be isochronous in its
action, and how the quiet glowing liquid drop, on assuming the conical
form, facilitated, as it were, the first action, are points, as to theory,
so evident, that I will not stop to speak of them. The principal thing to
observe at present is, the formation of the carrying current of air, and
the manner in which it exhibits its existence and influence by giving form
to the drop.

1587. That the drop, when of water, or a better conductor than water, is
formed into a cone principally by the current of air, is shown amongst
other ways (1594.) thus. A sharp point being held opposite the conical
drop, the latter soon lost its pointed form; was retraced and became round;
the current of air from it ceased, and was replaced by one from the point
beneath, which, if the latter were held near enough to the drop, actually
blew it aside, and rendered it concave in form.

1588. It is hardly necessary to say what happened with still worse
conductors than water, as oil, or oil of turpentine; the fluid itself was
then spun out into threads and carried off, not only because the air
rushing over its surface helped to sweep it away, but also because its
insulating particles assumed the same charged state as the particles of
air, and, not being able to discharge to them in a much greater decree than
the air particles themselves could do, were carried off by the same causes
which urged those in their course. A similar effect with melted sealing-wax
on a metal point forms an old and well-known experiment.

1589. A drop of gum water in the exhausted receiver of the air-pump was not
sensibly affected in its form when electrified. When air was let in, it
begun to show change of shape when the pressure was ten inches of mercury.
At the pressure of fourteen or fifteen inches the change was more sensible,
and as the air increased in density the effects increased, until they were
the same as those in the open atmosphere. The diminished effect in the rare
air I refer to the relative diminished energy of its current; that
diminution depending, in the first place, on the lower electric condition
of the electrified ball in the rarefied medium, and in the next, on the
attenuated condition of the dielectric, the cohesive force of water in
relation to rarefied air being something like that of mercury to dense air
(1581.), whilst that of water in dense air may be compared to that of
mercury in oil of turpentine (1597.).

1590. When a ball is covered with a thick conducting fluid, as treacle or
syrup, it is easy by inductive action to determine the wind from almost any
part of it (1577.); the experiment, which before was of rather difficult
performance, being rendered facile in consequence of the fluid enabling
that part, which at first was feeble in its action, to rise into an exalted
condition by assuming a pointed form.

1591. To produce the current, the electric intensity must rise and continue
at _one spot_, namely, at the origin of the current, higher than elsewhere,
and then, air having a uniform and ready access, the current is produced.
If no current be allowed (1574.), then discharge may take place by brush or
spark. But whether it be by brush or spark, or wind, it seems very probable
that the initial intensity or tension at which a particle of a given
gaseous dielectric charges, or commences discharge, is, under the
conditions before expressed, always the same (1410.).

1592. It is not supposed that all the air which enters into motion is
electrified; on the contrary, much that is not charged is carried on into
the stream. The part which is really charged may be but a small proportion
of that which is ultimately set in motion (1442.).


1593. When a drop of gum water (1584.) is made _negative_, it presents a
larger cone than when made positive; less of the fluid is thrown off, and
yet, when a ball is approached, sparks can hardly be obtained, so pointed
is the cone, and so free the discharge. A point held opposite to it did not
cause the retraction of the cone to such an extent as when it was positive.
All the effects are so different from those presented by the positive cone,
that I have no doubt such drops would present a very instructive method of
investigating the difference of positive and negative discharge in air and
other dielectrics (1480. 1501.).

1594. That I may not be misunderstood (1587.), I must observe here that I
do not consider the cones produced as the result _only_ of the current of
air or other insulating dielectric over their surface. When the drop is of
badly conducting matter, a part of the effect is due to the electrified
state of the particles, and this part constitutes almost the whole when the
matter is melted sealing-wax, oil of turpentine, and similar insulating
bodies (1588.). But even when the drop is of good conducting matter, as
water, solutions, or mercury, though the effect above spoken of will then
be insensible (1607.), still it is not the mere current of air or other
dielectric which produces all the change of form; for a part is due to
those attractive forces by which the charged drop, if free to move, would
travel along the line of strongest induction, and not being free to move,
has its form elongated until the _sum_ of the different forces tending to
produce this form is balanced by the cohesive attraction of the fluid. The
effect of the attractive forces are well shown when treacle, gum water, or
syrup is used; for the long threads which spin out, at the same time that
they form the axes of the currents of air, which may still be considered as
determined at their points, are like flexible conductors, and show by their
directions in what way the attractive forces draw them.

1595. When the phenomena of currents are observed in dense insulating
dielectrics, they present us with extraordinary degrees of mechanical
force. Thus, if a pint of well-rectified and filtered (1571.) oil of
turpentine be put into a glass vessel, and two wires be dipped into it in
different places, one leading to the electrical machine, and the other to
the discharging train, on working the machine the fluid will be thrown into
violent motion throughout its whole mass, whilst at the same time it will
rise two, three or four inches up the machine wire, and dart off jets from
it into the air.

1596. If very clean uninsulated mercury be at the bottom of the fluid, and
the wire from the machine be terminated either by a ball or a point, and
also pass through a glass tube extending both above and below the surface
of the oil of turpentine, the currents can be better observed, and will be
seen to rush down the wire, proceeding directly from it towards the
mercury, and there, diverging in all directions, will ripple its surface
strongly, and mounting up at the sides of the vessel, will return to
re-enter upon their course.

1597. A drop of mercury being suspended from an amalgamated brass ball,
preserved its form almost unchanged in air (1581.); but when immersed in
the oil of turpentine it became very pointed, and even particles of the
metal could be spun out and carried off by the currents of the dielectric.
The form of the liquid metal was just like that of the syrup in air
(1584.), the point of the cone being quite as fine, though not so long. By
bringing a sharp uninsulated point towards it, it could also be effected in
the same manner as the syrup drop in air (1587.), though not so readily,
because of the density and limited quantity of the dielectric.

1598. If the mercury at the bottom of the fluid be connected with the
electrical machine, whilst a rod is held in the hand terminating in a ball
three quarters of an inch, less or more, in diameter, and the ball be
dipped into the electrified fluid, very striking appearances ensue. When
the ball is raised again so as to be at a level nearly out of the fluid,
large portions of the latter will seem to cling to it (fig. 138.). If it be
raised higher, a column of the oil of turpentine will still connect it with
that in the basin below (fig. 139.). If the machine be excited into more
powerful action, this will become more bulky, and may then also be raised
higher, assuming the form (fig. 140); and all the time that these effects
continue, currents and counter-currents, sometimes running very close
together, may be observed in the raised column of fluid.

1599. It is very difficult to decide by sight the direction of the currents
in such experiments as these. If particles of silk are introduced they
cling about the conductors; but using drops of water and mercury the course
of the fluid dielectric seems well indicated. Thus, if a drop of water be
placed at the end of a rod (1571.) over the uninsulated mercury, it is soon
swept away in particles streaming downwards towards the mercury. If another
drop be placed on the mercury beneath the end of the rod, it is quickly
dispersed in all directions in the form of streaming particles, the
attractive forces drawing it into elongated portions, and the currents
carrying them away. If a drop of mercury be hung from a ball used to raise
a column of the fluid (1598.), then the shape of the drop seems to show
currents travelling in the fluid in the direction indicated by the arrows
(fig. 141.).

1600. A very remarkable effect is produced on these phenomena, connected
with positive and negative charge and discharge, namely, that a ball
charged positively raises a much higher and larger column of the oil of
turpentine than when charged negatively. There can be no doubt that this is
connected with the difference of positive and negative action already
spoken of (1480. 1525.), and tends much to strengthen the idea that such
difference is referable to the particles of the dielectric rather than to
the charged conductors, and is dependent upon the mode in which these
particles polarize (1503. 1523.).

1601. Whenever currents travel in insulating dielectrics they really effect
discharge; and it is important to observe, though a very natural result,
that it is indifferent which way the current or particles travel, as with
reversed direction their state is reversed. The change is easily made,
either in air or oil of turpentine, between two opposed rods, for an
insulated ball being placed in connexion with either rod and brought near
its extremity, will cause the current to set towards it from the opposite
end.

1602. The two currents often occur at once, as when both terminations
present brushes, and frequently when they exhibit the glow (1531.). In such
cases, the charged particles, or many of them, meet and mutually discharge
each other (1518. 1612.). If a smoking wax taper be held at the end of an
insulating rod towards a charged prime conductor, it will very often happen
that two currents will form, and be rendered visible by its vapour, one
passing as a fine filament of smoky particles directly to the charged
conductor, and the other passing as directly from the same taper wick
outwards, and from the conductor: the principles of inductric action and
charge, which were referred to in considering the relation of a carrier
ball and a conductor (1566.), being here also called into play.

       *       *       *       *       *

1603. The general analogy and, I think I may say, identity of action found
to exist as to insulation and conduction (1338. 1561.) when bodies, the
best and the worst in the classes of insulators or conductors, were
compared, led me to believe that the phenomena of _convection_ in badly
conducting media were not without their parallel amongst the best
conductors, such even as the metals. Upon consideration, the cones produced
by Davy[A] in fluid metals, as mercury and tin, seemed to be cases in
point, and probably also the elongation of the metallic medium through
which a current of electricity was passing, described by Ampère (1113)[B];
for it is not difficult to conceive, that the diminution of convective
effect, consequent upon the high conducting power of the metallic media
used in these experiments, might be fully compensated for by the enormous
quantity of electricity passing. In fact, it is impossible not to expect
_some_ effect, whether sensible or not, of the kind in question, when such
a current is passing through a fluid offering a sensible resistance to the
passage of the electricity, and, thereby, giving proof of a certain degree
of insulating power (1328.).

  [A] Philosophical Transactions, 1823, p. 155.

  [B] Bibliothèque Universelle, xxi, 417.

1604. I endeavoured to connect the convective currents in air, oil of
turpentine, &c. and those in metals, by intermediate cases, but found this
not easy to do. On taking bodies, for instance, which, like water, adds,
solutions, fused salts or chlorides, &c., have intermediate conducting
powers, the minute quantity of electricity which the common machine can
supply (371. 861.) is exhausted instantly, so that the cause of the
phenomenon is kept either very low in intensity, or the instant of time
during which the effect lasts is so small, that one cannot hope to observe
the result sought for. If a voltaic battery be used, these bodies are all
electrolytes, and the evolution of gas, or the production of other changes,
interferes and prevents observation of the effect required.

1605. There are, nevertheless, some experiments which illustrate the
connection. Two platina wires, forming the electrodes of a powerful voltaic
battery, were placed side by side, near each other, in distilled water,
hermetically sealed up in a strong glass tube, some minute vegetable fibres
being present in the water. When, from the evolution of gas and the
consequent increased pressure, the bubbles formed on the electrodes were so
small as to produce but feebly ascending currents, then it could be
observed that the filaments present were attracted and repelled between the
two wires, as they would have been between two oppositely charged surfaces
in air or oil of turpentine, moving so quickly as to displace and disturb
the bubbles and the currents which these tended to form. Now I think it
cannot be doubted, that under similar circumstances, and with an abundant
supply of electricity, of sufficient tension also, convective currents
might have been formed; the attractions and repulsions of the filaments
were, in fact, the elements of such currents (1572.), and therefore water,
though almost infinitely above air or oil of turpentine as a conductor, is
a medium in which similar currents can take place.

1606. I had an apparatus made (fig. 142.) in which _a_ is a plate of
shell-lac, _b_ a fine platina wire passing through it, and having only the
section of the wire exposed above; _c_ a ring of bibulous paper resting on
the shell-lac, and _d_ distilled water retained by the paper in its place,
and just sufficient in quantity to cover the end of the wire _b_; another
wire, _e_, touched a piece of tinfoil lying in the water, and was also
connected with a discharging train; in this way it was easy, by rendering
_b_ either positive or negative, to send a current of electricity by its
extremity into the fluid, and so away by the wire _e_.

1607. On connecting _b_ with the conductor of a powerful electrical
machine, not the least disturbance of the level of the fluid over the end
of the wire during the working of the machine could be observed; but at the
same time there was not the smallest indication of electrical charge about
the conductor of the machine, so complete was the discharge. I conclude
that the quantity of electricity passed in a _given time_ had been too
small, when compared with the conducting power of the fluid to produce the
desired effect.

1608. I then charged a large Leyden battery (291.), and discharged it
through the wire _b_, interposing, however, a wet thread, two feet long, to
prevent a spark in the water, and to reduce what would else have been a
sudden violent discharge into one of more moderate character, enduring for
a sensible length of time (334.). I now did obtain a very brief elevation
of the water over the end of the wire; and though a few minute bubbles of
gas were at the same time formed there, so as to prevent me from asserting
that the effect was unequivocally the same as that obtained by DAVY in the
metals, yet, according to my best judgement, it was partly, and I believe
principally, of that nature.

1609. I employed a voltaic battery of 100 pair of four-inch plates for
experiments of a similar nature with electrolytes. In these cases the
shell-lac was cupped, and the wire _b_ 0.2 of an inch in diameter.
Sometimes I used a positive amalgamated zinc wire in contact with dilute
sulphuric acid; at others, a negative copper wire in a solution of sulphate
of copper; but, because of the evolution of gas, the precipitation of
copper, &c., I was not able to obtain decided results. It is but right to
mention, that when I made use of mercury, endeavouring to repeat DAVY's
experiment, the battery of 100 pair was not sufficient to produce the
elevations[A].

  [A] In the experiments at the Royal Institution, Sir H. Davy used, I
  think, 500 or 600 pairs of plates. Those at the London Institution
  were made with the apparatus of Mr. Pepys (consisting of an enormous
  single pair of plates), described in the Philosophical Transactions
  for 1832, p. 187.

1610. The latter experiments (1609.) may therefore be considered as failing
to give the hoped-for proof, but I have much confidence in the former
(1605. 1608.), and in the considerations (1603.) connected with them. If I
have rightly viewed them, and we may be allowed to compare the currents at
points and surfaces in such extremely different bodies as air and the
metals, and admit that they are effects of the _same_ kind, differing only
in degree and in proportion to the insulating or conducting power of the
dielectric used, what great additional argument we obtain in favour of that
theory, which in the phenomena of insulation and conduction also, as in
these, would link _the same_ apparently dissimilar substances together
(1336. 1561.); and how completely the general view, which refers all the
phenomena to the direct action of the molecules of matter, seems to embrace
the various isolated phenomena as they successively come under
consideration!

       *       *       *       *       *

1611. The connection of this convective or carrying effect, which depends
upon a certain degree of insulation, with conduction; i.e. the occurrence
of both in so many of the substances referred to, as, for instance, the
metals, water, air, &c., would lead to many very curious theoretical
generalizations, which I must not indulge in here. One point, however, I
shall venture to refer to. Conduction appears to be essentially an action
of contiguous particles, and the considerations just stated, together with
others formerly expressed (1326, 1336, &c.), lead to the conclusion, that
all bodies conduct, and by the same process, air as well as metals; the
only difference being in the necessary degree of force or tension between
the particles which must exist before the act of conduction or transfer
from one particle to another can take place.

1612. The question then arises, what is this limiting condition which
separates, as it were, conduction and insulation from each other? Does it
consist in a difference between the two contiguous particles, or the
contiguous poles of these particles, in the nature and amount of positive
and negative force, no communication or discharge occurring unless that
difference rises up to a certain degree, variable for different bodies, but
always the same for the same body? Or is it true that, however small the
difference between two such particles, if _time_ be allowed, equalization
of force will take place, even with the particles of such bodies as air,
sulphur or lac? In the first case, insulating power in any particular body
would be proportionate to the degree of the assumed necessary difference of
force; in the second, to the _time_ required to equalize equal degrees of
difference in different bodies. With regard to airs, one is almost led to
expect a permanent difference of force; but in all other bodies, time seems
to be quite sufficient to ensure, ultimately, complete conduction. The
difference in the modes by which insulation may be sustained, or conduction
effected, is not a mere fanciful point, but one of great importance, as
being essentially connected with the molecular theory of induction, and the
manner in which the particles of bodies assume and retain their polarized
state.

       *       *       *       *       *

¶ xi. _Relation of a vacuum to electrical phenomena._

1613. It would seem strange, if a theory which refers all the phenomena of
insulation and conduction, i.e. all electrical phenomena, to the action of
contiguous particles, were to omit to notice the assumed possible case of a
_vacuum_. Admitting that a vacuum can be produced, it would be a very
curious matter indeed to know what its relation to electrical phenomena
would be; and as shell-lac and metal are directly opposed to each other,
whether a vacuum would be opposed to them both, and allow neither of
induction or conduction across it. Mr. Morgan[A] has said that a vacuum
does not conduct. Sir H. Davy concluded from his investigations, that as
perfect a vacuum as could be made[B] did conduct, but does not consider the
prepared spaces which he used as absolute vacua. In such experiments I
think I have observed the luminous discharge to be principally on the inner
surface of the glass; and it does not appear at all unlikely, that, if the
vacuum refused to conduct, still the surface of glass next it might carry
on that action.

  [A] Philosophical Transactions, 1785, p. 272

  [B] Ibid. 1822, p. 64.

1614. At one time, when I thought inductive force was exerted in right
lines, I hoped to illustrate this important question by making experiments
on induction with metallic mirrors (used only as conducting vessels)
exposed towards a very clear sky at night time, and of such concavity that
nothing but the firmament could be visible from the lowest part of the
concave _n_, fig. 143. Such mirrors, when electrified, as by connexion with
a Leyden jar, and examined by a carrier ball, readily gave electricity at
the lowest part of their concavity if in a room; but I was in hopes of
finding that, circumstanced as before stated, they would give little or
none at the same spot, if the atmosphere above really terminated in a
vacuum. I was disappointed in the conclusion, for I obtained as much
electricity there as before; but on discovering the action of induction in
curved lines (1231.), found a full and satisfactory explanation of the
result.

1615. My theory, as far as I have ventured it, does not pretend to decide
upon the consequences of a vacuum. It is not at present limited
sufficiently, or rendered precise enough, either by experiments relating to
spaces void of matter, or those of other kinds, to indicate what would
happen in the vacuum case. I have only as yet endeavoured to establish,
what all the facts seem to prove, that when electrical phenomena, as those
of induction, conduction, insulation and discharge occur, they depend on,
and are produced by the action of _contiguous_ particles of matter, the
next existing particle being considered as the contiguous one; and I have
further assumed, that these particles are polarized; that each exhibits the
two forces, or the force in two directions (1295. 1298.); and that they act
at a distance, only by acting on the _contiguous_ and intermediate
particles.

1616. But assuming that a perfect vacuum were to intervene in the course of
the lines of inductive action (1304.), it does not follow from this theory,
that the particles on opposite sides of such a vacuum could not act on each
other. Suppose it possible for a positively electrified particle to be in
the centre of a vacuum an inch in diameter, nothing in my present views
forbids that the particle should act at the distance of half an inch on all
the particles forming the inner superficies of the bounding sphere, and
with a force consistent with the well-known law of the squares of the
distance. But suppose the sphere of an inch were full of insulating matter,
the electrified particle would not then, according to my notion, act
directly on the distant particles, but on those in immediate association
with it, employing _all_ its power in polarizing them; producing in them
negative force equal in amount to its own positive force and directed
towards the latter, and positive force of equal amount directed outwards
and acting in the same manner upon the layer of particles next in
succession. So that ultimately, those particles in the surface of a sphere
of half an inch radius, which were acted on _directly_ when that sphere was
a vacuum, will now be acted on _indirectly_ as respects the central
particle or source of action, i.e. they will be polarized in the same way,
and with the same amount of force.


§ 19. _Nature of the electric current._


1617. The word _current_ is so expressive in common language, that when
applied in the consideration of electrical phenomena we can hardly divest
it sufficiently of its meaning, or prevent our minds from being prejudiced
by it (283. 511.). I shall use it in its common electrical sense, namely,
to express generally a certain condition and relation of electrical forces
supposed to be in progression.

1618. A current is produced both by excitement and discharge; and
whatsoever the variation of the two general causes may be, the effect
remains the same. Thus excitement may occur in many ways, as by friction,
chemical action, influence of heat, change of condition, induction, &c.;
and discharge has the forms of conduction, electrolyzation, disruptive
discharge, and convection; yet the current connected with these actions,
when it occurs, appears in all cases to be the same. This constancy in the
character of the current, notwithstanding the particular and great
variations which may be made in the mode of its occurrence, is exceedingly
striking and important; and its investigation and development promise to
supply the most open and advantageous road to a true and intimate
understanding of the nature of electrical forces.

1619. As yet the phenomena of the current have presented nothing in
opposition to the view I have taken of the nature of induction as an action
of contiguous particles. I have endeavoured to divest myself of prejudices
and to look for contradictions, but I have not perceived any in conductive,
electrolytic, convective, or disruptive discharge.

1620. Looking at the current as a _cause_, it exerts very extraordinary and
diverse powers, not only in its course and on the bodies in which it
exists, but collaterally, as in inductive or magnetic phenomena.

