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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



Online LibraryMichael FaradayExperimental Researches in Electricity, Volume 1 → online text (page 14 of 57)