Received June 18,—Read June 20, 1833.
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. fi.); 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 (fi.).
463. A similar piece of litmus
paper, dipped in solution of sulphate of soda n,
fi, 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 fi, 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 (fi.) 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.
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:”
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.”
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,
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; 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.
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. 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.
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. 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. 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, 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.
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 p and 29 of the
Philosophical Transactions for 1807, also at .
Again at 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; and supposes that the attractive and
repellent agencies may be communicated from the metallic
surfaces throughout the whole of the menstruum,
being communicated from one particle to another
particle of the same kind, and diminishing in
strength from the place of the poles to the middle
point, which is necessarily neutral. 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.
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, and uses the
terms attraction and repulsion apparently in the same
sense as before.
485. Messrs. Riffault and Chompre
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.
486. M. Biot is very cautious
in expressing an opinion as to the cause of the separation
of the elements of a compound body. 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. 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.
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, 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.
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.
489. M.A. de la Rive investigated
this subject particularly, and published a paper on
it in 1825. 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. 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, 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.
In this respect M. de la Rive’s hypothesis accords
in part with that of MM. Riffault and Chompre
(485.).
490. M. de la Rive considers
the portions of matter which are decomposed to be
those contiguous to both poles. He does
not admit with others the successive decompositions
and recompositions in the whole course of the electricity
through the humid conductor, 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.
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.
491. The latest communication
that I am aware of on the subject is by M. Hachette:
its date is October 1832. 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.”
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 (fi.),
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 (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.
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.
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 Chompre (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 Chompre (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 Chompre,
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, the nitrate of soda being, under similar
circumstances, decomposed by the former, but not by
the latter.
513. M. Hachette states 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.
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.). 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.
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.
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.
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, fi, 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, fi, 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 fi,
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.
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 fi; 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.
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. Yet such is not the case, as has been
shown by the experiments on free and combined acid
(526. 528.).
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. 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.
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.
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, were those in which acids
were passed through alkalies, and alkalies or earths
through acids; 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.
551. The instances in which sulphuric
acid could not be passed though baryta, or baryta
through sulphuric acid, 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.
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. 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.
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.
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.
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