1621. _Electrolytic action._--One of its direct actions is the exertion of
pure chemical force, this being a result which has now been examined to a
considerable extent. The effect is found to be _constant_ and _definite_
for the quantity of electric force discharged (783. &c.); and beyond that,
the _intensity_ required is in relation to the intensity of the affinity or
forces to be overcome (904. 906. 911.). The current and its consequences
are here proportionate; the one may be employed to represent the other; no
part of the effect of either is lost or gained; so that the case is a
strict one, and yet it is the very case which most strikingly illustrates
the doctrine that induction is an action of contiguous particles (1164.
1343.).

1622. The process of electrolytic discharge appears to me to be in close
analogy, and perhaps in its nature identical with another process of
discharge, which at first seems very different from it, I mean _convection_
(1347. 1572.). In the latter case the particles may travel for yards across
a chamber; they may produce strong winds in the air, so as to move
machinery; and in fluids, as oil of turpentine, may even shake the hand,
and carry heavy metallic bodies about[A]; and yet I do not see that the
force, either in kind or action, is at all different to that by which a
particle of hydrogen leaves one particle of oxygen to go to another, or by
which a particle of oxygen travels in the contrary direction.

  [A] If a metallic vessel three or four inches deep, containing oil of
  turpentine, be insulated and electrified, and a rod with a ball (an
  inch or more in diameter) at the end have the ball immersed in the
  fluid whilst the end is held in the hand, the mechanical force
  generated when the ball is moved to and from the sides of the vessel
  will soon be evident to the experimenter.

1623. Travelling particles of the air can effect chemical changes just as
well as the contact of a fixed platina electrode, or that of a combining
electrode, or the ions of a decomposing electrolyte (453. 471.); and in the
experiment formerly described, where eight places of decomposition were
rendered active by one current (469.), and where charged particles of air
in motion were the only electrical means of connecting these parts of the
current, it seems to me that the action of the particles of the electrolyte
and of the air were essentially the same. A particle of air was rendered
positive; it travelled in a certain determinate direction, and coming to an
electrolyte, communicated its powers; an equal amount of positive force was
accordingly acquired by another particle (the hydrogen), and the latter, so
charged, travelled as the former did, and in the same direction, until it
came to another particle, and transferred its power and motion, making that
other particle active. Now, though the particle of air travelled over a
visible and occasionally a large space, whilst the particle of the
electrolyte moved over an exceedingly small one; though the air particle
might be oxygen, nitrogen, or hydrogen, receiving its charge from force of
high intensity, whilst the electrolytic particle of hydrogen had a natural
aptness to receive the positive condition with extreme facility; though the
air particle might be charged with very little electricity at a very high
intensity by one process, whilst the hydrogen particle might be charged
with much electricity at a very low intensity by another process; these are
not differences of kind, as relates to the final discharging action of
these particles, but only of degree; not essential differences which make
things unlike, but such differences as give to things, similar in their
nature, that great variety which fits them for their office in the system
of the universe.

1624. So when a particle of air, or of dust in it, electrified at a
negative point, moves on through the influence of the inductive forces
(1572.) to the next positive surface, and after discharge passes away, it
seems to me to represent exactly that particle of oxygen which, having been
rendered negative in the electrolyte, is urged by the same disposition of
inductive forces, and going to the positive platina electrode, is there
discharged, and then passes away, as the air or dust did before it.

1625. _Heat_ is another direct effect of the _current_ upon substances in
which it occurs, and it becomes a very important question, as to the
relation of the electric and heating forces, whether the latter is always
definite in amount[A]. There are many cases, even amongst bodies which
conduct without change, that at present are irreconcileable with the
assumption that it is[B]; but there are also many which indicate that, when
proper limitations are applied, the heat produced is definite. Harris has
shown this for a given length of current in a metallic wire, using common
electricity[C]; and De la Rive has proved the same point for voltaic
electricity by his beautiful application of Breguet's thermometer[D].

  [A] See De la Rive's Researches, Bib. Universelle, 1829, xl. p. 40.

  [B] Amongst others, Davy, Philosophical Transactions, 1821, p. 438.
  Pelletier's important results, Annales de Chimie, 1834, lvi. p. 371.
  and Becquerel's non-heating current, Bib. Universelle, 1835, lx. 218.

  [C] Philosophical Transactions, 1824, pp. 225. 228.

  [D] Annales de Chimie, 1836, lxii. 177.

1626. When the production of heat is observed in electrolytes under
decomposition, the results are still more complicated. But important steps
have been taken in the investigation of this branch of the subject by De la
Rive[A] and others; and it is more than probable that, when the right
limitations are applied, constant and definite results will here also be
obtained.

  [A] Bib. Universelle, 1829, xl. 49; and Ritchie, Phil. Trans. 1832. p.
  296.

       *       *       *       *       *

1627. It is a most important part of the character of the current, and
essentially connected with its very nature, that it is always the same. The
two forces are everywhere in it. There is never one current of force or one
fluid only. Any one part of the current may, as respects the presence of
the two forces there, be considered as precisely the same with any other
part; and the numerous experiments which imply their possible separation,
as well as the theoretical expressions which, being used daily, assume it,
are, I think, in contradiction with facts (511, &c.). It appears to me to
be as impossible to assume a current of positive or a current of negative
force alone, or of the two at once with any predominance of one over the
other, as it is to give an absolute charge to matter (516. 1169. 1177.).

1628. The establishment of this truth, if, as I think, it be a truth, or on
the other hand the disproof of it, is of the greatest consequence. If, as a
first principle, we can establish, that the centres of the two forces, or
elements of force, never can be separated to any sensible distance, or at
all events not further than the space between two contiguous particles
(1615.), or if we can establish the contrary conclusion, how much more
clear is our view of what lies before us, and how much less embarrassed the
ground over which we have to pass in attaining to it, than if we remain
halting between two opinions! And if, with that feeling, we rigidly test
every experiment which bears upon the point, as far as our prejudices will
let us (1161.), instead of permitting them with a theoretical expression to
pass too easily away, are we not much more likely to attain the real truth,
and from that proceed with safety to what is at present unknown?

1629. I say these things, not, I hope, to advance a particular view, but to
draw the strict attention of those who are able to investigate and judge of
the matter, to what must be a turning point in the theory of electricity;
to a separation of two roads, one only of which can be right: and I hope I
may be allowed to go a little further into the facts which have driven me
to the view I have just given.

1630. When a wire in the voltaic circuit is heated, the temperature
frequently rises first, or most at one end. If this effect were due to any
relation of positive or negative as respects the current, it would be
exceedingly important. I therefore examined several such cases; but when,
keeping the contacts of the wire and its position to neighbouring things
unchanged, I altered the direction of the current, I found that the effect
remained unaltered, showing that it depended, not upon the direction of the
current, but on other circumstances. So there is here no evidence of a
difference between one part of the circuit and another.

1631. The same point, i.e. uniformity in every part, may be illustrated by
what may be considered as the inexhaustible nature of the current when
producing particular effects; for these effects depend upon transfer only,
and do not consume the power. Thus a current which will heat one inch of
platina wire will heat a hundred inches (853. note). If a current be
sustained in a constant state, it will decompose the fluid in one
voltameter only, or in twenty others if they be placed in the circuit, in
each to an amount equal to that in the single one.

1632. Again, in cases of disruptive discharge, as in the spark, there is
frequently a dark part (1422.) which, by Professor Johnson, has been called
the neutral point[A]; and this has given rise to the use of expressions
implying that there are two electricities existing separately, which,
passing to that spot, there combine and neutralize each other[B]. But if
such expressions are understood as correctly indicating that positive
electricity alone is moving between the positive ball and that spot, and
negative electricity only between the negative ball and that spot, then
what strange conditions these parts must be in; conditions, which to my
mind are every way unlike those which really occur! In such a case, one
part of a current would consist of positive electricity only, and that
moving in one direction; another part would consist of negative electricity
only, and that moving in the other direction; and a third part would
consist of an accumulation of the two electricities, not moving in either
direction, but mixing up together! and being in a relation to each other
utterly unlike any relation which could be supposed to exist in the two
former portions of the discharge. This does not seem to me to be natural.
In a current, whatever form the discharge may take, or whatever part of the
circuit or current is referred to, as much positive force as is there
exerted in one direction, so much negative force is there exerted in the
other. If it were not so we should have bodies electrified not merely
positive and negative, but on occasions in a most extraordinary manner, one
being charged with five, ten, or twenty times as much of both positive and
negative electricity in equal quantities as another. At present, however,
there is no known fact indicating such states.

  [A] Silliman's Journal, 1834, xxv. p. 57.

  [B] Thomson on Heat and Electricity, p. 171.

1633. Even in cases of convection, or carrying discharge, the statement
that the current is everywhere the same must in effect be true (1627.); for
how, otherwise, could the results formerly described occur? When currents
of air constituted the mode of discharge between the portions of paper
moistened with iodide of potassium or sulphate of soda (465. 469.),
decomposition occurred; and I have since ascertained that, whether a
current of positive air issued from a spot, or one of negative air passed
towards it, the effect of the evolution of iodine or of acid was the same,
whilst the reversed currents produced alkali. So also in the magnetic
experiments (307.) whether the discharge was effected by the introduction
of a wire, or the occurrence of a spark, or the passage of convective
currents either one way or the other (depending on the electrified state of
the particles), the result was the same, being in all cases dependent upon
the perfect current.

1634. Hence, the section of a current compared with other sections of the
same current must be a constant quantity, if the actions exerted be of the
same kind; or if of different kinds, then the forms under which the effects
are produced are equivalent to each other, and experimentally convertible
at pleasure. It is in sections, therefore, we must look for identity of
electrical force, even to the sections of sparks and carrying actions, as
well as those of wires and electrolytes.

1635. In illustration of the utility and importance of establishing that
which may be the true principle, I will refer to a few cases. The doctrine
of unipolarity, as formerly stated, and I think generally understood[A], is
evidently inconsistent with my view of a current (1627.); and the later
singular phenomena of poles and flames described by Erman and others[B]
partake of the same inconsistency of character. If a unipolar body could
exist, i.e. one that could conduct the one electricity and not the other,
what very new characters we should have a right to expect in the currents
of single electricities passing through them, and how greatly ought they to
differ, not only from the common current which is supposed to have both
electricities travelling in opposite directions in equal amount at the same
time, but also from each other! The facts, which are excellent, have,
however, gradually been more correctly explained by Becquerel[C],
Andrews[D], and others; and I understand that Professor Ohms[E] has
perfected the work, in his close examination of all the phenomena; and
after showing that similar phenomena can take place with good conductors,
proves that with soap, &c. many of the effects are the mere consequences of
the bodies evolved by electrolytic action.

  [A] Erman, Annales de Chimie, 1807. lxi. p. 115. Davy's Elements, p.
  168. Biot, Ency. Brit. Supp, iv. p. 444. Becquerel, Traité, i. p. 167.
  De la Rive, Bib. Univ. 1837. vii. 392.

  [B] Erman, Annales de Chimie, 1824. xxv. 278. Becquerel, Ibid. xxxvi.
  p. 329

  [C] Becquerel, Annales de Chimie, 1831. xlvi. p. 283.

  [D] Andrews, Philosophical Magazine, 1836. ix. 182.

  [E] Schweigger's Jahrbuch de Chimie, &c. 1830. Heft 8. Not
  understanding German, it is with extreme regret I confess I have not
  access, and cannot do justice, to the many most valuable papers in
  experimental electricity published in that language. I take this
  opportunity also of stating another circumstance which occasions me
  great trouble, and, as I find by experience, may make, me seemingly
  regardless of the labours of others:--it is a gradual loss of memory
  for some years past; and now, often when I read a memoir, I remember
  that I have seen it before, and would have rejoiced if at the right
  time I could have recollected and referred to it in the progress of my
  own papers.--M.F.

1636. I conclude, therefore, that the _facts_ upon which the doctrine of
unipolarity was founded are not adverse to that unity and indivisibility of
character which I have stated the current to possess, any more than the
phenomena of the pile itself (which might well bear comparison with those
of unipolar bodies,) are opposed to it. Probably the effects which have
been called effects of unipolarity, and the peculiar differences of the
positive and negative surface when discharging into air, gases, or other
dielectrics (1480. 1525.) which have been already referred to, may have
considerable relation to each other[A].

  [A] See also Hare in Silliman's Journal, 1833. xxiv. 246.

       *       *       *       *       *

1637. M. de la Rive has recently described a peculiar and remarkable effect
of heat on a current when passing between electrodes and a fluid[A]. It is,
that if platina electrodes dip into acidulated water, no change is produced
in the passing current by making the positive electrode hotter or colder;
whereas making the negative electrode hotter increased the deflexion of a
galvanometer affected by the current, from 12° to 30° and even 45°, whilst
making it colder diminished the current in the same high proportions.

  [A] Bibliothèque Universelle, 1837, vii. 388.

1638. That one electrode should have this striking relation to heat whilst
the other remained absolutely without, seem to me as incompatible with what
I conceived to be the character of a current as unipolarity (1627. 1635.),
and it was therefore with some anxiety that I repeated the experiment. The
electrodes which I used were platina; the electrolyte, water containing
about one sixth of sulphuric acid by weight: the voltaic battery consisted
of two pairs of amalgamated zinc and platina plates in dilute sulphuric
acid, and the galvanometer in the circuit was one with two needles, and
gave when the arrangement was complete a deflexion of 10° or 12°.

1639. Under these circumstances heating either electrode increased the
current; heating both produced still more effect. When both were heated, if
either were cooled, the effect on the current fell in proportion. The
proportion of effect due to heating this or that electrode varied, but on
the whole heating the negative seemed to favour the passage of the current
somewhat more than heating the positive. Whether the application of heat
were by a flame applied underneath, or one directed by a blowpipe from
above, or by a hot iron or coal, the effect was the same.

1640. Having thus removed the difficulty out of the way of my views
regarding a current, I did not pursue this curious experiment further. It
is probable, that the difference between my results and those of M. de la
Rive may depend upon the relative values of the currents used; for I
employed only a weak one resulting from two pairs of plates two inches long
and half an inch wide, whilst M. de la Rive used four pairs of plates of
sixteen square inches in surface.

       *       *       *       *       *

1641. Electric discharges in the atmosphere in the form of balls of fire
have occasionally been described. Such phenomena appear to me to be
incompatible with all that we know of electricity and its modes of
discharge. As _time_ is an element in the effect (1418. 1436.) it is
possible perhaps that an electric discharge might really pass as a ball
from place to place; but as every thing shows that its velocity must be
almost infinite, and the time of its duration exceedingly small, it is
impossible that the eye should perceive it as anything else than a line of
light. That phenomena of balls of fire may appear in the atmosphere, I do
not mean to deny; but that they have anything to do with the discharge of
ordinary electricity, or are at all related to lightning or atmospheric
electricity, is much more than doubtful.

       *       *       *       *       *

1642. All these considerations, and many others, help to confirm the
conclusion, drawn over and over again, that the current is an indivisible
thing; an axis of power, in every part of which both electric forces are
present in equal amount[A] (517. 1627.). With conduction and
electrolyzation, and even discharge by spark, such a view will harmonize
without hurting any of our preconceived notions; but as relates to
convection, a more startling result appears, which must therefore be
considered.

  [A] I am glad to refer here to the results obtained by Mr. Christie
  with magneto-electricity, Philosophical Transactions, 1833, p. 113
  note. As regards the current in a wire, they confirm everything that I
  am contending for.

1643. If two balls A and B be electrified in opposite states and held
within each other's influence, the moment they move towards each other, a
current, or those effects which are understood by the word current, will be
produced. Whether A move towards B, or B move in the opposite direction
towards A, a current, and in both cases having the same _direction_, will
result. If A and B move from each other, then a _current_ in the opposite
direction, or equivalent effects, will be produced.

1644. Or, as charge exists only by induction (1178. 1299.), and a body when
electrified is necessarily in relation to other bodies in the opposite
state; so, if a ball be electrified positively in the middle of a room and
be then moved in any direction, effects will be produced, as _current_ in
the same direction (to use the conventional mode of expression) had
existed: or, if the ball be negatively electrified, and then moved, effects
as if a current in a direction contrary to that of the motion had been
formed, will be produced.

1645. I am saying of a single particle or of two what I have before said,
in effect, of many (1633.). If the former account of currents be true, then
that just stated must be a necessary result. And, though the statement may
seem startling at first, it is to be considered that, according to my
theory of induction, the charged conductor or particle is related to the
distant conductor in the opposite state, or that which terminates the
extent of the induction, by all the intermediate particles (1165, 1295.),
these becoming polarized exactly as the particles of a solid electrolyte do
when interposed between the two electrodes. Hence the conclusion regarding
the unity and identity of the current in the case of convection, jointly
with the former cases, is not so strange as it might at first appear.

       *       *       *       *       *

1646. There is a very remarkable phenomenon or effect of the electrolytic
discharge, first pointed out, I believe, by Mr. Porrett, of the
accumulation of fluid under decomposing action in the current on one side
of an interposed diaphragm[A]. It is a mechanical result; and as the liquid
passes from the positive towards the negative electrode in all the known
cases, it seems to establish a relation to the polar condition of the
dielectric in which the current exists (1164. 1525.). It has not as yet
been sufficiently investigated by experiment; for De la Rive says[B], it
requires that the water should be a bad conductor, as, for instance,
distilled water, the effect not happening with strong solutions; whereas,
Dutrochet says[C] the contrary is the case, and that, the effect is not
directly due to the electric current.

  [A] Annals of Philosophy, 1816. viii. p. 75.

  [B] Annales de Chimie, 1835. xxviii. p. 196.

  [C] Annales de Chimie, 1832, xlix. p. 423.

1647. Becquerel, in his Traité de l'Electricité, has brought together the
considerations which arise for and against the opinion, that the effect
generally is an electric effect[A]. Though I have no decisive fact to quote
at present, I cannot refrain from venturing an opinion, that the effect is
analogous both to combination and convection (1623.), being a case of
carrying due to the relation of the diaphragm and the fluid in contact with
it, through which the electric discharge is jointly effected; and further,
that the peculiar relation of positive and negative small and large
surfaces already referred to (1482. 1503. 1525.), may be the direct cause
of the fluid and the diaphragm travelling in contrary but determinate
directions. A very valuable experiment has been made by M. Becquerel with
particles of clay[B], which will probably bear importantly on this point.

  [A] Vol. iv. p. 192, 197.

  [B] Traité de l'Electricité, i. p. 285.

       *       *       *       *       *

1648. _As long as_ the terms _current_ and _electro-dynamic_ are used to
express those relations of the electric forces in which progression of
either fluids or effects are supposed to occur (283.), _so long_ will the
idea of velocity be associated with them; and this will, perhaps, be more
especially the case if the hypothesis of a fluid or fluids be adopted.

1649. Hence has arisen the desire of estimating this velocity either
directly or by some effect dependent on it; and amongst the endeavours to
do this correctly, may be mentioned especially those of Dr. Watson[A] in
1748, and of Professor Wheatstone[B] in 1834; the electricity in the early
trials being supposed to travel from end to end of the arrangement, but in
the later investigations a distinction occasionally appearing to be made
between the transmission of the effect and of the supposed fluid by the
motion of whose particles that effect is produced.

  [A] Philosophical Transactions, 1748.

  [B] Ibid. 1834, p. 583.

1650. Electrolytic action has a remarkable bearing upon this question of
the velocity of the current, especially as connected with the theory of an
electric fluid or fluids. In it there is an evident transfer of power with
the transfer of each particle of the anion or cathion present, to the next
particles of the cathion or anion; and as the amount of power is definite,
we have in this way a means of localizing as it were the force, identifying
it by the particle and dealing it out in successive portions, which leads,
I think, to very striking results.

1651. Suppose, for instance, that water is undergoing decomposition by the
powers of a voltaic battery. Each particle of hydrogen as it moves one way,
or of oxygen as it moves in the other direction, will transfer a certain
amount of electrical force associated with it in the form of chemical
affinity (822. 852. 918.) onwards through a distance, which is equal to
that through which the particle itself has moved. This transfer will be
accompanied by a corresponding movement in the electrical forces throughout
every part of the circuit formed (1627. 1634.), and its effects may be
estimated, as, for instance, by the heating of a wire (853.) at any
particular section of the current however distant. If the water be a cube
of an inch in the side, the electrodes touching, each by a surface of one
square inch, and being an inch apart, then, by the time that a tenth of it,
or 25.25 grs., is decomposed, the particles of oxygen and hydrogen
throughout the mass may be considered as having moved relatively to each
other in opposite directions, to the amount of the tenth of an inch; i.e.
that two particles at first in combination will after the motion be the
tenth of an inch apart. Other motions which occur in the fluid will not at
all interfere with this result; for they have no power of accelerating or
retarding the electric discharge, and possess in fact no relation to it.

1652. The quantity of electricity in 25.25 grains of water is, according to
an estimate of the force which I formerly made (861.), equal to above 24
millions of charges of a large Leyden battery; or it would have kept any
length of a platina wire 1/104 of an inch in diameter red-hot for an hour
and a half (853.). This result, though given only as an approximation, I
have seen no reason as yet to alter, and it is confirmed generally by the
experiments and results of M. Pouillet[A]. According to Mr. Wheatstone's
experiments, the influence or effects of the current would appear at a
distance of 576,000 miles in a second[B]. We have, therefore, in this view
of the matter, on the one hand, an enormous quantity of power equal to a
most destructive thunder-storm appearing instantly at the distance of
576,000 miles from its source, and on the other, a quiet effect, in
producing which the power had taken an hour and a half to travel through
the tenth of an inch: yet these are the equivalents to each other, being
effects observed at the sections of one and the same current (1634.).

  [A] Becquerel, Traité de l'Electricité, v. p. 278.

  [B] Philosophical Transactions, 1834, p. 589.

       *       *       *       *       *

1653. It is time that I should call attention to the lateral or transverse
forces of the _current_. The great things which have been achieved by
Oersted, Arago, Ampère, Davy, De la Rive, and others, and the high degree
of simplification which has been introduced into their arrangement by the
theory of Ampère, have not only done their full service in advancing most
rapidly this branch of knowledge, but have secured to it such attention
that there is no necessity for urging on its pursuit. I refer of course to
magnetic action and its relations; but though this is the only recognised
lateral action of the current, there is great reason for believing that
others exist and would by their discovery reward a close search for them
(951.).

1654. The magnetic or transverse action of the current seems to be in a
most extraordinary degree independent of those variations or modes of
action which it presents directly in its course; it consequently is of the
more value to us, as it gives us a higher relation of the power than any
that might have varied with each mode of discharge. This discharge, whether
it be by conduction through a wire with infinite velocity (1652.), or by
electrolyzation with its corresponding and exceeding slow motion (1651.),
or by spark, and probably even by convection, produces a transverse
magnetic action always the same in kind and direction.

1655. It has been shown by several experimenters, that whilst the discharge
is of the _same kind_ the amount of lateral or magnetic force is very
constant (216. 366. 367. 368. 376.). But when we wish to compare discharge
of different kinds, for the important purpose of ascertaining whether the
same amount of current will in its _different forms_ produce the same
amount of transverse action, we find the data very imperfect. Davy noticed,
that when the electric current was passing through an aqueous solution it
affected a magnetic needle[A], and Dr. Ritchie says, that the current in
the electrolyte is as magnetic as that in a metallic wire[B], and has
caused water to revolve round a magnet as a wire carrying the current would
revolve.

  [A] Philosophical Transactions, 1821, p. 426.

  [B] Ibid. 1832, p. 294.

1656. Disruptive discharge produces its magnetic effects: a strong spark,
passed transversely to a steel needle, will magnetise it as well as if the
electricity of the spark were conducted by a metallic wire occupying the
line of discharge; and Sir H. Davy has shown that the discharge of a
voltaic battery in vacuo is affected and has motion given to it by
approximated magnets[A].

  [A] Philosophical Transactions, 1821, p. 427.

1657. Thus the three very different modes of discharge, namely, conduction,
electrolyzation, and disruptive discharge, agree in producing the important
transverse phenomenon of magnetism. Whether convection or carrying
discharge will produce the same phenomenon has not been determined, and the
few experiments I have as yet had time to make do not enable me to answer
in the affirmative.

       *       *       *       *       *

1658. Having arrived at this point in the consideration of the current and
in the endeavour to apply its phenomena as tests of the truth or fallacy of
the theory of induction which I have ventured to set forth, I am now very
much tempted to indulge in a few speculations respecting its lateral action
and its possible connexion with the transverse condition of the lines of
ordinary induction (1165, 1304.)[A]. I have long sought and still seek for
an effect or condition which shall be to statical electricity what magnetic
force is to current electricity (1411.); for as the lines of discharge are
associated with a certain transverse effect, so it appeared to me
impossible but that the lines of tension or of inductive action, which of
necessity precede that discharge, should also have their correspondent
transverse condition or effect (951.).

  [A] Refer for further investigations to 1709.--1736.--_Dec. 1838._

1659. According to the beautiful theory of Ampère, the transverse force of
a current may be represented by its attraction for a similar current and
its repulsion of a contrary current. May not then the equivalent transverse
force of static electricity be represented by that lateral tension or
repulsion which the lines of inductive action appear to possess (1304.)?
Then again, when current or discharge occurs between two bodies, previously
under inductrical relations to each other, the lines of inductive force
will weaken and fade away, and, as their lateral repulsive tension
diminishes, will contract and ultimately disappear in the line of
discharge. May not this be an effect identical with the attractions of
similar currents? i.e. may not the passage of static electricity into
current electricity, and that of the lateral tension of the lines of
inductive force into the lateral attraction of lines of similar discharge,
have the same relation and dependences, and run parallel to each other?

1660. The phenomena of induction amongst currents which I had the good
fortune to discover some years ago (6. &c. 1048.) may perchance here form a
connecting link in the series of effects. When a current is first formed,
it tends to produce a current in the contrary direction in all the matter
around it; and if that matter have conducting properties and be fitly
circumstanced, such a current is produced. On the contrary, when the
original current is stopped, one in the same direction tends to form all
around it, and, in conducting matter properly arranged, will be excited.

1661. Now though we perceive the effects only in that portion of matter
which, being in the neighbourhood, has conducting properties, yet
hypothetically it is probable, that the nonconducting matter has also its
relations to, and is affected by, the disturbing cause, though we have not
yet discovered them. Again and again the relation of conductors and
non-conductors has been shown to be one not of opposition in kind, but only
of degree (1334, 1603.); and, therefore, for this, as well as for other
reasons, it is probable, that what will affect a conductor will affect an
insulator also; producing perhaps what may deserve the term of the
electrotonic state (60. 242. 1114.).

1662. It is the feeling of the necessity of some lateral connexion between
the lines of electric force (1114.); of some link in the chain of effects
as yet unrecognised, that urges me to the expression of these speculations.
The same feeling has led me to make many experiments on the introduction of
insulating dielectrics having different inductive capacities (1270. 1277.)
between magnetic poles and wires carrying currents, so as to pass across
the lines of magnetic force. I have employed such bodies both at rest and
in motion, without, as yet, being able to detect any influence produced by
them; but I do by no means consider the experiments as sufficiently
delicate, and intend, very shortly, to render them more decisive[A].

  [A] See onwards 1711.--1726.--_Dec. 1838._

1663. I think the hypothetical question may at present be put thus: can
such considerations as those already generally expressed (1658.) account
for the transverse effects of electrical currents? are two such currents in
relation to each other merely by the inductive condition of the particles
of matter between them, or are they in relation by some higher quality and
condition (1654.), which, acting at a distance and not by the intermediate
particles, has, like the force of gravity, no relation to them?

1664. If the latter be the case, then, when electricity is acting upon and
in matter, its direct and its transverse action are essentially different
in their nature; for the former, if I am correct, will depend upon the
contiguous particles, and the latter will not. As I have said before, this
may be so, and I incline to that view at present; but I am desirous of
suggesting considerations why it may not, that the question may be
thoroughly sifted.

1665. The transverse power has a character of polarity impressed upon it.
In the simplest forms it appears as attraction or repulsion, according as
the currents are in the same or different directions: in the current and
the magnet it takes up the condition of tangential forces; and in magnets
and their particles produces poles. Since the experiments have been made
which have persuaded me that the polar forces of electricity, as in
induction and electrolytic action (1298. 1343.), show effects at a distance
only by means of the polarized contiguous and intervening particles, I have
been led to expect that _all polar forces_ act in the same general manner;
and the other kinds of phenomena which one can bring to bear upon the
subject seem fitted to strengthen that expectation. Thus in
crystallizations the effect is transmitted from particle to particle; and
in this manner, in acetic acid or freezing water a crystal a few inches or
even a couple of feet in length will form in less than a second, but
progressively and by a transmission of power from particle to particle.
And, as far as I remember, no case of polar action, or partaking of polar
action, except the one under discussion, can be found which does not act by
contiguous particles[A]. It is apparently of the nature of polar forces
that such should be the case, for the one force either finds or developed
the contrary force near to it, and has, therefore, no occasion to seek for
it at a distance.

  [A] I mean by contiguous particles those which are next to each other,
  not that there is _no_ space between them. See (1616.).

1666. But leaving these hypothetical notions respecting the nature of the
lateral action out of sight, and returning to the direct effects, I think
that the phenomena examined and reasoning employed in this and the two
preceding papers tend to confirm the view first taken (1464.), namely, that
ordinary inductive action and the effects dependent upon it are due to an
action of the contiguous particles of the dielectric interposed between the
charged surfaces or parts which constitute, as it were, the terminations of
the effect. The great point of distinction and power (if it have any) in
the theory is, the making the dielectric of essential and specific
importance, instead of leaving it as it were a mere accidental circumstance
or the simple representative of space, having no more influence over the
phenomena than the space occupied by it. I have still certain other results
and views respecting the nature of the electrical forces and excitation,
which are connected with the present theory; and, unless upon further
consideration they sink in my estimation, I shall very shortly put them
into form as another series of these electrical researches.

_Royal Institution.
February 14th, 1838._




FOURTEENTH SERIES.


§ 20. _Nature of the electric force or forces._ § 21. _Relation of the
electric and magnetic forces._ § 22. _Note on electrical excitation._

Received June 21, 1838.--Read June 21, 1838.

§ 20. _Nature of the electric force or forces._


1667. The theory of induction set forth and illustrated in the three
preceding series of experimental researches does not assume anything new as
to the nature of the electric force or forces, but only as to their
distribution. The effects may depend upon the association of one electric
fluid with the particles of matter, as in the theory of Franklin, Epinus,
Cavendish, and Mossotti; or they may depend upon the association of two
electric fluids, as in the theory of Dufay and Poisson; or they may not
depend upon anything which can properly be called the electric fluid, but
on vibrations or other affections of the matter in which they appear. The
theory is unaffected by such differences in the mode of viewing the nature
of the forces; and though it professes to perform the important office of
stating _how_ the powers are arranged (at least in inductive phenomena), it
does not, as far as I can yet perceive, supply a single experiment which
can be considered as a distinguishing test of the truth of any one of these
various views,

1668. But, to ascertain how the forces are arranged, to trace them in their
various relations to the particles of matter, to determine their general
laws, and also the specific differences which occur under these laws, is as
important as, if not more so than, to know whether the forces reside in a
fluid or not; and with the hope of assisting in this research, I shall
offer some further developments, theoretical and experimental, of the
conditions under which I suppose the particles of matter are placed when
exhibiting inductive phenomena.

1669. The theory assumes that all the _particles_, whether of insulating or
conducting matter, are as wholes conductors.

1670. That not being polar in their normal state, they can become so by the
influence of neighbouring charged particles, the polar state being
developed at the instant, exactly as in an insulated conducting _mass_
consisting of many particles.

1671. That the particles when polarized are in a forced state, and tend to
return to their normal or natural condition.

1672. That being as wholes conductors, they can readily be charged, either
_bodily_ or _polarly_.

1673. That particles which being contiguous[A] are also in the line of
inductive action can communicate or transfer their polar forces one to
another _more_ or _less_ readily.

  [A] See note to 1164.--_Dec. 1838._

1674. That those doing so less readily require the polar forces to be
raised to a higher degree before this transference or communication takes
place.

1675. That the _ready_ communication of forces between contiguous particles
constitutes _conduction_, and the _difficult_ communication _insulation_;
conductors and insulators being bodies whose particles naturally possess
the property of communicating their respective forces easily or with
difficulty; having these differences just as they have differences of any
other natural property.

1676. That ordinary induction is the effect resulting from the action of
matter charged with excited or free electricity upon insulating matter,
tending to produce in it an equal amount of the contrary state.

1677. That it can do this only by polarizing the particles contiguous to
it, which perform the same office to the next, and these again to those
beyond; and that thus the action is propagated from the excited body to the
next conducting mass, and there renders the contrary force evident in
consequence of the effect of communication which supervenes in the
conducting mass upon the polarization of the particles of that body
(1675.).

1678. That therefore induction can only take place through or across
insulators; that induction is insulation, it being the necessary
consequence of the state of the particles and the mode in which the
influence of electrical forces is transferred or transmitted through or
across such insulating media.

1679. The particles of an insulating dielectric whilst under induction may
be compared to a series of small magnetic needles, or more correctly still
to a series of small insulated conductors. If the space round a charged
globe were filled with a mixture of an insulating dielectric, as oil of
turpentine or air, and small globular conductors, as shot, the latter being
at a little distance from each other so as to be insulated, then these
would in their condition and action exactly resemble what I consider to be
the condition and action of the particles of the insulating dielectric
itself (1337.). If the globe were charged, these little conductors would
all be polar; if the globe were discharged, they would all return to their
normal state, to be polarized again upon the recharging of the globe. The
state developed by induction through such particles on a mass of conducting
mutter at a distance would be of the contrary kind, and exactly equal in
amount to the force in the inductric globe. There would be a lateral
diffusion of force (1224. 1297.), because each polarized sphere would be in
an active or tense relation to all those contiguous to it, just as one
magnet can affect two or more magnetic needles near it, and these again a
still greater number beyond them. Hence would result the production of
curved lines of inductive force if the inducteous body in such a mixed
dielectric were an uninsulated metallic ball (1219. &c.) or other properly
shaped mass. Such curved lines are the consequences of the two electric
forces arranged as I have assumed them to be: and, that the inductive force
can be directed in such curved lines is the strongest proof of the presence
of the two powers and the polar condition of the dielectric particles.

1680. I think it is evident, that in the case stated, action at a distance
can only result through an action of the contiguous conducting particles.
There is no reason why the inductive body should polarize or affect
_distant_ conductors and leave those _near_ it, namely the particles of the
dielectric, unaffected: and everything in the form of fact and experiment
with conducting masses or particles of a sensible size contradicts such a
supposition.

1681. A striking character of the electric power is that it is limited and
exclusive, and that the two forces being always present are exactly equal
in amount. The forces are related in one of two ways, either as in the
natural normal condition of an uncharged insulated conductor; or as in the
charged state, the latter being a case of induction.

1682. Cases of induction are easily arranged so that the two forces being
limited in their direction shall present no phenomena or indications
external to the apparatus employed, Thus, if a Leyden jar, having its
external coating a little higher than the internal, be charged and then its
charging ball and rod removed, such jar will present no electrical
appearances so long as its outside is uninsulated. The two forces which may
be said to be in the coatings, or in the particles of the dielectric
contiguous to them, are entirely engaged to each other by induction through
the glass; and a carrier ball (1181.) applied either to the inside or
outside of the jar will show no signs of electricity. But if the jar be
insulated, and the charging ball and rod, in an uncharged state and
suspended by an insulating thread of white silk, be restored to their
place, then the part projecting above the jar will give electrical
indications and charge the carrier, and at the same time the _outside_
coating of the jar will be found in the opposite state and inductric
towards external surrounding objects.

1683. These are simple consequences of the theory. Whilst the charge of the
inner coating could induce only through the glass towards the outer
coating, and the latter contained no more of the contrary force than was
equivalent to it, no induction external to the jar could be perceived; but
when the inner coating was extended by the rod and ball so that it could
induce through the air towards external objects, then the tension of the
polarized glass molecules would, by their tendency to return to the normal
state, fall a little, and a portion of the charge passing to the surface of
this new part of the inner conductor, would produce inductive action
through the air towards distant objects, whilst at the same time a part of
the force in the outer coating previously directed inwards would now be at
liberty, and indeed be constrained to induct outwards through the air,
producing in that outer coating what is sometimes called, though I think
very improperly, free charge. If a small Leyden jar be converted into that
form of apparatus usually known by the name of the electric well, it will
illustrate this action very completely.

1684. The terms _free charge_ and _dissimulated electricity_ convey
therefore erroneous notions if they are meant to imply any difference as to
the mode or kind of action. The charge upon an insulated conductor in the
middle of a room is in the same relation to the walls of that room as the
charge upon the inner coating of a Leyden jar is to the outer coating of
the same jar. The one is not more _free_ or more _dissimulated_ than the
other; and when sometimes we make electricity appear where it was not
evident before, as upon the outside of a charged jar, when, after
insulating it, we touch the inner coating, it is only because we divert
more or less of the inductive force from one direction into another; for
not the slightest change is in such circumstances impressed upon the
character or action of the force.

       *       *       *       *       *

1685. Having given this general theoretical view, I will now notice
particular points relating to the nature of the assumed electric polarity
of the insulating dielectric particles.

1686. The polar state may be considered in common induction as a forced
state, the particles tending to return to their normal condition. It may
probably be raised to a very high degree by approximation of the inductric
and inducteous bodies or by other circumstances; and the phenomena of
electrolyzation (861. 1652. 1796.) seem to imply that the quantity of power
which can thus be accumulated on a single particle is enormous. Hereafter
we may be able to compare corpuscular forces, as those of gravity,
cohesion, electricity, and chemical affinity, and in some way or other from
their effects deduce their relative equivalents; at present we are not able
to do so, but there seems no reason to doubt that their electrical, which
are at the same time their chemical forces (891. 918.), will be by far the
most energetic.

1687. I do not consider the powers when developed by the polarization as
limited to two distinct points or spots on the surface of each particle to
be considered as the poles of an axis, but as resident on large portions of
that surface, as they are upon the surface of a conductor of sensible size
when it is thrown into a polar state. But it is very probable,
notwithstanding, that the particles of different bodies may present
specific differences in this respect, the powers not being equally diffused
though equal in quantity; other circumstances also, as form and quality,
giving to each a peculiar polar relation. It is perhaps to the existence of
some such differences as these that we may attribute the specific actions
of the different dielectrics in relation to discharge(1394. 1508.). Thus
with respect to oxygen and nitrogen singular contrasts were presented when
spark and brush discharge were made to take place in these gases, as may be
seen by reference to the Table in paragraph 1518 of the Thirteenth Series;
for with nitrogen, when the small, negative or the large positive ball was
rendered inductric, the effects corresponded with those which in oxygen
were produced when the small positive or the large negative ball was
rendered inductric.

1688. In such solid bodies as glass, lac, sulphur, &c., the particles
appear to be able to become polarized in all directions, for a mass when
experimented upon so as to ascertain its inductive capacity in three or
more directions (1690.), gives no indication of a difference. Now as the
particles are fixed in the mass, and as the direction of the induction
through them must change with its change relative to the mass, the constant
effect indicates that they can be polarized electrically in any direction.
This accords with the view already taken of each particle as a whole being
a conductor (1669.), and, as an experimental fact, helps to confirm that
view.

1689. But though particles may thus be polarized in _any_ direction under
the influence of powers which are probably of extreme energy (1686.), it
does not follow that each particle may not tend to polarize to a greater
degree, or with more facility, in one direction than another; or that
different kinds may not have specific differences in this respect, as they
have differences of conducting and other powers (1296. 1326. 1395.). I
sought with great anxiety for a relation of this nature; and selecting
crystalline bodies as those in which all the particles are symmetrically
placed, and therefore best fitted to indicate any result which might depend
upon variation of the direction of the forces to the direction of the
particles in which they were developed, experimented very carefully with
them. I was the more strongly stimulated to this inquiry by the beautiful
electrical condition of the crystalline bodies tourmaline and boracite, and
hoped also to discover a relation between electric polarity and that of
crystallization, or even of cohesion itself (1316.). My experiments have
not established any connexion of the kind sought for. But as I think it of
equal importance to show either that there is or is not such a relation, I
shall briefly describe the results.

1690. The form of experiment was as follows. A brass ball 0.73 of an inch
in diameter, fixed at the end of a horizontal brass rod, and that at the
end of a brass cylinder, was by means of the latter connected with a large
Leyden battery (291.) by perfect metallic communications, the object being
to keep that ball, by its connexion with the charged battery in an
electrified state, very nearly uniform, for half an hour at a time. This
was the inductric ball. The inducteous ball was the carrier of the torsion
electrometer (1229. 1314.); and the dielectric between them was a cube cut
from a crystal, so that two of its faces should be perpendicular to the
optical axis, whilst the other four were parallel to it. A small projecting
piece of shell-lac was fixed on the inductric ball at that part opposite to
the attachment of the brass rod, for the purpose of preventing actual
contact between the ball and the crystal cube. A coat of shell-lac was also
attached to that side of the carrier ball which was to be towards the cube,
being also that side which was furthest from the repelled ball in the
electrometer when placed in its position in that instrument. The cube was
covered with a thin coat of shell-lac dissolved in alcohol, to prevent the
deposition of damp upon its surface from the air. It was supported upon a
small table of shell-lac fixed on the top of a stem of the same substance,
the latter being of sufficient strength to sustain the cube, and yet
flexible enough from its length to act as a spring, and allow the cube to
bear, when in its place, against the shell-lac on the inductric ball.

[Illustration:]

1691. Thus it was easy to bring the inducteous ball always to the same
distance from the inductric bull, and to uninsulate and insulate it again
in its place; and then, after measuring the force in the electrometer
(1181.), to return it to its place opposite to the inductric ball for a
second observation. Or it was easy by revolving the stand which supported
the cube to bring four of its faces in succession towards the inductric
ball, and so observe the force when the lines of inductive action (1304.)
coincided with, or were transverse to, the direction of the optical axis of
the crystal. Generally from twenty to twenty-eight observations were made
in succession upon the four vertical faces of a cube, and then an average
expression of the inductive force was obtained, and compared with similar
averages obtained at other times, every precaution being taken to secure
accurate results.

1692. The first cube used was of _rock crystal_; it was 0.7 of an inch in
the side. It presented a remarkable and constant difference, the average of
not less than 197 observations, giving 100 for the specific inductive
capacity in the direction coinciding with the optical axis of the cube,
whilst 93.59 and 93.31 were the expressions for the two transverse
directions.

1693. But with a second cube of rock crystal corresponding results were not
obtained. It was 0.77 of an inch in the side. The average of many
experiments gave 100 for the specific inductive capacity coinciding with
the direction of the optical axis, and 98.6 and 99.92 for the two other
directions.

1694. Lord Ashley, whom I have found ever ready to advance the cause of
science, obtained for me the loan of three globes of rock crystal belonging
to Her Grace the Duchess of Sutherland for the purposes of this
investigation. Two had such fissures as to render them unfit for the
experiments (1193. 1698.). The third, which was very superior, gave me no
indications of any difference in the inductive force for different
directions.

1695. I then used cubes of Iceland spar. One 0.5 of an inch in diameter
gave 100 for the axial direction, and 98.66 and 95.74 for the two cross
directions. The other, 0.8 of an inch in the side, gave 100 for the axial
direction, whilst 101.73 and 101.86 were the numbers for the cross
direction.

1696. Besides these differences there were others, which I do not think it
needful to state, since the main point is not confirmed. For though the
experiments with the first cube raised great expectation, they have not
been generalized by those which followed. I have no doubt of the results as
to that cube, but they cannot as yet be referred to crystallization. There
are in the cube some faintly coloured layers parallel to the optical axis,
and the matter which colours them may have an influence; but then the
layers are also nearly parallel to a cross direction, and if at all
influential should show some effect in that direction also, which they did
not.

1697. In some of the experiments one half or one part of a cube showed a
superiority to another part, and this I could not trace to any charge the
different parts had received. It was found that the varnishing of the cubes
prevented any communication of charge to them, except (in a few
experiments) a small degree of the negative state, or that which was
contrary to the state of the inductric ball (1564. 1566.).

1698. I think it right to say that, as far as I could perceive, the
insulating character of the cubes used was perfect, or at least so nearly
perfect, as to bear a comparison with shell-lac, glass, &c. (1255). As to
the cause of the differences, other than regular crystalline structure,
there may be several. Thus minute fissures in the crystal insensible to the
eye may be so disposed as to produce a sensible electrical difference
(1193.). Or the crystallization may be irregular; or the substance may not
be quite pure; and if we consider how minute a quantity of matter will
alter greatly the conducting power of water, it will seem not unlikely that
a little extraneous matter diffused through the whole or part of a cube,
may produce effects sufficient to account for all the irregularities of
action that have been observed.

1699. An important inquiry regarding the electrical polarity of the
particles of an insulating dielectric, is, whether it be the molecules of
the particular substance acted on, or the component or ultimate particles,
which thus act the part of insulated conducting polarizing portions
(1669.).

1700. The conclusion I have arrived at is, that it is the molecules of the
substance which polarize as wholes (1347.); and that however complicated
the composition of a body may be, all those particles or atoms which are
held together by chemical affinity to form one molecule of the resulting
body act as one conducting mass or particle when inductive phenomena and
polarization are produced in the substance of which it is a part.

1701. This conclusion is founded on several considerations. Thus if we
observe the insulating and conducting power of elements when they are used
as dielectrics, we find some, as sulphur, phosphorus, chlorine, iodine,
&c., whose particles insulate, and therefore polarize in a high degree;
whereas others, as the metals, give scarcely any indication of possessing a
sensible proportion of this power (1328.), their particles freely
conducting one to another. Yet when these enter into combination they form
substances having no direct relation apparently, in this respect, to their
elements; for water, sulphuric acid, and such compounds formed of
insulating elements, conduct by comparison freely; whilst oxide of lead,
flint glass, borate of lead, and other metallic compounds containing very
high proportions of conducting matter, insulate excellently well. Taking
oxide of lead therefore as the illustration, I conceive that it is not the
particles of oxygen and lead which polarize separately under the act of
induction, but the molecules of oxide of lead which exhibit this effect,
all the elements of one particle of the resulting body, being held together
as parts of one conducting individual by the bonds of chemical affinity;
which is but another term for electrical force (918.).

1702. In bodies which are electrolytes we have still further reason for
believing in such a state of things. Thus when water, chloride of tin,
iodide of lead, &c. in the solid state are between the electrodes of the
voltaic battery, their particles polarize as those of any other insulating
dielectric do (1164.); but when the liquid state is conferred on these
substances, the polarized particles divide, the two halves, each in a
highly charged state, travelling onwards until they meet other particles in
an opposite and equally charged state, with which they combine, to the
neutralization of their chemical, i.e. their electrical forces, and the
reproduction of compound particles, which can again polarize as wholes, and
again divide to repeat the same series of actions (1347.).

1703. But though electrolytic particles polarize as wholes, it would appear
very evident that in them it is not a matter of entire indifference _how_
the particle polarizes (1689.), since, when free to move (380, &c.) the
polarities are ultimately distributed in reference to the elements; and
sums of force equivalent to the polarities, and very definite in kind and
amount, separate, as it were, from each other, and travel onwards with the
elementary particles. And though I do not pretend to know what an atom is,
or how it is associated or endowed with electrical force, or how this force
is arranged in the cases of combination and decomposition, yet the strong
belief I have in the electrical polarity of particles when under inductive
action, and the hearing of such an opinion on the general effects of
induction, whether ordinary or electrolytic, will be my excuse, I trust,
for a few hypothetical considerations.

1704 In electrolyzation it appears that the polarized particles would
(because of the gradual change which has been induced upon the chemical,
i.e. the electrical forces of their elements (918.)) rather divide than
discharge to each other without division (1348.); for if their division,
i.e. their decomposition and recombination, be prevented by giving them the
solid state, then they will insulate electricity perhaps a hundredfold more
intense than that necessary for their electrolyzation (419, &c.). Hence the
tension necessary for direct conduction in such bodies appears to be much
higher than that for decomposition (419. 1164. 1344.).

1705. The remarkable stoppage of electrolytic conduction by solidification
(380. 1358.), is quite consistent with these views of the dependence of
that process on the polarity which is common to all insulating matter when
under induction, though attended by such peculiar electro-chemical results
in the case of electrolytes. Thus it may be expected that the first effect
of induction is so to polarize and arrange the particles of water that the
positive or hydrogen pole of each shall be from the positive electrode and
towards the negative electrode, whilst the negative or oxygen pole of each
shall be in the contrary direction; and thus when the oxygen and hydrogen
of a particle of water have separated, passing to and combining with other
hydrogen and oxygen particles, unless these new particles of water could
turn round they could not take up that position necessary for their
successful electrolytic polarization. Now solidification, by fixing the
water particles and preventing them from assuming that essential
preliminary position, prevents also their electrolysis (413.); and so the
transfer of forces in that manner being prevented (1347. 1703.), the
substance acts as an ordinary insulating dielectric (for it is evident by
former experiments (419. 1704.) that the insulating tension is higher than
the electrolytic tension), induction through it rises to a higher degree,
and the polar condition of the molecules as wholes, though greatly exalted,
is still securely maintained.

1706. When decomposition happens in a fluid electrolyte, I do not suppose
that all the molecules in the same sectional plane (1634.) part with and
transfer their electrified particles or elements at once. Probably the
_discharge force_ for that plane is summed up on one or a few particles,
which decomposing, travelling and recombining, restore the balance of
forces, much as in the case of spark disruptive discharge (1406.); for as
those molecules resulting from particles which have just transferred power
must by their position (1705.) be less favourably circumstanced than
others, so there must be some which are most favourably disposed, and
these, by giving way first, will for the time lower the tension and produce
discharge.

1707. In former investigations of the action of electricity (821, &c.) it
was shown, from many satisfactory cases, that the quantity of electric
power transferred onwards was in proportion to and was definite for a given
quantity of matter moving as anion or cathion onwards in the electrolytic
line of action; and there was strong reason to believe that each of the
particles of matter then dealt with, had associated with it a definite
amount of electrical force, constituting its force of chemical affinity,
the chemical equivalents and the electro-chemical equivalents being the
same (836.). It was also found with few, and I may now perhaps say with no
exceptions (1341.), that only those compounds containing elements in single
proportions could exhibit the characters and phenomena of electrolytes
(697.); oxides, chlorides, and other bodies containing more than one
proportion of the electro-negative element refusing to decompose under the
influence of the electric current.

1708. Probable reasons for these conditions and limitations arise out of
the molecular theory of induction. Thus when a liquid dielectric, as
chloride of tin, consists of molecules, each composed of a single particle
of each of the elements, then as these can convey equivalent opposite
forces by their separation in opposite directions, both decomposition and
transfer can result. But when the molecules, as in the bichloride of tin,
consist of one particle or atom of one element, and two of the other, then
the simplicity with which the particles may be supposed to be arranged and
to act, is destroyed. And, though it may be conceived that when the
molecules of bichloride of tin are polarized as wholes by the induction
across them, the positive polar force might accumulate on the one particle
of tin whilst the negative polar force accumulated on the two particles of
chlorine associated with it, and that these might respectively travel right
and left to unite with other two of chlorine and one of tin, in analogy
with what happens in cases of compounds consisting of single proportions,
yet this is not altogether so evident or probable. For when a particle of
tin combines with two of chlorine, it is difficult to conceive that there
should not be some relation of the three in the resulting molecule
analogous to fixed position, the one particle of metal being perhaps
symmetrically placed in relation to the two of chlorine: and, it is not
difficult to conceive of such particles that they could not assume that
position dependent both on their polarity and the relation of their
elements, which appears to be the first step in the process of
electrolyzation (1345. 1705.).


§ 21. _Relation of the electric and magnetic forces._


1709. I have already ventured a few speculations respecting the probable
relation of magnetism, as the transverse force of the current, to the
divergent or transverse force of the lines of inductive action belonging to
static electricity (1658, &c.).

1710. In the further consideration of this subject it appeared to me to be
of the utmost importance to ascertain, if possible, whether this lateral
action which we call magnetism, or sometimes the induction of electrical
currents (26. 1048, &c.), is extended to a distance _by the action of the
intermediate particles_ in analogy with the induction of static
electricity, or the various effects, such as conduction, discharge, &c.,
which are dependent on that induction; or, whether its influence at a
distance is altogether independent of such intermediate particles (1662.).

1711. I arranged two magneto-electric helices with iron cores end to end,
but with an interval of an inch and three quarters between them, in which
interval was placed the end or pole of a bar magnet. It is evident, that on
moving the magnetic pole from one core towards the other, a current would
tend to form in both helices, in the one because of the lowering, and in
the other because of the strengthening of the magnetism induced in the
respective soft iron cores. The helices were connected together, and also
with a galvanometer, so that these two currents should coincide in
direction, and tend by their joint force to deflect the needle of the
instrument. The whole arrangement was so effective and delicate, that
moving the magnetic pole about the eighth of an inch to and fro two or
three times, in periods equal to those required for the vibrations of the
galvanometer needle, was sufficient to cause considerable vibration in the
latter; thus showing readily the consequence of strengthening the influence
of the magnet on the one core and helix, and diminishing it on the other.

1712. Then without disturbing the distances of the magnet and cores, plates
of substances were interposed. Thus calling the two cores A and B, a plate
of shell-lac was introduced between the magnetic pole and A for the time
occupied by the needle in swinging one way; then it was withdrawn for the
time occupied in the return swing; introduced again for another equal
portion of time; withdrawn for another portion, and so on eight or nine
times; but not the least effect was observed on the needle. In other cases
the plate was alternated, i.e. it was introduced between the magnet and A
for one period of time, withdrawn and introduced between the magnet and B
for the second period, withdrawn and restored to its first place for the
third period, and so on, but with no effect on the needle.

1713. In these experiments _shell-lac_ in plates 0.9 of an inch in
thickness, _sulphur_ in a plate 0.9 of an inch in thickness, and _copper_
in a plate 0.7 of an inch in thickness were used without any effect. And I
conclude that bodies, contrasted by the extremes of conducting and
insulating power, and opposed to each other as strongly as metals, air, and
sulphur, show no difference with respect to magnetic forces when placed in
their lines of action, at least under the circumstances described.

1714. With a plate of iron, or even a small piece of that metal, as the
head of a nail, a very different effect was produced, for then the
galvanometer immediately showed its sensibility, and the perfection of the
general arrangement.

1715. I arranged matters so that a plate of _copper_ 0.2 of an inch in
thickness, and ten inches in diameter, should have the part near the edge
interposed between the magnet and the core, in which situation it was first
rotated rapidly, and then held quiescent alternately, for periods according
with that required for the swinging of the needle; but not the least effect
upon the galvanometer was produced.

1716. A plate of shell-lac 0.6 of an inch in thickness was applied in the
same manner, but whether rotating or not it produced no effect.

1717. Occasionally the plane of rotation was directly across the magnetic
curve: at other times it was made as oblique as possible; the direction of
the rotation being also changed in different experiments, but not the least
effect was produced.

1718. I now removed the helices with their soft iron cores, and replaced
them by two _flat helices_ wound upon card board, each containing forty-two
feet of silked copper wire, and having no associated iron. Otherwise the
arrangement was as before, and exceedingly sensible; for a very slight
motion of the magnet between the helices produced an abundant vibration of
the galvanometer needle.

1719. The introduction of plates of shell-lac, sulphur, or copper into the
intervals between the magnet and these helices (1713.), produced not the
least effect, whether the former were quiescent or in rapid revolution
(1715.). So here no evidence of the influence of the intermediate particles
could be obtained (1710.).

1720. The magnet was then removed and replaced by a flat helix,
corresponding to the two former, the three being parallel to each other.
The middle helix was so arranged that a voltaic current could be sent
through it at pleasure. The former galvanometer was removed, and one with a
double coil employed, one of the lateral helices being connected with one
coil, and the other helix with the other coil, in such manner that when a
voltaic current was sent through the middle helix its inductive action
(26.) on the lateral helices should cause currents in them, having contrary
directions in the coils of the galvanometer. By a little adjustment of the
distances these induced currents were rendered exactly equal, and the
galvanometer needle remained stationary notwithstanding their frequent
production in the instrument. I will call the middle coil C, and the
external coils A and B.

1721. A plate of copper 0.7 of an inch thick and six inches square, was
placed between coils C and B, their respective distances remaining
unchanged; and then a voltaic current from twenty pairs of 4 inch plates
was sent through the coil C, and intermitted, in periods fitted to produce
an effect on the galvanometer (1712.). if any difference had been produced
in the effect of C on A and B. But notwithstanding the presence of air in
one interval and copper in the other, the inductive effect was exactly
alike on the two coils, and as if air had occupied both intervals. So that
notwithstanding the facility with which any induced currents might form in
the thick copper plate, the coil outside of it was just as much affected by
the central helix C as if no such conductor as the copper had been there
(65.).

1722. Then, for the copper plate was substituted one of sulphur 0.9 of an
inch thick; still the results were exactly the same, i.e. there was no
action at the galvanometer.

1723. Thus it appears that when a voltaic current in one wire is exerting
its inductive action to produce a contrary or a similar current in a
neighbouring wire, according as the primary current is commencing or
ceasing, it makes not the least difference whether the intervening space is
occupied by such insulating bodies as air, sulphur and shell-lac, or such
conducting bodies as copper, and the other non-magnetic metals.

1724. A correspondent effect was obtained with the like forces when
resident in a magnet thus. A single flat helix (1718.) was connected with a
galvanometer, and a magnetic pole placed near to it; then by moving the
magnet to and from the helix, or the helix to and from the magnet, currents
were produced indicated by the galvanometer.

1725. The thick copper plate (1721.) was afterwards interposed between the
magnetic pole and the helix; nevertheless on moving these to and fro,
effects, exactly the same in direction and amount, were obtained as if the
copper had not been there. So also on introducing a plate of sulphur into
the interval, not the least influence on the currents produced by motion of
the magnet or coils could be obtained.

1726. These results, with many others which I have not thought it needful
to describe, would lead to the conclusion that (judging by the _amount_ of
effect produced at a distance by forces transverse to the electric current,
i.e. magnetic forces,) the intervening matter, and therefore the
intervening particles, have nothing to do with the phenomena; or in other
words, that though the inductive force of static electricity is transmitted
to a distance by the action of the intermediate particles (1164. 1666.),
the transverse inductive force of currents, which can also act at a
distance, is not transmitted by the intermediate particles in a similar
way.

1727. It is however very evident that such a conclusion cannot be
considered as proved. Thus when the metal copper is between the pole and
the helix (1715. 1719. 1725.) or between the two helices (1721.) we know
that its particles are affected, and can by proper arrangements make their
peculiar state for the time very evident by the production of either
electrical or magnetical effects. It seems impossible to consider this
effect on the particles of the intervening matter as independent of that
produced by the inductric coil or magnet C, on the inducteous coil or core
A (1715. 1721.); for since the inducteous body is equally affected by the
inductric body whether these intervening and affected particles of copper
are present or not (1723. 1725.), such a supposition would imply that the
particles so affected had no reaction back on the original inductric
forces. The more reasonable conclusion, as it appears to me, is, to
consider these affected particles as efficient in continuing the action
onwards from the inductric to the inducteous body, and by this very
communication producing the effect of _no loss_ of induced power at the
latter.

1728. But then it may be asked what is the relation of the particles of
insulating bodies, such as air, sulphur, or lac, when _they_ intervene in
the line of magnetic action? The answer to this is at present merely
conjectural. I have long thought there must be a particular condition of
such bodies corresponding to the state which causes currents in metals and
other conductors (26. 53. 191. 201. 213.); and considering that the bodies
are insulators one would expect that state to be one of tension. I have by
rotating non-conducting bodies near magnetic poles and poles near them, and
also by causing powerful electric currents to be suddenly formed and to
cease around and about insulators in various directions, endeavoured to
make some such state sensible, but have not succeeded. Nevertheless, as any
such state must be of exceedingly low intensity, because of the feeble
intensity of the currents which are used to induce it, it may well be that
the state may exist, and may be discoverable by some more expert
experimentalist, though I have not been able to make it sensible.

1729. It appears to me possible, therefore, and even probable, that
magnetic action may be communicated to a distance by the action of the
intervening particles, in a manner having a relation to the way in which
the inductive forces of static electricity are transferred to a distance
(1677.); the intervening particles assuming for the time more or less of a
peculiar condition, which (though with a very imperfect idea) I have
several times expressed by the term _electro-tonic state_ (60. 242. 1114.
1661.). I hope it will not be understood that I hold the settled opinion
that such is the case. I would rather in fact have proved the contrary,
namely, that magnetic forces are quite independent of the matter
intervening between the inductric and the inductions bodies; but I cannot
get over the difficulty presented by such substances as copper, silver,
lead, gold, carbon, and even aqueous solutions (201. 213.), which though
they are known to assume a peculiar state whilst intervening between the
bodies acting and acted upon (1727.), no more interfere with the final
result than those which have as yet had no peculiarity of condition
discovered in them.

1730. A remark important to the whole of this investigation ought to be
made here. Although I think the galvanometer used as I have described it
(1711. 1720.) is quite sufficient to prove that the final amount of action
on each of the two coils or the two cores A and B (1713. 1719.) is equal,
yet there is an effect which _may_ be consequent on the difference of
action of two interposed bodies which it would not show. As time enters as
an element into these actions[A] (125.), it is very possible that the
induced actions on the helices or cores A, B, though they rise to the same
degree when air and copper, or air and lac are contrasted as intervening
substances, do not do so in the same time; and yet, because of the length
of time occupied by a vibration of the needle, this difference may not be
visible, both effects rising to their maximum in periods so short as to
make no sensible portion of that required for a vibration of the needle,
and so exert no visible influence upon it.

  [A] See Annnles de Chimie, 1833, tom. li. pp. 422, 428.

       *       *       *       *       *

1731. If the lateral or transverse force of electrical currents, or what
appears to be the same thing, magnetic power, could be proved to be
influential at a distance independently of the intervening contiguous
particles, then, as it appears to me, a real distinction of a high and
important kind, would be established between the natures of these two
forces (1654. 1664.). I do not mean that the powers are independent of each
other and might be rendered separately active, on the contrary they are
probably essentially associated (1654.), but it by no means follows that
they are of the same nature. In common statical induction, in conduction,
and in electrolyzation, the forces at the opposite extremities of the
particles which coincide with the lines of action and have commonly been
distinguished by the term electric, are polar, and in the cases of
contiguous particles act only to insensible distances; whilst those which
are transverse to the direction of these lines, and are called magnetic,
are circumferential, act at a distance, and if not through the mediation of
the intervening particles, have their relations to ordinary matter entirely
unlike those of the electrical forces with which they are associated.

1732. To decide this question of the identity or distinction of the two
kinds of power, and establish their true relation, would be exceedingly
important. The question seems fully within the reach of experiment, and
offers a high reward to him who will attempt its settlement.

1733. I have already expressed a hope of finding an effect or condition
which shall be to statical electricity what magnetic force is to current
electricity (1658.). If I could have proved to my own satisfaction that
magnetic forces extended their influence to a distance by the conjoined
action of the intervening particles in a manner analogous to that of
electrical forces, then I should have thought that the natural tension of
the lines of inductive action (1659.), or that state so often hinted at as
the electro-tonic state (1661. 1662.), was this related condition of
statical electricity.

1734. It may be said that the state of _no lateral action_ is to static or
inductive force the equivalent of _magnetism_ to current force; but that
can only be upon the view that electric and magnetic action are in their
nature essentially different (1664.). If they are the same power, the whole
difference in the results being the consequence of the difference of
_direction_, then the normal or _undeveloped_ state of electric force will
correspond with the state of _no lateral action_ of the magnetic state of
the force; the electric current will correspond with the lateral effects
commonly called magnetism; but the state of static induction which is
between the normal condition and the current will still require a
corresponding lateral condition in the magnetic series, presenting its own
peculiar phenomena; for it can hardly be supposed that the normal electric,
and the inductive or polarized electric, condition, can both have the same
lateral relation. If magnetism be a separate and a higher relation of the
powers developed, then perhaps the argument which presses for this third
condition of that force would not be so strong.

1735. I cannot conclude these general remarks upon the relation of the
electric and magnetic forces without expressing my surprise at the results
obtained with the copper plate (1724. 1725.). The experiments with the flat
helices represent one of the simplest cases of the induction of electrical
currents (1720.); the effect, as is well known, consisting in the
production of a momentary current in a wire at the instant when a current
in the contrary direction begins to pass through a neighbouring parallel
wire, and the production of an equally brief current in the reverse
direction when the determining current is stopped (26.). Such being the
case, it seems very extraordinary that this induced current which takes
place in the helix A when there is only air between A and C (1720.). should
be equally strong when that air is replaced by an enormous mass of that
excellently conducting metal copper (1721.). It might have been supposed
that this mass would have allowed of the formation and discharge of almost
any quantity of currents in it, which the helix C was competent to induce,
and so in some degree have diminished if not altogether prevented the
effect in A: instead of which, though we can hardly doubt that an infinity
of currents are formed at the moment in the copper plate, still not the
smallest diminution or alteration of the effect in A appears (65.). Almost
the only way of reconciling this effect with generally received notions is,
as it appears to me, to admit that magnetic action is communicated by the
action of the intervening particles (1729. 1733.).

1736. This condition of things, which is very remarkable, accords perfectly
with the effects observed in solid helices where wires are coiled over
wires to the amount of five or six or more layers in succession, no
diminution of effect on the outer ones being occasioned by those within.


§ _22. Note on electrical excitation._


1737. That the different modes in which electrical excitement takes place
will some day or other be reduced under one common law can hardly be
doubted, though for the present we are bound to admit distinctions. It will
be a great point gained when these distinctions are, not removed, but
understood.

1738. The strict relation of the electrical and chemical powers renders the
chemical mode of excitement the most instructive of all, and the case of
two isolated combining particles is probably the simplest that we possess.
Here however the action is local, and we still want such a test of
electricity as shall apply to it, to cases of current electricity, and also
to those of static induction. Whenever by virtue of the previously combined
condition of some of the acting particles (923.) we are enabled, as in the
voltaic pile, to expand or convert the local action into a current, then
chemical action can be traced through its variations to the production of
_all_ the phenomena of tension and the static state, these being in every
respect the same as if the electric forces producing them had been
developed by friction.

1739. It was Berzelius, I believe, who first spoke of the aptness of
certain particles to assume opposite states when in presence of each other
(959.). Hypothetically we may suppose these states to increase in intensity
by increased approximation, or by heat, &c. until at a certain point
combination occurs, accompanied by such an arrangement of the forces of the
two particles between themselves as is equivalent to a discharge, producing
at the same time a particle which is throughout a conductor (1700.).

1740. This aptness to assume an excited electrical state (which is probably
polar in those forming non-conducting matter) appears to be a primary fact,
and to partake of the nature of induction (1162.), for the particles do not
seem capable of retaining their particular state independently of each
other (1177.) or of matter in the opposite state. What appears to be
definite about the particles of matter is their assumption of a
_particular_ state, as the positive or negative, in relation to each other,
and not of either one or other indifferently; and also the acquirement of
force up to a certain amount.

1741. It is easily conceivable that the same force which causes local
action between two free particles shall produce current force if one of the
particles is previously in combination, forming part of an electrolyte
(923. 1738.). Thus a particle of zinc, and one of oxygen, when in presence
of each other, exert their inductive forces (1740.), and these at last rise
up to the point of combination. If the oxygen be previously in union with
hydrogen, it is held so combined by an analogous exertion and arrangement
of the forces; and as the forces of the oxygen and hydrogen are for the
time of combination mutually engaged and related, so when the superior
relation of the forces between the oxygen and zinc come into play, the
induction of the former or oxygen towards the metal cannot be brought on
and increased without a corresponding deficiency in its induction towards
the hydrogen with which it is in combination (for the amount of force in a
particle is considered as definite), and the latter therefore has its force
turned towards the oxygen of the next particle of water; thus the effect
may be considered as extended to sensible distances, and thrown into the
condition of static induction, which being discharged and then removed by
the action of other particles produces currents.

1742. In the common voltaic battery, the current is occasioned by the
tendency of the zinc to take the oxygen of the water from the hydrogen, the
effective action being at the place where the oxygen leaves the previously
existing electrolyte. But Schoenbein has arranged a battery in which the
effective action is at the other extremity of this essential part of the
arrangement, namely, where oxygen goes to the electrolyte[A]. The first may
be considered as a case where the current is put into motion by the
abstraction of oxygen from hydrogen, the latter by that of hydrogen from
oxygen. The direction of the electric current is in both cases the same,
when referred to the direction in which the elementary particles of the
electrolyte are moving (923. 962.), and both are equally in accordance with
the hypothetical view of the inductive action of the particles just
described (1740.).

  [A] Philosophical Magazine, 1838, xii. 225, 315. also De la Rive's
  results with peroxide of manganese. Annales de Chimie, 1836, lxi. p.
  40.--_Dec. 1838._

1743. In such a view of voltaic excitement, the action of the particles may
be divided into two parts, that which occurs whilst the force in a particle
of oxygen is rising towards a particle of zinc acting on it, and falling
towards the particle of hydrogen with which it is associated (this being
the progressive period of the inductive action), and that which occurs when
the change of association takes place, and the particle of oxygen leaves
the hydrogen and combines with the zinc. The former appears to be that
which produces the current, or if there be no current, produces the state
of tension at the termination of the battery; whilst the latter, by
terminating for the time the influence of the particles which have been
active, allows of others coming into play, and so the effect of current is
continued.

1744. It seems highly probable, that excitement by friction may very
frequently be of the same character. Wollaston endeavoured to refer such
excitement to chemical action[A]; but if by chemical action ultimate union
of the acting particles is intended, then there are plenty of cases which
are opposed to such a view. Davy mentions some such, and for my own part I
feel no difficulty in admitting other means of electrical excitement than
chemical action, especially if by chemical action is meant a final
combination of the particles.

  [A] Philosophical Transactions, 1801, p. 427.

1745. Davy refers experimentally to the opposite states which two particles
having opposite chemical relations can assume when they are brought into
the close vicinity of each other, but _not_ allowed to combine[A]. This, I
think, is the first part of the action already described (1743.); but in my
opinion it cannot give rise to a continuous current unless combination take
place, so as to allow other particles to act successively in the same
manner, and not even then unless one set of the particles be present as an
element of an electrolyte (923. 963.); i.e. mere quiescent contact alone
without chemical action does not in such cases produce a _current_.

  [A] Philosophical Transactions, 1807, p. 31.

1746. Still it seems very possible that such a relation may produce a high
charge, and thus give rise to excitement by friction. When two bodies are
rubbed together to produce electricity in the usual way, one at least must
be an insulator. During the act of rubbing, the particles of opposite kinds
must be brought more or less closely together, the few which are most
favourably circumstanced being in such close contact as to be short only of
that which is consequent upon chemical combination. At such moments they
may acquire by their mutual induction (1740.) and partial discharge to each
other, very exalted opposite states, and when, the moment after, they are
by the progress of the rub removed from each other's vicinity, they will
retain this state if both bodies be insulators, and exhibit them upon their
complete separation.

1747. All the circumstances attending friction seem to me to favour such a
view. The irregularities of form and pressure will cause that the particles
of the two rubbing surfaces will be at very variable distances, only a few
at once being in that very close relation which is probably necessary for
the development of the forces; further, those which are nearest at one time
will be further removed at another, and others will become the nearest, and
so by continuing the friction many will in succession be excited. Finally,
the lateral direction of the separation in rubbing seems to me the best
fitted to bring many pairs of particles, first of all into that close
vicinity necessary for their assuming the opposite states by relation to
each other, and then to remove them from each other's influence whilst they
retain that state.

1748. It would be easy, on the same view, to explain hypothetically, how,
if one of the rubbing bodies be a conductor, as the amalgam of an
electrical machine, the state of the other when it comes from under the
friction is (as a mass) exalted; but it would be folly to go far into such
speculation before that already advanced has been confirmed or corrected by
fit experimental evidence. I do not wish it to be supposed that I think all
excitement by friction is of this kind; on the contrary, certain
experiments lead me to believe, that in many cases, and perhaps in all,
effects of a thermo-electric nature conduce to the ultimate effect; and
there are very probably other causes of electric disturbance influential at
the same time, which we have not as yet distinguished.

_Royal Institution.
June, 1838._




INDEX.

       *       *       *       *       *

N.B. A dash rule represents the _italics_ immediately preceding it. The
references are sometimes to the individual paragraph, and sometimes to that
in conjunction with those which follow.

       *       *       *       *       *

_Absolute_ charge of matter, 1169.
---- quantity of electricity in matter, 852, 861, 873.
Acetate of potassa, its electrolysis, 749.
Acetates, their electrolysis, 774.
Acetic acid, its electrolysis, 773.
_Acid_, nitric, formed in air by a spark, 324.
----, or alkali, alike in exciting the pile, 932.
----, transference of, 525.
---- _for battery_, its nature and strength, 1128, 1137.
---- ----, nitric, the best, 1138.
---- ----, effect of different strengths, 1139.
---- _in voltaic pile_, does not evolve the electricity, 925, 933.
---- ----, its use, 925.
Acids and bases, their relation in the voltaic pile, 927, 933.
Active battery, general remarks on, 1034, 1136.
Adhesion of fluids to metals, 1038.
Advantages of a new voltaic battery, 1132.
_Affinities, chemical_, opposed voltaically, 891, 904, 910.
----, their relation in the active pile, 949.
_Air_, its attraction by surfaces, 622.
----, _charge of_, 1173.
----, ----, by brush, 1434, 1441.
----, ----, by glow, 1537, 1543.
----, convective currents in, 1572, 1576, 1581.
----, dark discharge in, 1548.
----, disruptive discharge in, 1359, 1406, 1425, 1526.
----, induction in, 1208, 1215, 1284, 1362.
----, its insulating and conducting power, 411, 1332, 1336, 1362.
----, its rarefaction facilitates discharge, 1375.
----, electrified, 1443.
----, electro-chemical decompositions in, 454, 1623.
----, hot, discharges voltaic battery, 271, 274.
----, poles of, 455, 461, 559.
----, _positive and negative_ brush in, 1467, 1472, 1476.
----, ---- glow in, 1526, 1530.
----, ---- spark in, 1485.
----, rarefied, brush in, 1451, 1456.
----, retention of electricity on conductors by, 1377, 1398.
----, _specific inductive capacity of_, 1284.
----, ----, not varied by temperature or pressure, 1287, 1288.
_Alkali_ has strong exciting power in voltaic pile, 884, 931, 941.
----, transference of, 525.
_Amalgamated zinc_, its condition, 1000.
----, how prepared, 863.
----, its valuable use, 863, 999.
---- battery, 1001.
_Ammonia_, nature of its electrolysis, 748.
----, solution of, a bad conductor, 554, 748.
Ampère's inductive results, 78, 255, 379 _note_.
_Anions_ defined, 665, 824.
----, table of, 847.
---- related through the entire circuit, 963.
----, their action in the voltaic pile, 924.
----, their direction of transfer, 962,
Anode defined, 663.
_Antimony_, its relation to magneto-electric induction, 139.
----, chloride of, not an electrolyte, 690, 796.
----, oxide of, how affected by the electric current, 801.
---- _supposed new_ protoxide, 693.
---- ----, sulphuret, 694.
_Animal electricity_, its general characters considered, 351.
---- is identical with other electricities, 354, 360.
----, its chemical force, 355.
----, enormous amount, 359.
----, evolution of heat, 353,
----, magnetic force, 351.
----, physiological effects, 357.
----, spark, 358.
----, tension, 352.
Apparatus, inductive, 1187. _See_ Inductive apparatus.
_Arago's magnetic phenomena_, their nature, 81, 120.
----, reason why no effect if no motion, 126,
----, direction of motion accounted for, 121.
----, due to induced electric currents, 119, 248.
----, like electro-magnetic rotations in principle, 121.
----, not due to direct induction of magnetism, 128, 138, 215, 243, 248.
----, obtained with electro-magnets, 129.
----, produced by conductors only, 130, 215.
----, time an element in, 124.
----, Babbage and Hershel's results explained, 127.
Arago's experiment, Sturgeon's form of, 219.
Associated voltaic circles, 989.
_Atmospheric_ balls of fire, 1611.
----, electricity, its chemical action, 336.
Atomic number judged of from electrochemical equivalent, 851.
_Atoms of matter_, 869, 1703.
----, their electric power, 856, 860.
Attraction of particles, its influence in Döbereiner's phenomena, 619.
_Attractions_, electric, their force, 1022 _note_.
----, _chemic, produce_ current force, 852, 918, 947, 996, 1741.
----, ---- local force, 852, 921, 947, 959, 1739.
----, hygrometric, 621.
Aurora borealis referred to magneto-electric induction, 192.
Axis of power, the electric current on, 517, 1627, 1642.

Balls of fire, atmospheric, 1611.
Barlow's revolving globe, magnetic effects explained, 137, 160.
Barry, decomposed bodies by atmospheric electricity, 338.
Bases and acids, their relation in the pile, 927.
Battery, Leyden, that generally used, 291.
_Battery, voltaic_, its nature, 856, 989.
----, _origin of its power_, 878, 989.
----, ---- not in contact, 887, 915,
----, ---- chemical, 879, 916, 919, 1741.
----, ----, oxidation of the zinc, 919, 944.
----, its circulating force, 858, 1120.
----, its local force, 1120.
----, quantity of electricity circulating, 990.
----, intensity of electricity circulating, 990, 993.
----, _intensity of its current_, 909, 994.
----, ---- increased, 905, 989.
----, _its diminution in power_, 1035.
----, ---- _from_ adhesion of fluid, 1003, 1136.
----, ---- ---- peculiar state of metal, 1040.
----, ---- ---- exhaustion of charge, 1042.
----, ---- ---- irregularity of plates, 1045, 1146.
----, use of metallic contact in, 893, 896.
----, _electrolytes essential to it_, 921.
----, ----, why, 858, 923.
----, state of metal and electrolyte before contact, 916.
----, conspiring action of associated affinities, 989.
----, purity of its zinc, 1144.
----, use of amalgamated zinc in, 999.
----, _plates, their_ number, 1151.
----, ---- size, 1154.
----, ---- vicinity, 1148.
----, ---- immersion, 1150.
----, ---- relative age, 1146.
----, ---- foulness, 1145.
----, _excited by_ acid, 880, 926, 1137.
----, ---- alkali, 931, 934, 941.
----, ---- sulphuretted solutions, 943.
----, the acid, its use, 925,
----, acid for, 1128, 1137.
----, nitric acid best for, 1137.
----, construction of, 989, 1001, 1121.
----, with numerous alternations, 989.
----, Hare's, 1123.
----, general remarks on, 1031. 1136.
----, simultaneous decompositions with, 1156.
----, practical results with, 1136.
----, _improved_, 1001, 1006, 1120.
----, ----, its construction, 1124.
----, ----, power, 1125, 1128.
----, ----, advantages, 1132.
----, ----, disadvantages, 1132.
Batteries, voltaic, compared, 1126.
Becquerel, his important secondary results, 745, 784.
Berzelius, his view of combustion, 870, 959.
Biot's theory of electro-chemical decomposition, 486.
Bismuth, its relation to magneto-electric induction, 139.
_Bodies_ classed in relation to the electric current, 823.
---- classed in relation to magnetism, 255.
Bodies electrolyzable, 824.
Bonijol decomposed substances by atmospheric electricity, 336.
Boracic acid a bad conductor, 408.
_Brush, electric_, 1425.
----, produced, 1425.
----, not affected by nature of conductors, 1454, 1473.
----, is affected by the dielectrics, 1455, 1463, 1475.
----, not dependent on current of air, 1440.
----, proves molecular action of dielectric, 1449, 1450.
----, its analysis, 1427, 1433.
----, nature, 1434, 1441, 1447.
----, form, 1428, 1449, 1451.
----, _ramifications_, 1439.
---- ----, their coalescence, 1453.
----, sound, 1426, 1431.
----, requisite intensity for, 1446.
---- has sensible duration, 1437.
---- is intermitting, 1427, 1431, 1451.
----, _light of_, 1444, 1445, 1451.
----, ----, in different gases, 1446, 1454.
----, dark? 1444, 1552.
----, passes into spark, 1448.
----, spark and glow relation of, 1533, 1539, 1542.
----, in gases, 1454, 1463, 1476.
----, oxygen, 1457, 1476.
----, nitrogen, 1458, 1476.
----, hydrogen, 1459, 1476.
----, coal-gas, 1460, 1476.
----, carbonic acid gas, 1461, 1476.
----, muriatic acid gas, 1462, 1476.
----, rare air, 1451, 1455, 1474.
----, oil of turpentine, 1452.
----, positive, 1455, 1467, 1484.
----, _negative_, 1468, 1472, 1484.
----, ----, of rapid recurrence, 1468, 1491.
----, positive and negative in different gases, 1455, 1475, 1506.

_Capacity, specific inductive_, 1252.
----. _See_ Specific inductive capacity.
_Carbonic acid gas_ facilitates formation of spark, 1463.
----, brush in, 1461, 1476.
----, glow in, 1534.
----, spark in, 1422, 1463.
----, _positive and negative_ brush in, 1476.
----, ---- discharge in, 1546.
----, non-interference of, 645, 652.
Carbonic oxide gas, interference of, 645, 652.
_Carrying discharge_, 1562.
----. _See_ Discharge convective.
Cathode described, 663, 824.
_Cations_, or cathions, described, 665, 824.
----, table of, 817.
----, direction of their transfer, 962.
Cations, are in relation through the entire circuit, 963.
_Characters of_ electricity, table of, 360.
---- the electric current, constant, 1618, 1627.
---- voltaic electricity, 268.
---- ordinary electricity, 284.
---- magneto-electricity, 343.
---- thermo-electricity, 349.
---- animal electricity, 351.
_Charge_, free, 1684.
---- is always induction, 1171, 1177, 1300, 1682.
---- on surface of conductors: why, 1301.
----. _influence of_ form on, 1302.
----, ---- distance on, 1303.
----, loss of, by convection, 1569.
----, removed from good insulators, 1203.
---- of matter, absolute, 1169.
---- _of air_, 1173.
---- ---- by brush, 1434, 1441.
---- ---- by glow, 1526, 1537, 1543.
---- of particles in air, 1564.
---- of oil of turpentine, 1172.
---- of inductive apparatus divided, 1208.
----, residual, of a Leyden jar, 1249.
----, _chemical, for battery_, good, 1137.
-----, ----, weak and exhausted, 1042, 1143.
_Chemical action_, the, exciting the pile is oxidation, 921.
---- _superinduced by_ metals, 564.
---- ---- platina, 564, 617, 630.
---- tested by iodide of potassium, 315.
Chemical actions, distant, opposed to each other, 891, 910, 1007.
_Chemical affinity_ influenced by mechanical forces, 656.
---- transferable through metals, 918.
---- statical or local, 852, 921, 917, 959.
---- current, 852, 918, 947, 996.
_Chemical decomposition by_ voltaic electricity, 278, 450, 661.
---- common electricity, 309, 453.
---- magneto-electricity, 346.
---- thermo-electricity, 349.
---- animal electricity, 355.
----. _See_ Decomposition electro-chemical.
Chemical and electrical forces identical, 877, 918, 947, 960, 965, 1031.
_Chloride of_ antimony not an electrolyte, 690.
---- _lead_, its electrolysis, 794, 815.
---- ----, electrolytic intensity for, 978.
---- _silver_, its electrolysis, 541, 813, 902.
---- ----, electrolytic intensity for, 979.
---- tin, its electrolysis, 789, 819.
_Chlorides in_ solution, their electrolysis, 766.
---- fusion, their electrolysis, 789, 813.
Circle of anions and cathions, 963.
_Circles_, simple voltaic, 875.
----, associated voltaic, 989.
Circuit, voltaic, relation of bodies in, 962.
_Classification of bodies in relation to_ magnetism, 255.
---- the electric current, 823, 817.
Cleanliness of metals and other solids, 633.
_Clean platina_, its characters, 633, 717.
----, _its power of effecting combination_, 590, 605, 617, 632.
----, ----. _See_ Plates of platina.
_Coal gas_, brush in, 1460.
----, dark discharge in, 1556.
----, positive and negative brush in, 1476.
----, positive and negative discharge in, 1515.
----, spark in, 1422.
Colladon on magnetic force of common electricity, 289.
Collectors, magneto-electric, 86.
_Combination effected by_ metals, 564, 608.
---- solids, 564, 618.
---- poles of platina, 566.
---- _platina_, 564, 568, 571, 590, 630.
---- ----, as plates, 569.
---- ----, as sponge, 609, 636.
---- ----, cause of, 590, 616, 625, 656.
---- ----, how, 630.
---- ----, interferences with, 638, 652, 655.
---- ---- _retarded by_ olefiant gas, 640.
---- ---- ---- carbonic oxide, 645, 652.
---- ---- ---- sulphuret of carbon, 650.
---- ---- ---- ether, 651.
---- ---- ---- other substances, 649, 653, 654.
Comparison of voltaic batteries, 1126, 1146.
_Conditions_, general, of voltaic decomposition, 669.
----, new, of electro-chemical decomposition, 453.
_Conducting power_ measured by a magnet, 216.
---- of solid electrolytes, 419.
---- of water, constant, 984.
_Conduction_, 418, 1320.
----, its nature, 1320, 1326, 1611.
----, of two kinds, 987.
----, preceded by induction, 1329, 1332, 1338.
---- and insulation, cases of the same kind, 1320, 1326, 1336, 1338, 1561.
----, its relation to the intensity of the current conducted, 419.
---- common to all bodies, 444, 449.
---- by a vacuum, 1613.
---- by lac, 1234, 1324.
---- by sulphur, 1241, 1328.
---- by glass, 1239, 1324.
---- by spermaceti, 1240, 1323.
---- by gases, 1336.
----, slow, 1233, 1245, 1328.
---- affected by temperature, 445, 1339.
---- by metals diminished by heat, 432, 445.
---- increased by heat, 432, 441, 445.
---- of electricity and heat, relation of, 416.
----, _simple, can occur in electrolytes_, 967, 983.
----, ---- with very feeble currents, 970.
---- by electrolytes without decomposition, 968, 1017, 1032.
---- and decomposition associated in electrolytes, 413, 676, 854.
---- facilitated in electrolytes, 1355.
---- _by water_ bad, 1159.
---- ---- improved by dissolved bodies, 984, 1355.
----, electrolytic, stopped, 380, 1358, 1705.
---- of currents stopped by ice, 381.
---- conferred by liquefaction, 394, 410.
---- _taken away by solidification_, 394, 1705.
---- ---- why, 910, 1705.
----, _new law of_, 380, 394, 410.
----, ----, supposed exception to, 691, 1340.
----, general results as to, 443.
Conductive discharge, 1320.
_Conductors_, electrolytic, 474.
----, magneto-electric, 86.
----, their nature does not affect the electric brush, 1454.
----, size of, affects discharge, 1372.
----, form of, affects discharge, 1374, 1425.
----, _distribution of electricity on_, 1368.
----, ----, _affected by_ form, 1374.
----, ----, ---- distance, 1364, 1371.
----, ----, ---- air pressure, 1375.
----, ----, irregular with equal pressure, 1378.
Constancy of electric current, 1618.
_Constitution of electrolytes as to_ proportions, 679, 697, 830, 1708.
---- liquidity, 394, 823.
_Contact of metals_ not necessary for electrolyzation, 879.
----, its use in the voltaic battery, 893.
---- not necessary for spark, 915, 956.
_Contiguous particles_, their relation to induction, 1165, 1679.
---- active in electrolysis, 1349, 1703.
_Convection_, 1562, 1642.
---- or convective discharge. _See_ Discharge convective.
Copper, iron, and sulphur circle, 943.
Coruscations of lightning, 1464.
_Coulomb's electrometer_, 1180.
----, precautions in its use, 1182, 1186, 1206.
Crystals, induction through, 1689.
Cube, large, electrified, 1173.
Cubes of crystals, induction through, 1692, 1695.
Current chemical affinity, 852, 918, 947, 996.
Current, voltaic, without metallic contact, 879, 887.
_Current, electric_, 1617.
----, defined, 282, 511.
----, nature of, 511, 667, 1617, 1627.
----, variously produced, 1618.
----, _produced by_ chemical action, 879, 916, 1741.
----, ---- animals, 351.
----, ---- friction, 301, 307, 311.
----, ---- heat, 349,
----, ---- discharge of static electricity, 296, 307, 363.
----, ---- _induction by_ other currents, 6, 1089.
----, ---- ---- magnets, 30, 88, 344.
----, evolved in the moving earth, 181.
----, in the earth, 187.
----, natural standard of direction, 663.
----, none of one electricity, 1627, 1632, 1635.
----, two forces everywhere in it, 1627, 1632, 1635, 1642.
----, one, and indivisible, 1627.
----, an axis of power, 517, 1642.
----, constant in its characters, 1618, 1627.
----, inexhaustibility of, 1631.
----, _its velocity in_ conduction, 1648.
----, ---- electrolyzation, 1651.
----, regulated by a fine wire, 853, _note_.
----, affected by heat, 1637.
----, stopped by solidification, 381.
----, _its section_, 498, 504, 1634.
----, ---- presents a constant force, 1634.
----, _produces_ chemical phenomena, 1621.
----, ---- heat, 1625.
----, its heating power uniform, 1630.
----, produces magnetism, 1653.
----, Porrett's effects produced by, 1646.
----, _induction of_, 1, 6, 232, 241, 1101, 1048.
----, ----, on itself, 1048.
----, ----. _See_ Induction of electric current.
----, its inductive force lateral, 1108.
----, induced in different metals, 193, 213, 201, 211.
----, _its transverse effects_, 1653.
----, ---- constant, 1655.
----, _its transverse forces_, 1658.
----, ---- are in relation to contiguous particles, 1664.
----, ---- their polarity of character, 1665.
---- and magnet, their relation remembered, 38, _note_.
_Currents_ in air by convection, 1572, 1581.
----, metals by convection, 1603.
----, oil of turpentine by convection, 1595, 1598.
Curved lines, induction in, 1215.
Curves, magnetic, their relation to dynamic induction, 217, 232.

Daniell on the size of the voltaic metals, 1525.
_Dark discharge_,1444, 1544.
----. _See_ Discharge, dark.
Dates of some facts and publications, 139, _note after_.
_Davy's_ theory of electro-chemical decomposition, 482, 500.
---- electro-chemical views, 965.
---- mercurial cones, convective phenomena, 1603.
_Decomposing force_ alike in every section of the current, 501, 505.
----, variation of, on each particle, 503.
_Decomposition_ and conduction associated in electrolytes, 413, 854.
----, primary and secondary results of, 742, 777.
---- _by common electricity_, 309, 454.
---- ----, precautions, 322.
_Decomposition, electro-chemical_, 450, 669.
----, nomenclature of, 661.
----, new terms relating to, 662.
----, its distinguishing character, 309.
----, by common electricity, 309, 454.
----, by a single pair of plates, 862, 897, 904, 931.
----, by the electric current, 1621.
----, without metallic contact, 880, 882.
----, its cause, 891, 904, 910.
----, not due to direct attraction or repulsion of poles, 493, 497, 536,
  542, 5460.
----, _dependent on_ previous induction, 1345.
----, ---- the electric current, 493, 510, 524, 854.
----, ---- intensity of current, 905.
----, ---- chemical affinity of particles, 519, 525, 519.
----, resistance to, 891, 910, 1007.
----, intensity requisite for, 966, 1354.
----, stopped by solidification, 380, 1358, 1705.
----, retarded by interpositions, 1007.
----, assisted by dissolved bodies, 1355.
----, division of the electrolyte, 1347, 1623, 1701.
----, transference, 519, 525, 538, 550, 1347, 1706.
----, why elements appear at the poles, 535.
----, uncombined bodies do not travel, 544, 546.
----, circular series of effects, 562, 962.
----, simultaneous, 1156,
----, _definite_, 329, 372, 377, 504, 704, 714, 722, 726, 732, 764, 783,
  807, 821, 960.
----, ---- independent of variations of electrodes, 714, 722, 807, 832.
----, necessary intensity of current, 911, 966, 1345, 1354.
----, influence of water in, 472.
----, in air, 451, 461, 469.
----, some general conditions of, 669.
----, new conditions of, 453.
----, primary results, 742.
----, secondary results, 702, 742, 748, 777.
----, of acetates, 774.
----, acetic acid, 773.
----, ammonia, 748.
----, _chloride of_ antimony, 690, 796.
----, ---- lead, 794, 815.
----, ---- silver, 541, 813, 979.
----, _chlorides in_ solution, 766.
----, ---- fusion, 789, 913.
----, fused electrolytes, 789.
----, hydriodic acid and iodides, 767, 787.
----, hydrocyanic acid and cyanides, 771.
----, hydrofluoric acid and fluorides, 770.
----, _iodide of_ lead, 802, 818.
----, ---- potassium, 805.
----, muriatic acid, 758, 780.
----, nitre, 753.
----, nitric acid, 752.
----, _oxide_ antimony, 801.
----, ---- lead, 797.
----, protochloride of tin, 789, 819.
----, protiodide of tin, 804.
----, sugar, gum, &c., 776.
----, of sulphate of magnesia, 495.
----, sulphuric acid, 757.
----, sulphurous acid, 755.
----, tartaric acid, 775.
----, water, 704, 785, 807.
----, _theory of_, 477, 1345.
----, ----, by A. de la Rive, 489, 507, 514, 543.
----, ----, Biot, 486.
----, ----, Davy, 482, 500.
----, ----, Grotthuss, 481, 499, 515.
----, ----, Hachette, 491, 513,
----, ----, Riffault and Chompré, 485, 507, 512.
----, author's theory, 518, 524, 1345, 1623, 1703, 1766.
_Definite_ decomposing action of electricity, 329, 372, 377, 504, 704, 783,
  821.
----, magnetic action of electricity, 216, 362, 367, 377.
----, _electro-chemical action_, 822, 869, 960.
----, ----, general principles of, 822, 862.
----, ----, _in chloride of lead_, 815.
----, ----, ---- silver, 813.
----, ----, in hydriodic acid, 767, 787.
----, ----, iodide of lead, 802, 818.
----, ----, muriatic acid, 758, 786,
----, ----, protochloride of tin, 819.
----, ----, water, 732, 785, 807.
Degree in measuring electricity, proposal for, 736.
_De la Rive_ on heat at the electrodes, 1637.
----, his theory of electro-chemical decomposition, 489, 507, 514, 543.
_Dielectrics_, what, 1168.
----, their importance in electrical actions, 1666.
----, their relation to static induction, 1296.
----, their condition under induction, 1369, 1679.
----, their nature affects the brush, 1455.
----, their specific electric actions, 1296, 1398, 1423, 1454, 1503, 1560.
Difference of positive and negative discharge, 1465, 1480, 1485.
Differential inductometer, 1307.
_Direction of_ ions in the circuit, 962.
----, the electric current, 563.
----, the magneto-electric current, 114, 116.
----, the induced volta-electric current, 19, 26, 1091.
Disruptive discharge, 1359, 1405. _See_ Discharge, disruptive.
_Discharge, electric_, as balls of fire, 1641.
----, of Leyden jar, 1300.
----, _of voltaic battery by_ hot air, 271, 274.
----, ---- points, 272.
----, velocity of, in metal, varied, 1333.
----, varieties of, 1319.
----, brush, 1425. _See_ Brush.
----, carrying, 1562. _See_ Discharge, convective.
----, conductive, 1320. _See_ Conduction.
----, dark, 1444, 1544.
----, disruptive, 1359, 1405.
----, electrolytic, 1343, 1622, 1704.
----, glow, 1526. _See_ Glow.
----, positive and negative, 1465.
----, spark, 1406. _See_ Spark, electric.
_Discharge, connective_, 1442, 1562, 1601, 1623, 1633, 1642.
----, in insulating media, 1562, 1572.
----, in good conductors, 1603.
----, _with fluid terminations in_ air, 1581, 1589.
----, ---- liquids, 1597.
----, from a ball, 1576, 1590.
----, influence of points in, 1573.
----, _affected by_ mechanical causes, 1579.
----, ---- flame, 1580.
----, with glow, 1576.
----, _charge of a particle in_ air, 1564.
----, ---- oil of turpentine, 1570.
----, charge of air by, 1442, 1592.
----, _currents produced in_ air, 1572, 1581, 1591.
----, ---- oil of turpentine, 1595, 1598.
----, direction of the currents, 1599, 1645.
----, Porrett's effects, 1646,
----, positive and negative, 1593, 1600, 1643.
----, related to electrolytic discharge, 1622, 1633.
_Discharge, dark_, 1444, 1544, 1560.
----, with negative glow, 1544.
----, between positive and negative glow, 1547.
----, in air, 1548.
----, muriatic acid gas, 1554.
----, coal gas, 1556.
----, hydrogen, 1558.
----, nitrogen, 1559.
_Discharge, disruptive_, 1405.
----, preceded by induction, 1362.
----, determined by one particle, 1370, 1409.
----, necessary intensity, 1409, 1553.
----, determining intensity constant, 1410.
----, related to particular dielectric, 1503.
----, facilitates like action, 1417, 1435, 1453, 1553.
----, its time, 1418, 1436, 1498, 1641.
----, _varied by_ form of conductors, 1302, 1372, 1374.
----, ---- change in the dielectric, 1395, 1422,1454.
----, ---- rarefaction of air, 1365, 1375, 1451.
----, ---- temperature, 1367, 1380.
----, ---- distance of conductors, 1303, 1364, 1371.
----, ---- size of conductors, 1372.
----, in liquids and solids, 1403.
----, in _different gases_, 1381, 1388, 1421.
----, ---- not alike, 1395.
----, ---- specific differences, 1399, 1422, 1687.
----, _positive and negative_, 1393, 1399, 1465, 1524.
----, ----, distinctions, 1467, 1475, 1482.
----, ----, differences, 1485, 1501.
----, ----, relative facility, 1496, 1520.
----, ----, dependent on the dielectric, 1503.
----, ----, in different gases, 1506, 1510, 1518, 1687.
----, ----, of voltaic current, 1524.
----, brush, 1425.
----, collateral, 1412.
----, dark, 1444, 1544, 1560.
----, glow, 1526.
----, spark, 1406.
----, theory of, 1308, 1406, 1434.
_Discharge, electrolytic_, 1164, 1343, 1621, 1703, 1706.
----, previous induction, 1345, 1351.
----, necessary intensity, 912, 966, 1346, 1354.
----, division of the electrolyte, 1347, 1704.
----, stopped by solidifying the electrolyte, 380, 1358, 1705.
----, facilitated by added bodies, 1355.
----, in curved lines, 521, 1216, 1351.
----, proves action of contiguous particles, 1349.
----, positive and negative, 1525.
----, velocity of electric current in, 1650.
----, related to convective discharge, 1622.
----, theory of, 1344, 1622, 1704.
Discharging train generally used, 292.
Disruptive discharge, 1405. _See_ Discharge, disruptive.
Dissimulated electricity, 1684.
_Distance, its influence_ in induction, 1303, 1364,1371.
---- over disruptive discharge, 1364, 1371.
Distant chemical actions, connected and opposed, 891, 909.
Distinction of magnetic and magneto-electric action, 138, 215, 243, 253.
Division of a charge by inductive apparatus, 1208.
Döbereiner on combination effected by platina, 609, 610.
Dulong and Thenard on combination by platina and solids, 609, 611.
Dust, charge of its particles, 1567.

Earth, natural magneto-electric induction in, 181, 190, 192.
_Elasticity of_ gases, 626.
---- gaseous particles, 658.
_Electric_ brush, 1425. _See_ Brush, electric.
---- condition of particles of matter, 862, 1669.
---- conduction, 1320. _See_ Conduction.
---- _current_ defined, 283, 511.
---- ----, nature of, 511, 1617, 1627.
---- ----. _See_ Current, electric.
---- ----, _induction of_, 6, 232, 241, 1048, 1101. _See_ Induction of
  electric current.
---- ----, ----, on itself, 1048.
---- discharge, 1319. _See_ Discharge.
---- force, nature of, 1667. _See_ Forces.
---- induction, 1162. _See_ Induction.
---- inductive capacity, 1252. _See_ Specific inductive capacity.
---- polarity, 1685. _See_ Polarity, electric.
---- spark, 1406. _See_ Spark, electric.
---- and magnetic forces, their relation, 118, 1411, 1653, 1658, 1709,
  1731.
Electrics, charge of, 1171, 1247.
_Electrical_ excitation, 1737. _See_ Excitation.
---- machine generally used, 290.
---- battery generally used, 291.
---- and chemical forces identical, 877, 917, 947, 960, 965, 1031.
_Electricities_, their identity, however excited, 265, 360.
----, one or two, 516, 1667.
----, _two_, 1163.
----, ----, their independent existence, 1168.
----, ----, their inseparability, 1168, 1177, 1244.
----, ----, never separated in the current, 1628.
_Electricity_, quantity of, in matter, 852, 861.
----, _its distribution on conductors_, 1368.
----, ---- _influenced by_ form, 1302, 1374.
----, ---- ---- distance, 1303, 1364, 1371.
----, ---- ---- air's pressure, 1375.
----, relation of a vacuum to, 1613.
----, dissimulated, 1684.
----, common and voltaic, measured, 361, 860.
----, _its definite_ decomposing action, 329, 377, 783, 1621.
----, ---- heating action, 1625.
----, ---- magnetic action, 216, 366.
----, animal, its characters, 351.
----, magneto-, its characters, 343.
----, ordinary, its characters, 284.
----, thermo-, its characters, 349.
----, voltaic, its characters, 268.
_Electricity from magnetism_, 27, 36, 57, 83, 135, 140.
----, _on magnetisation of soft iron by_ currents, 27, 34, 57, 113.
---- ---- magnets, 36, 44.
----, _employing_ permanent magnets, 39, 84, 112.
----, ---- terrestrial magnetic force, 140, 150, 161.
----, ---- _moving conductors_, 55, 83, 132, 139, 149, 161, 171.
----, ---- ---- essential condition, 217.
---- _by revolving plate_, 83, 149, 240.
---- ---- a constant source of electricity, 89, 90, 154.
---- ----, law of evolution, 114.
---- ----, direction of the current evolved, 91, 99, 110, 116, 117.
---- ----, course of the currents in the plate, 123, 150.
---- by a revolving globe, 137, 160.
---- by plates, 94, 101.
---- by a wire, 49, 55, 109, 112, 137.
----, conductors and magnet may move together, 218.
----, _current produced_ in a single wire, 49, 55, 170.
----, ---- a ready source of electricity, 46, _note_.
----, ---- momentary, 28, 30, 47.
----, ---- permanent, 89, 154.
----, ---- deflects galvanometer, 30, 39, 46.
----, ---- makes magnets, 34.
----, ----, shock of, 56.
----, ----, spark of, 32.
----, ---- traverses fluids, 23, 33.
----, ----, its direction, 30, 38, 41, 52, 53, 54, 78, 91, 99, 114, 142,
  166, 220, 222.
----, effect of approximation and recession, 18, 39, 50.
----, the essential condition, 217.
----, general expression of the effects, 256.
----, from magnets alone, 220.
_Electricity of the voltaic pile_, 875.
---- _its source_, 875.
---- ---- not metallic contact, 887, 915.
---- ---- is in chemical action, 879, 916, 919, 1738, 1741.
_Electro-chemical decomposition_, 450, 661.
----, nomenclature, 661.
----, general conditions of, 669.
----, new conditions of, 453,
----, influence of water in, 472.
----, primary and secondary results, 742.
----, definite, 732, 783.
----, theory of, 477.
----. _See_ also Decomposition, electrochemical.
_Electro-chemical equivalents_, 824, 833, 835, 855.
----, table of, 847.
----, how ascertained, 837.
---- always consistent, 835.
---- same as chemical equivalents, 836, 839.
---- able to determine atomic number, 851.
Electro-chemical excitation, 878, 919, 1738.
Electrode defined, 662.
_Electrodes_ affected by heat, 1637.
----, _varied in_ size, 714, 722.
----, ---- nature, 807.
----. _See_ Poles.
Electrolysis, resistance to, 1007.
_Electrolyte_ defined, 664.
---- _exciting, solution of_ acid, 881, 925.
---- ---- alkali, 931, 941.
---- _exciting_, water, 944, 945.
---- ---- sulphuretted solution, 943.
_Electrolytes_, their necessary constitution, 669, 823, 829, 858, 921,
  1347, 1708.
---- consist of single proportionals of elements, 679, 697, 830, 1707.
---- _essential to voltaic pile_, 921.
---- ----, why, 858, 923.
---- conduct and decompose simultaneously, 413.
---- can conduct feeble currents without decomposition, 967.
----, as ordinary conductors, 970, 983, 1344.
----, solid, their insulating and conducting power, 419.
----, real conductive power not affected by dissolved matters, 1356.
----, needful conducting power, 1158.
---- are good conductors when fluid, 394, 823.
_Electrolytes non-conductors when solid_ 381, 394.
----, why, 910, 1705.
----, the exception, 1032.
_Electrolytes_, their particles polarize as wholes, 1700.
----, polarized light sent across, 951.
----, relation of their moving elements to the passing current, 923, 1704.
----, their resistance to decomposition, 891, 1007, 1705.
----, and metal, their states in the voltaic pile, 946.
----, salts considered as, 698.
----, acids not of this class, 681.
_Electrolytic_ action of the current, 478, 518, 1620.
---- conductors, 474.
---- discharge, 1343. _See_ Discharge, electrolytic.
---- induction, 1164, 1343.
---- _intensity_, 911, 966, 983.
---- ---- varies for different bodies, 912, 986, 1354.
---- ---- of chloride of lead, 978.
---- ---- chloride of silver, 979.
---- ---- sulphate of soda, 975.
---- ---- water, 968, 981.
---- ---- its natural relation, 987.
_Electrolyzation_, 450, 661, 1164, 1347, 1704. _See_ Decomposition
  electro-chemical.
---- defined, 664.
---- facilitated, 394, 417, 549, 1355.
---- in a single circuit, 863, 879.
----, intensity needful for, 919, 966,
---- of chloride of silver, 541, 979.
---- sulphate of magnesia, 495.
---- and conduction associated, 413, 676.
Electro-magnet, inductive effects in, 1060.
Electro-magnetic induction definite, 216, 366.
_Electrometer, Coulomb's_, described, 1180.
----, how used, 1183.
_Electro-tonic state_, 60, 231, 242, 1114, 1661, 1729.
---- _considered common to all_ metals, 66.
---- ---- conductors, 76.
---- is a state of tension, 71.
---- is dependent on particles, 73.
Elementary bodies probably ions, 849.
_Elements evolved_ by force of the current, 493, 520, 524.
---- at the poles, why, 535.
---- determined to either pole, 552, 681, 757.
----, transference of, 454, 538.
----, if not combined, do not travel, 544, 546.
_Equivalents_, electro-chemical, 824, 833, 855.
----, chemical and electro-chemical, the same, 836, 839.
Ether, interference of, 651.
_Evolution_ of electricity, 1162, 1737.
---- of one electric force impossible, 1175.
---- of elements at the poles, why, 535.
_Excitation_, electrical, 1737.
---- by chemical action, 878, 916, 1739.
---- by friction, 1744.
Exclusive induction, 1681.

Flame favours convectivc discharge, 1580.
Flowing water, electric currents in, 190.
Fluid terminations for convection, 1581.
Fluids, their adhesion to metals, 1038.
Fluoride of lead, hot, conducts well, 1340.
_Force, chemical_, local, 947, 959, 1739.
----, circulating, 917, 947, 996, 1120.
_Force_, electric, nature of, 1163, 1667.
----, inductive, of currents, its nature, 60, 1113, 1735.
_Forces, electric_, two, 1163.
----, inseparable, 1163, 1177, 1244, 1627.
---- and chemical, are the same, 877, 916.
---- _and magnetic_, relation of, 1411, 1653, 1658, 1709.
---- ----, are they essentially different? 1663, 1731.
_Forces, exciting, of voltaic apparatus_, 887, 916.
----, exalted, 905, 994, 1138, 1148.
_Forces_, polar, 1665.
---- _of the current_, direct, 1620.
---- ----, lateral or transverse, 1653, 1709.
_Form, its influence on_ induction, 1302, 1374.
---- discharge, 1372, 1374.
Fox, his terrestrial electric currents, 187.
_Friction_ electricity, its characters, 284.
----, excitement by, 1744.
Fusion, conduction consequent upon, 394, 402.
Fusinieri, on combination effected by platina, 613.

_Galvanometer_, affected by common electricity, 289, 366.
----, a correct measure of electricity, 367, _note_.
_Gases_, their elasticity, 626, 657.
----, conducting power, 1336.
----, _insulating power_, 1381, 1507.
----, ---- not alike, 1395, 1508.
----, _specific inductive capacity_, 1283, 1290.
----, ---- alike in all, 1292.
----, specific influence on brush and spark, 1463, 1687.
----, discharge, disruptive, through, 1381.
----, brush in, 1454.
----, spark in, 1421.
----, _positive and negative brushes in_, 1475.
----, ----, their differences, 1476.
----, positive and negative discharge in, 1393, 1506, 1687.
----, solubility of, in cases of electrolyzation, 717, 728.
----, from water, spontaneous recombination of, 566.
----, mixtures of, affected by platina plates, 571.
----, mixed, relation of their particles, 625.
_General_ principles of definite electrolytic action, 822.
---- remarks on voltaic batteries, 1031, 1136.
---- _results as to_ conduction, 443.
---- ---- induction, 1295.
_Glass_, its conducting power, 1239.
----, its specific inductive capacity, 1271.
----, _its attraction for_ air, 622.
----, ---- water, 1251.
_Globe, revolving of Barlow_, effects explained, 137, 160.
----, is magnetic, 164.
_Glow_, 1405, 1525.
----, produced, 1527.
----, positive, 1527.
----, negative, 1530.
----, favoured by rarefaction of air, 1529.
----, is a continuous charge of air, 1526, 1537, 1543.
----, occurs in all gases, 1534.
----, accompanied by a wind, 1535.
----, its nature, 1543,
----, discharge, 1526.
----, brush and spark relation of, 1533, 1538, 1539, 1542.
Grotthuss' theory of electro-chemical decomposition, 481, 499, 515.
_Growth of a_ brush, 1437.
---- spark, 1553.

Hachette's view of electro-chemical decomposition, 491.
Hare's voltaic trough, 1123, 1132.
Harris on induction in air, 1363.
_Heat_ affects the two electrodes, 1637.
---- increases the conducting power of some bodies, 432, 438, 1340.
----, its conduction related to that of electricity, 416.
----, as a result of the electric current, 853, _note_, 1625, 1630.
---- _evolved by_ animal electricity, 353.
---- ---- common electricity, 287.
---- ---- magneto-electricity, 344.
---- ---- thermo-electricity, 349.
---- ---- voltaic electricity, 276.
Helix, inductive effects in, 1061, 1094.
Hydriodic acid, its electrolyses, 767, 787.
Hydrocyanic acid, its electrolyses, 771, 788.
Hydrofluoric acid, not electrolysable, 770.
_Hydrogen_, brush in, 1459.
----, _positive and negative_ brush in, 1476.
----, ---- discharge in, 1514.
_Hydrogen and oxygen combined by_ platina plates, 570, 605.
---- spongy platina, 609.

_Ice_, its conducting power, 419.
---- a non-conductor of voltaic currents, 381.
Iceland crystal, induction across, 1695.
_Identity_, of electricities, 265, 360.
---- of chemical and electrical forces, 877, 917, 947, 961, 1031.
Ignition of wire by electric current, 853, _note_, 1630.
Improved voltaic battery, 1006, 1120.
Increase of cells in voltaic battery, effect of, 990.
Inducteous surfaces, 1483.
_Induction apparatus_, 1187.
----, fixing the stem, 1190, 1193, 1200.
----, precautions, 1194, 1199, 1213, 1232, 1250.
----, removal of charge, 1203.
----, retention of charge, 1205, 1207.
----, a charge divided, 1208.
----, peculiar effects with, 1233.
_Induction, static_, 1161.
----, an action of contiguous particles, 1165, 1231, 1253, 1295, 1450,
  1668, 1679.
----, consists in a polarity of particles, 1298, 1670, 1679.
----, continues only in insulators, 1298, 1324, 1338.
----, intensity of, sustained, 1362.
----, _influenced by the_ form of conductors, 1302.
----, ---- distance of conductors, 1303.
----, ---- relation of the bounding surfaces, 1483.
----, charge, a case of, 1171, 1177, 1300.
----, exclusive action, 1681.
----, towards space, 1614.
----, across a vacuum, 1614.
---- _through_ air, 1217, 1284.
---- ---- different gases, 1381, 1395.
---- ---- crystals, 1689,
---- ---- lac, 1228, 1255, 1308.
---- ---- metals, 1329, 1332.
---- ---- all bodies, 1331, 1334.
----, _its relation to_ other electrical actions, 1165, 1178.
----, ---- insulation, 1324, 13602, 1368, 1678.
----, ---- conduction, 1320.
----, ---- discharge, 1319, 1323, 1362.
----, ---- electrolyzation, 1164, 1343.
----, ---- intensity, 1178, 1362.
----, ---- excitation, 1178, 1740.
----, its relation to charge, 1177, 1299.
---- an essential general electric function, 1178, 1299.
----, general results as to, 1295.
----, theory of, 1165, 1231, 1295, 1667, 1669.
---- _in curved lines_, 1166, 1215, 1679.
---- ----, _through_ air, 1218, 1449.
---- ----, ---- other gases, 1226.
---- ----, ---- lac, 1228.
---- ----, ---- sulphur, 1228.
---- ----, ---- oil of turpentine, 1227.
_induction, specific_, 1167, 1252, 1307.
----, _the problem_ stated, 1252.
----, ---- solved, 1307.
----, _of air_, 1284.
----, ----, invariable, 1287, 1288.
----, _of gases_, 1283, 1290.
----, ---- alike in all, 1292.
----, of shell-lac, 1256, 1269.
----, glass, 1271.
----, sulphur, 1275.
----, spermaceti, 1279.
----, certain fluid insulators, 1280.
_Induction of electric currents_, 6, 34, 232, 241, 1048, 1089, 1101, 1660,
  1718.
----, on aiming the principal current, 10, 238, 1101.
----, on stopping the principal current, 10, 17, 238, 1087, 1100.
---- by approximation, 18, 236.
---- by increasing distance, 19, 237.
---- _effective through_ conductors, 1719, 1721, 1735.
---- ---- insulators, 1719, 1722, 1735.
---- in different metals, 193, 202, 211, 213.
---- in the moving earth, 181.
---- in flowing water, 190.
---- in revolving plates, 85, 240.
----, _the induced current, its_ direction, 26, 232.
----, ---- duration, 19, 47, 89.
----, ----, traverses fluids, 20, 23.
----, ----, its intensity in different conductors, 183, 193, 201, 211, 213.
----, ----, not obtained by Leyden discharge, 24.
----, Ampère's results, 78, 255, 379, _note_.
_Induction of a current on itself_, 1048, 1109.
----, apparatus used, 1052.
----, _in a_ long wire, 1064, 1068, 1092, 1118.
----, ---- doubled wire, 1096.
----, ---- helix, 1053, 1061.
---- in doubled helices, 1096.
---- in an electro-magnet, 1056, 1060.
----, wire and helix compared, 1065.
----, short wire, effects with, 1067.
----, action momentary, 1070, 1091, 1100.
----, causes no permanent change in the current, 1071.
----, not due to momentum, 1077.
----, induced current separated, 1078, 1089.
----, _effect at_ breaking contact, 1060, 1081, 1084, 1087.
----, ---- making contact, 1101, 1106.
----, _effects produced_, shock, 1060, 1064, 1079.
----, ---- spark, 1060, 1064, 1080.
----, ---- chemical decomposition, 1084.
----, ---- ignition of wire, 1081, 1104.
----, cause is in the conductor, 1059, 1070.
----, general principles of the action, 1093, 1107.
----, direction of the forces lateral, 1108.
_induction, magnetic_, 255, 1658, 1710.
----, by intermediate particles, 1663, 1710, 1729, 1735.
----, _through_ quiescent bodies, 1712, 1719, 1720, 1735.
----, ---- moving bodies, 1715, 1716, 1719.
---- and magneto-electric, distinguished, 138, 215, 243, 253.
_Induction_, magneto-electric, 27, 58, 81, 140, 193, 1709. _See_ Arago's
  magnetic phenomena.
----, magnelectric, 58.
----, electrolytic, 1164, 1345, 1702, 1740.
----, volta-electric, 26.
Inductive capacity, specific, 1167, 1252.
_Inductive force of currents_ lateral, 26, 1108.
----, its nature, 1113, 1660, 1663, 1709.
_Inductive force, lines of_, 1231, 1297, 1304.
----, often curved, 1219, 1224, 1230.
----, exhibited by the brush, 1449.
----, their lateral relation, 1231, 1297, 1304.
----, their relation to magnetism, 1411, 1658, 1709.
Inductometer, differential, 1307, 1317.
Inductric surfaces, 1483.
Inexhaustible nature of the electric current, 1631.
Inseparability of the two electric forces, 1163, 1177, 1244, 1628.
Insulating power of different gases, 1388, 1395, 1507.
_Insulation_, 1320, 1359, 1361.
----, its nature, 1321.
---- is sustained induction, 1324.
----, degree of induction sustained, 1362.
---- _dependent on the_ dielectrics, 1368.
---- ---- distance in air, 1303, 1364, 1371.
---- ---- density of air, 1365, 1375.
---- ---- induction, 1368.
---- ---- form of conductors, 1302, 1374.
----, as affected by temperature of air, 1367, 1380.
---- _in different gases_, 1381, 1388.
---- ---- differs, 1395.
---- in liquids and solids, 1403.
---- in metals, 1328, 1331, 1332.
---- and conduction not essentially different, 1320, 1326, 1336, 1338,
  1561.
----, its relation to induction, 1324, 1362, 1368, 1678.
_Insulators_, liquid, good, 1172.
----, solid, good, 1254.
----, the best conduct, 1233, 1241, 1245, 1247, 1254.
---- tested as to conduction, 1255.
---- and conductors, relation of, 1328, 1334, 1338.
_Intensity_, its influence in conduction, 419.
----, inductive, how represented, 1370.
----, relative, of magneto-electric currents, 183, 193, 211, 213.
---- of disruptive discharge constant, 1410.
----, electrolytic, 912, 966, 983, 1354.
---- necessary for electrolyzation, 911, 966.
---- _of the current of single circles_, 904.
---- ---- increased, 906.
---- of electricity in the voltaic battery, 990, 993.
---- of voltaic current increased, 906, 990.
_Interference with combining power of platina_, 638, 655.
---- by olefiant gas, 640.
---- carbonic oxide, 645.
---- sulphuret of carbon, 650.
---- ether, 651.
Interpositions, their retarding effects, 1018.
_Iodides in_ solution, their electrolysis, 769.
---- fusion, their electrolysis, 802, 813.
_Iodide_ of lead, electrolysed, 802, 818.
---- of potassium, test of chemical action, 316.
_Ions_, what, 665, 824, 833, 834, 849.
---- not transferable alone, 542, 547, 826.
----, table of, 847.
_Iron_, both magnetic and magneto-electric at once, 138, 254.
----, copper and sulphur circles, 943.

Jenkin, his shock by one pair of plates, 1049.

Kemp, his amalgam of zinc, 999.
Knight, Dr. Gowin, his magnet, 44.

_Lac_, charge removed from, 1203.
----, induction through, 1255.
----, specific inductive capacity of, 1256, 1269.
----, effects of its conducting power, 1234.
----, its relation to conduction and insulation, 1324.
_Lateral_ direction of inductive forces of currents, 26, 1108.
---- forces of the current, 1653, 1709.
_Law of_ conduction, new, 380, 394, 410.
---- magneto-electric induction, 114.
---- volta-electric induction, 26.
_Lead_, chloride of, electrolysed, 794, 815.
----, fluoride of, conducts well when heated, 1340.
----, iodide of, electrolysed, 802, 818.
----, oxide of, electrolysed, 797.
_Leyden jar_, condition of its charge, 1682.
----, its charge, nature of, 1300.
----, its discharge, 1300.
----, its residual charge, 1249.
_Light_, polarized, passed across electrolytes, 951.
----, _electric_, 1405, 1445, 1560, _note_.
----, ----, spark, 1406, 1553.
----, ----, brush, 1425, 1444, 1445.
----, ----, glow, 1526.
Lightning, 1420, 1404, 1641.
_Lines of inductive force_, 1231, 1304,
---- often curved, 1219, 1224, 1230.
----, as shown by the brush, 1449.
----, their lateral relation, 1231, 1297, 1304.
----, their relation to magnetism, 1411, 1658, 1709.
Liquefaction, conduction consequent upon, 380, 394, 410.
Liquid bodies which are non-conductors, 405.
Local chemical affinity, 947, 959, 961, 1739.

_Machine_, electric, evolution of electricity by, 1748.
------, magneto-electric, 135, 154, 158, 1118.
_Magnelectric_ induction, 58.
----, collectors or conductors, 86.
_Magnesia_, sulphate, decomposed against water, 494, 533.
----, transference of, 495.
_Magnet_, a measure of conducting power, 216.
---- _and_ current, their relation remembered, 38, _note_.
---- ---- plate revolved together, 218.
---- ---- cylinder revolved together, 219.
---- revolved alone, 220, 223.
---- and moving conductors, their general relation, 256.
---- made by induced current, 13, 14.
----, electricity from, 36, 220, 223.
_Magnetic_ bodies, but few, 255.
----, curves, their inductive relation, 217, 232.
---- _effects of_ voltaic electricity, 277.
---- ---- common electricity, 288, 367.
---- ---- magneto-electricity, 27, 83, 345.
---- ---- thermo-electricity, 349.
---- ---- animal electricity, 354.
---- and electric forces, their relation, 118, 1411, 1653, 1658, 1709,
  1731.
---- forces active through intermediate particles, 1663, 1710, 1729, 1735.
---- _forces of the current_, 1653.
---- ---- very constant, 1654.
---- deflection by common electricity, 289, 296.
---- phenomena of Arago explained, 81.
---- induction. _See_ Induction, magnetic.
---- _induction through_ quiescent bodies, 1712, 1719, 1720, 1735.
---- ---- moving bodies, 1715, 1719.
---- and magneto-electric action distinguished, 138, 215, 243, 253.
_Magnetism_, electricity evolved by, 27.
----, its relation to the lines of inductive force, 1411, 1658, 1709.
---- bodies classed in relation to, 255.
_Magneto-electric currents_, their intensity, 183, 193, 211, 213.
----, their direction, 114, 110.
---- traverse fluids, 33.
---- momentary, 30.
---- permanent, 89.
---- in all conductors, 193, 213.
_Magneto-electric induction_, 27, 58.
----, terrestrial, 110, 181.
----, law of, 114.
----. _See_ Arago's magnetic phenomena.
_Magneto-electric machines_, 135, 154, 158.
----, inductive effects in their wires, 1118,
_Magneto-electricity_, its general characters considered, 343, &c.
---- identical with other electricities, 360.
----, its tension, 343.
----, evolution of heat, 344.
----, magnetic force, 345.
----, chemical force, 346.
----, spark, 348.
----, physiological effects, 347.
----. _See_ Induction, magnetic.
_Matter_, atoms of, 869, 1703.
----, new condition of, 60, 231, 242, 1114, 1661, 1729.
----, quantity of electricity in, 852, 861, 873, 1652.
----, absolute charge of, 1169.
_Measures of electricity_, galvanometer, 367, _note_.
----, voltameter, 704, 736, 739.
----, metal precipitated, 740, 842.
Measure of specific inductive capacity, 1307, 1600.
_Measurement of_ common and voltaic electricities, 361, 860, 1652.
---- _electricity_, degree, 736, 738.
---- ---- by voltameter, 704, 736, 739.
---- ---- by galvanometer, 367, _note_.
---- ---- by metal precipitated, 740, 842.
Mechanical forces affect chemical affinity, 656.
Mercurial terminations for convection, 1581.
_Mercury_, periodide of, an exception to the law of conduction? 691, 1341.
----, perchloride of, 692, 1341.
_Metallic contact_ not necessary for electrolyzation, 879.
---- not essential to the voltaic current, 879, 887, 915.
---- its use in the pile, 893, 896.
Metallic poles, 557.
Metal and electrolyte, their state, 946.
_Metals_, adhesion of fluids to, 1038.
----, _their power of inducing combination_ 564, 608.
----, ---- interfered with, 638.
----, static induction in, 1329, 1332.
----, different, currents induced in, 193, 211.
----, generally secondary results of electrolysis, 746.
---- transfer chemical force, 918.
----, transference of, 539, 545.
---- insulate in a certain degree, 1328.
----, convective currents in, 1603.
----, but few magnetic, 255.
Model of relation of magnetism and electricity, 116.
Molecular inductive action, 1164, 1669.
_Motion_ essential to magneto-electric induction, 39, 217, 256.
---- across magnetic curves, 217.
---- _of conductor and magnet, relative_, 114.
---- ---- not necessary, 218.
Moving magnet is electric, 220.
_Muriatic acid gas_, its high insulating power, 1395.
----, brush in, 1462.
----, dark discharge in, 1554.
----, glow in, 1534.
----, positive and negative brush in, 1476.
----, _spark in_, 1422, 1463.
----, ----, has no dark interval, 1463, 1555.
_Muriatic acid_ decomposed by common electricity, 314.
----, its electrolysis (primary), 758, 786.

Nascent state, its relation to combination, 658, 717.
_Natural_ standard of direction for current, 663.
---- relation of electrolytic intensity, 987.
_Nature of the electric_ current, 1617.
---- force or forces, 1667.
_Negative_ current, none, 1627, 1632.
---- _discharge_, 1465, 1484.
---- ----, as Spark, 1467, 1482.
---- ----, as brush, 1466, 1502.
---- spark or brush, 1484, 1502.
_Negative and positive discharge_, 1465, 1482, 1525
---- in different gases, 1393.
_New_ electrical condition of matter, 60, 231, 242, 1114, 1661, 1729.
---- law of conduction, 380, 394, 410.
_Nitric acid_ formed by spark in air, 324.
---- _favours_ excitation of current, 906, 1138
---- ---- transmission of current, 1020.
---- is best for excitation of battery, 1137.
----, nature of its electrolysis, 752.
_Nitrogen_ determined to either pole, 554, 748, 752.
---- a secondary result of electrolysis, 746, 748.
----, brush in, 1458.
----, dark discharge in, 1559.
----, glow in, 1534.
----, spark in, 1422, 1463.
----, _positive and negative_ brush in, 1476.
----, ---- discharge in, 1512.
----, its influence on lightning, 1464.
Nomenclature, 662, 1483.
Nonconduction by solid electrolytes, 381, 1358, 1705.
Note on electrical excitation, 1737.
Nuclei, their action, 623, 657.

Olefiant gas, interference of, 610, 652.
_Ordinary electricity_, its tension, 285.
---- evolution of heat, 287.
---- magnetic force, 288, 362.
---- _chemical force_, 309, 454.
---- ----, precautions, 322.
---- spark, 333.
---- physiological effect, 332.
---- general characters considered, 284.
----, identity with other electricities, 360.
Origin of the force of the voltaic pile, 878, 910, 919.
Oxidation the origin of the electric current in the voltaic pile, 919, 930.
Oxide of lead electrolysed, 797.
_Oxygen_, brush in, 1457.
----, _positive and negative_ brush in, 1476,
----, ---- discharge in, 1513.
----, solubility of, in cases of electrolyzation, 717, 728.
----, spark in, 1422.
---- _and hydrogen combined by_ platina plates, 570, 605, 630.
---- ---- spongy platina, 609, 636.
---- ---- other metals, 608.

_Particles_, their nascent state, 658.
---- in air, how charged, 1564.
----, neighbouring, their relation to each other, 619, 624, 657.
----, contiguous, active in induction, 1165, 1677.
---- of a dielectric, their inductive condition, 1369, 1410, 1669.
----, polarity of, when under induction, 1298, 1676.
----, _how polarised_, 1669, 1679.
----, ----, in any direction, 1689.
----, ----, as wholes or elements, 1699.
----, ----, in electrolytes, 1702.
----, crystalline, 1689.
----, contiguous, active in electrolysis, 1349, 1702.
----, _their_ action in electrolyzation, 520, 1343, 1703.
----, ---- local chemical action, 961, 1739.
----, ---- relation to electric action, 73.
----, ---- electric action, 1669, 1679, 1740.
Path of the electric spark, 1107.
Phosphoric acid not an electrolyte, 682.
_Physiological effects of_ voltaic-electricity, 279.
---- common electricity, 332.
---- magneto-electricity, 56, 347.
---- thermo-electricity, 349.
---- animal electricity, 357.
_Pile, voltaic_, electricity of, 875.
----. _See_ Battery, voltaic.
_Plates of platina_ effect combination, 568, 571, 590, 630.
---- _prepared by_ electricity, 570, 585, 588.
---- ---- friction, 591.
---- ---- heat, 595.
---- ---- chemical cleansing, 599, 605,
----, clean, their general properties, 633, 717.
----, _their power preserved_, 576.
----, ---- in water, 580.
----, _their power diminished by_ action, 581.
----, ---- exposure to air, 636.
----, _their power affected by_ washing in water, 582.
----, ---- heat, 584, 597.
----, ---- presence of certain gases, 638, 655.
----, their power, cause of, 590, 616, 630.
----, _theory of their action_, Döbereiner's, 610.
----, ----, Dulong and Thenard's, 611.
----, ----, Fusinieri's, 613.
----, ----, author's, 619, 626, 630, 656.
_Plates of voltaic battery_ foul, 1145.
----, new and old, 1116.
----, vicinity of, 1148.
----, immersion of, 1003, 1150.
----, number of, 989, 1151.
----, large or small, 1154.
_Platina_, clean, its characters, 633, 717.
---- attracts matter from the air, 634.
----, spongy, its state, 637.
----, _clean, its power of effecting combination_, 564, 590, 605, 617, 630.
----, ---- interfered with, 638.
----, _its action retarded by_ olefiant gas, 640, 652.
----, ----, carbonic oxide, 645, 652.
----. _See_ Combination, Plates of platina, and Interference.
---- poles, recombination effected by, 567, 588.
Plumbago poles for chlorides, 794.
Poisson's theory of electric induction, 1305.
_Points_, favour convective discharge, 1573.
----, fluid for convection, 1581.
_Polar_ forces, their character, 1665.
---- decomposition by common electricity, 312, 321, 469.
_Polarity_, meaning intended, 1304, 1685.
---- of particles under induction, 1298, 1676.
----, _electric_, 1070, 1085.
----, ----, its direction, 1688, 1703,
----, ----, its variation, 1687.
----, ----, its degree, 1686.
----, ----, in crystals, 1689.
----, ----, in molecules or atoms, 1699.
----, ----, in electrolytes, 1702.
Polarized light across electrolytes, 951.
_Poles, electric_, their nature, 461, 498, 556, 662.
----, appearance of evolved bodies at, accounted for, 535.
---- one element to either? 552, 681, 757.
----, of air, 455, 461, 559.
----, of water, 491, 558.
----, of metal, 557.
----, of platina, recombination effected by, 567, 588.
----, of plumbago, 794.
Poles, magnetic, distinguished, 44, _note_.
Porrett's peculiar effects, 1646.
_Positive_ current none, 1627, 1632.
---- _discharge_, 1465, 1480.
---- ----, as spark, 1467, 1482.
---- ----, as brush, 1467, 1476.
---- spark or brush, 1484, 1502.
---- _and negative_, convective discharge, 1600.
---- ---- _disruptive discharge_, 1465, 1482, 1485, 1525.
---- ---- ---- in different gases, 1393.
---- ---- voltaic discharge, 1524.
---- ---- electrolytic discharge, 1525.
Potassa acetate, nature of its electrolysis, 749.
Potassium, iodide of, electrolysed, 805.
Power of voltaic batteries estimated, 1126.
Powers, their state of tension in the pile, 949.
Practical results with the voltaic battery, 1136.
Pressure of air retains electricity, explained, 1377, 1398.
Primary electrolytical results, 742.
Principles, general, of definite electrolytic action, 822.
Proportionals in electrolytes, single, 679, 697.

_Quantity of electricity in_ matter, 852, 861, 873, 1652.
---- voltaic battery, 990.

Rarefaction of air facilitates discharge, why, 1375.
Recombination, spontaneous, of gases from water, 566.
_Relation_, by measure, of electricities, 361.
---- of magnets and moving conductors, 256.
---- of magnetic induction to intervening bodies, 1662, 1728.
---- of a current and magnet, to remember, 38, _note_.
---- of electric and magnetic forces, 118, 1411, 1653, 1658,1709, 1731.
---- of conductors and insulators, 1321, 1326, 1334, 1338.
---- of conduction and induction, 1320, 1337.
---- _of induction and_ disruptive discharge, 1362.
---- ---- electrolyzation, 1164, 1343.
---- ---- excitation, 1178, 1740.
---- ---- charge, 1171, 1177, 1300.
---- of insulation and induction, 1324, 1362, 1368, 1678.
----, lateral, of lines of inductive force, 1231, 1297, 1304.
---- of a vacuum to electricity, 1613.
---- of spark, brush, and glow, 1533, 1539, 1542.
---- of gases to positive and negative discharge, 1510.
---- of neighbouring particles to each other, 619, 624.
---- _of elements in_ decomposing electrolytes, 923, 1702.
---- ---- exciting electrolytes, 921.
---- of acids and bases voltaically, 927, 933.
Remarks on the active battery, 1034, 1136.
Residual charge of a Leyden jar, 1249.
_Resistance_ to electrolysis, 891, 904, 911, 1007.
---- of an electrolyte to decomposition, 1007.
_Results_ of electrolysis, primary or secondary, 742, 777.
----, practical, with the voltaic battery, 1136.
----, general, as to induction, 1295, 1669.
Retention of electricity by pressure of the atmosphere explained, 1377,
  1398.
_Revolving_ plate. _See_ Arago's phenomena.
---- _globe, Barlow's_, effect explained, 137, 160, 169.
---- ----, magnetic, 164.
---- ----, direction of currents in, 161, 166.
Riffault's and Chompré's theory of electro-chemical decomposition, 485,
  507, 512.
Rock crystal, induction across, 1692.
Room, insulated and electrified, 1173.
Rotation of the earth a cause of magneto-electric induction, 181.

Salts considered as electrolytes, 698.
Scale of electrolytic intensities, 912.
_Secondary electrolytical results_, 702, 742, 748, 777.
---- become measures of the electric current, 843.
_Sections of the current_, 498, 1634.
----, decomposing force alike in all, 501, 1621.
_Sections of lines of inductive action_, 1369.
----, amount of force constant, 1369.
Shock, strong, with one voltaic pair, 1049.
_Silver, chloride of_, its electrolyzation, 541, 813, 902.
----, electrolytic intensity for, 979.
Silver, sulphuret of, hot, conducts well, 433.
_Simple voltaic circles_, 875.
----, decomposition effected by, 897, 904, 931.
Single and many pairs of plates, relation of, 990.
_Single voltaic circuits_, 875.
---- without metallic contact, 879.
---- with metallic contact, 893.
---- their force exalted, 906.
---- _give_ strong shocks, 1049.
---- ---- a bright spark, 1050.
_Solid electrolytes are non-conductors_, 394, 402, 1358.
----, why, 910, 1705.
_Solids, their power of inducing combination_, 564, 618.
---- interfered with, 638.
Solubility of gases in cases of electrolyzation, 717, 728.
_Source of electricity in the voltaic pile_, 875.
---- is chemical action, 879, 916, 919, 1741.
Spark, 1360, 1406.
_Spark, electric, its_ conditions, 1360, 1406, 1553.
---- path, 1407.
---- light, 1553.
---- insensible duration or time, 1438.
---- accompanying dark parts, 1547, 1632.
---- determination, 1370. 1409.
_Spark is affected by the_ dielectrics, 1395, 1421.
---- size of conductor, 1372.
---- form of conductor, 1302, 1374.
---- rarefaction of air, 1375.
_Spark_, atmospheric or lightning, 1464, 1641.
----, negative, 1393, 1467, 1482, 1484, 1502.
----, positive, 1393, 1448, 1467, 1482, 1484, 1502.
----, ragged, 1420, 1448.
----, when not straight, why, 1568.
----, variation in its length, 1381.
----, tendency to its repetition, 1392.
----, facilitates discharge, 1417, 1553.
----, passes into brush, 1448.
----, preceded by induction, 1362.
----, forms nitric acid in air, 324.
----, in gases, 1383, 1421.
----, in air, 1422.
----, in nitrogen, 1422, 1463.
----, in oxygen, 1422.
----, in hydrogen, 1422.
----, in carbonic acid, 1422, 1463.
----, in muriatic acid gas, 1422, 1463.
----, in coal-gas, 1422.
----, in liquids, 1424.
----, precautions, 958, 1074.
----, voltaic, without metallic contact, 915, 956.
---- from single voltaic pair, 1050.
---- from common and voltaic electricity assimilated, 334.
----, first magneto-electric, 32.
---- of voltaic electricity, 280.
---- of common electricity, 333.
---- of magneto-electricity, 348.
---- of thermo-electricity, 349.
---- of animal electricity, 358.
----, brush and glow related, 1533, 1539, 1542.
Sparks, their expected coalition, 1412.
Specific induction. _See_ Induction, specific, 1252.
_Specific inductive capacity_, 1252.
----, apparatus for, 1187.
---- of lac, 1256, 1270, 1308.
---- of sulphur, 1275, 1310.
---- of air, 1284.
---- of gases, 1283, 1290.
---- of glass, 1271.
_Spermaceti_, its conducting power, 1240, 1323.
----, its relation to conduction and insulation, 1322.
Standard of direction in the current, 663.
State, electrotonic, 60, 231, 242, 1114, 1661, 1729.
Static induction. _See_ Induction, static.
_Sturgeon_, his form of Arago's experiment, 249.
----, use of amalgamated zinc by, 863, 999.
_Sulphate of soda_, decomposed by common electricity, 317.
----, electrolytic intensity for, 975.
_Sulphur_ determined to either pole, 552, 681, 757.
----, its conducting power, 1241, 1245.
----, its specific inductive capacity, 1275.
----, copper and iron, circle, 943.
_Sulphuret of_ carbon, interference of, 650.
---- silver, hot, conducts well, 433.
Sulphuretted solution excites the pile, 943.
_Sulphuric acid_, conduction by, 409, 681.
----, magneto-electric induction on, 200, 213.
---- in voltaic pile, its use, 925.
---- not an electrolyte, 681.
----, its transference, 525.
----, its decomposition, 681, 757.
Sulphurous acid, its decomposition, 755.
_Summary of_ conditions of conduction, 443.
---- molecular inductive theory, 1669.

_Table of_ discharge in gases, 1388.
---- electric effects, 360.
---- electro-chemical equivalents, 847.
---- electrolytes affected by fusion, 402.
---- insulation in gases, 1388.
---- ions, anions, and cathions, 847.
Tartaric acid, nature of its electrolysis, 775.
_Tension_, inductive, how represented, 1370.
---- of voltaic electricity, 268.
---- of common electricity, 285.
---- of thermo-electricity, 349.
---- of magneto-electricity, 343.
---- of animal electricity, 352.
---- of zinc and electrolyte in the voltaic pile, 949.
Terrestrial electric currents, 187.
_Terrestrial magneto-electric induction_, 140.
---- cause of aurora borealis, 192.
----, _electric currents produced by_, 141, 150.
----, ----, _in helices_ alone, 148.
----, ----, ---- with iron, 141, 146.
----, ----, ---- with a magnet, 147.
----, ---- a single wire, 170.
----, ---- a revolving plate, 149.
----, ---- a revolving ball, 160.
----, ---- the earth, 173.
Test between magnetic and magneto-electric action, 215, 243.
_Theory of_ combination of gases by clean platina, 619, 626, 630, 656.
---- electro-chemical decomposition, 477, 661, 1623, 1704.
---- the voltaic apparatus, 875, 1741.
---- static induction, 1165, 1231, 1295, 1666, 1667.
---- disruptive discharge, 1368, 1406, 1434.
---- Arago's phenomena, 120.
_Thermo-electricity_, its general characters, 349.
---- identical with other electricities, 360.
----, its evolution of heat, 349.
----, magnetic, force, 349.
----, physiological effects, 349.
----, spark, 349.
Time, 59, 68, 124, 1248, 1328, 1346, 1418, 1431, 1436, 1439, 1612, 1641,
  1730.
_Tin_, iodide of, electrolysed, 804.
----, protochloride, electrolysis of, definite, 789, 819.
_Torpedo_, nature of its electric discharge, 359.
----, its enormous amount of electric force, 359.
Transfer of elements and the current, their relation, 923, 962.
_Transference_ is simultaneous in opposite directions, 542, 828.
----, uncombined bodies do not travel, 544, 546, 826.
---- _of elements_, 454, 507, 539, 550, 826.
---- ---- across great intervals, 455, 468.
---- ----, its nature, 519, 525, 538, 549.
---- of chemical force, 918.
Transverse forces of the current, 1653, 1709.
Travelling of charged particles, 1563.
Trough, voltaic. _See_ Battery, voltaic.
_Turpentine, oil of_, a good fluid insulator, 1172.
----, its insulating power destroyed, 1571.
---- charged, 1172.
----, brush in, 1452,
----, electric motions in, 1588, 1595,
----, convective currents in, 1595, 1598.

Unipolarity, 1635.

Vacuum, its relation to electricity, 1613.
Vaporization, 657.
_Velocity of_ conduction in metals varied, 1333.
---- the electric discharge, 1641, 1649.
---- conductive and electrolytic discharge, difference of, 1650.
Vicinity of plates in voltaic battery, 1148.
Volta-electric induction, 26.
_Volta-electrometer_, 704, 736.
----, fluid decomposed in it, water, 706, 728, 732.
----, forms of, 707, 734.
---- _tested for variation of_ electrodes, 714, 722.
---- ---- fluid within, 727.
---- ---- intensity, 723.
----, strength of acid used in, 728,733.
----, _its indications by_ oxygen and hydrogen, 736.
----, ---- hydrogen, 734.
----, ---- oxygen, 735.
----, how used, 737.
Voltameter, 704.
_Voltaic battery_, its nature, 875, 989.
----, remarks on, 1034, 1136.
----, improved, 1001, 1119.
----, practical results with, 1136.
----. _See_ Battery, voltaic.
_Voltaic circles, simple_, 875.
----, decomposition by, 897.
Voltaic circles associated, or battery, 989.
_Voltaic circuit_, relation of bodies in, 962.
----, defined, 282, 511.
----, origin of, 916, 1741.
----, its direction, 663, 925,
----, intensity increased, 905, 990.
----, produced by oxidation of zinc, 919, 930.
---- not due to combination of oxide and acid, 925, 933.
----, _its relation to the_ combining oxygen, 921, 962.
----, ---- combining sulphur, 943.
----, ---- the transferred elements, 923, 962.
----, relation of bodies in, 962.
Voltaic current, 1617. _See_ Current, electric.
Voltaic discharge, positive and negative, 1524.
Voltaic decomposition, 450, 600. _See_ Decomposition, electro-chemical.
_Voltaic electricity_, identical with electricity, otherwise evolved, 268,
  360.
----, _discharged by_ points, 272.
----, ---- hot air, 271, 274.
----, its tension, 268, 275.
----, evolution of heat by, 276.
----, its magnetic force, 277.
----, its chemical force, 278.
----, its spark, 280.
----, its physiological effects, 279.
----, its general characters considered, 268.
_Voltaic pile_ distinguished, 856, _note_.
----, electricity of, 875.
----, depends on chemical action, 872.
----, relation of acid and bases in the, 927.
----. _See_ Battery, voltaic.
_Voltaic spark_ without contact, 915, 956.
----, precautions, 958, 1074.
Voltaic trough, 989. _See_ Battery, voltaic.


_Water_, flowing, electric currents in, 190.
----, retardation of current by, 1159.
----, _its direct conducting power_, 1017, 1159, 1355.
----, ---- constant, 984.
----, electro-chemical decomposition against, 494, 532.
----, poles of, 494, 533, 558.
----, its influence in electro-chemical decomposition, 472.
---- is the great electrolyte, 924.
----, _the exciting electrolyte when_ pure, 944.
----, ---- acidulated, 880, 926, 1137.
----, ---- alkalized, 931, 934, 941.
----, electrolytic intensity for, 968, 981, 1017.
---- electrolyzed in a single circuit, 862.
----, its electrolysis definite, 732, 785, 807.
----, decomposition of by fine wires, 327.
----, quantity of electricity in its elements, 853, 861.
----, determined to either pole, 553.
_Wheatstone's_ analysis of the electric brush, 1427.
---- measurement of conductive velocity in metals, 1328.
_Wire, ignition of, by the electric current_, 853, _note_, 1631.
---- is uniform throughout, 1630.
_Wire_ a regulator of the electric current, 853, _note_.
----, velocity of conduction in, varied, 1333.
----, single, a current induced in, 170.
----, long, inductive effects in, 1064, 1118.
_Wollaston on_ decomposition by common electricity, 309.
---- decomposition of water by points, 327.

_Zinc, amalgamated_, its condition, 863, 1000.
----, used in pile, 999.
_Zinc_, how amalgamated, 863.
----, of troughs, its purity, 1144.
----, its relation to the electrolyte, 949.
----, its oxidation is the source of power in the pile, 919.
---- _plates of troughs_, foul, 1145.
---- ----, new and old, 1146.
----, waste of, in voltaic batteries, 997.


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