§ 11. _On Electro-chemical Decomposition._ ¶ i. _New conditions of
Electro-chemical Decomposition._ ¶ ii. _Influence of Water in
Electro-chemical Decomposition._ ¶ iii. _Theory of Electro-chemical
Decomposition._
Received June 18,--Read June 20, 1833.
§ 11. _On Electro-chemical Decomposition._[A]
[A] Refer to the note after 1047, Series viii.--_Dec. 1838._
450. I have in a recent series of these Researches (265.) proved (to my own
satisfaction, at least,) the identity of electricities derived from
different sources, and have especially dwelt upon the proofs of the
sameness of those obtained by the use of the common electrical machine and
the voltaic battery.
451. The great distinction of the electricities obtained from these two
sources is the very high tension to which the small quantity obtained by
aid of the machine may be raised, and the enormous quantity (371. 376.) in
which that of comparatively low tension, supplied by the voltaic battery,
may be procured; but as their actions, whether magnetical, chemical, or of
any other nature, are essentially the same (360.), it appeared evident that
we might reason from the former as to the manner of action of the latter;
and it was, to me, a probable consequence, that the use of electricity of
such intensity as that afforded by the machine, would, when applied to
effect and elucidate electro-chemical decomposition, show some new
conditions of that action, evolve new views of the internal arrangements
and changes of the substances under decomposition, and perhaps give
efficient powers over matter as yet undecomposed.
452. For the purpose of rendering the bearings of the different parts of
this series of researches more distinct, I shall divide it into several
heads.
¶ i. _New conditions of Electro-chemical Decomposition._
453. The tension of machine electricity causes it, however small in
quantity, to pass through any length of water, solutions, or other
substances classing with these as conductors, as fast as it can be
produced, and therefore, in relation to quantity, as fast as it could have
passed through much shorter portions of the same conducting substance. With
the voltaic battery the case is very different, and the passing current of
electricity supplied by it suffers serious diminution in any substance, by
considerable extension of its length, but especially in such bodies as
those mentioned above.
454. I endeavoured to apply this facility of transmitting the current of
electricity through any length of a conductor, to an investigation of the
transfer of the elements in a decomposing body, in contrary directions,
towards the poles. The general form of apparatus used in these experiments
has been already described (312. 316); and also a particular experiment
(319.), in which, when a piece of litmus paper and a piece of turmeric
paper were combined and moistened in solution of sulphate of soda, the
point of the wire from the machine (representing the positive pole) put
upon the litmus paper, and the receiving point from the discharging train
(292. 316.), representing the negative pole, upon the turmeric paper, a
very few turns of the machine sufficed to show the evolution of acid at the
former, and alkali at the latter, exactly in the manner effected by a
volta-electric current.
455. The pieces of litmus and turmeric paper were _now_ placed each upon a
separate plate of glass, and connected by an insulated string four feet
long, moistened in the same solution of sulphate of soda: the terminal
decomposing wire points were placed upon the papers as before. On working
the machine, the same evolution of acid and alkali appeared as in the
former instance, and with equal readiness, notwithstanding that the places
of their appearance were four feet apart from each other. Finally, a piece
of string, seventy feet long, was used. It was insulated in the air by
suspenders of silk, so that the electricity passed through its entire
length: decomposition took place exactly as in former cases, alkali and
acid appearing at the two extremities in their proper places.
456. Experiments were then made both with sulphate of soda and iodide of
potassium, to ascertain if any diminution of decomposing effect was
produced by such great extension as those just described of the moist
conductor or body under decomposition; but whether the contact of the
decomposing point connected with the discharging train was made with
turmeric paper touching the prime conductor, or with other turmeric paper
connected with it through the seventy feet of string, the spot of alkali
for an equal number of turns of the machine had equal intensity of colour.
The same results occurred at the other decomposing wire, whether the salt
or the iodide were used; and it was fully proved that this great extension
of the distance between the poles produced no effect whatever on the amount
of decomposition, provided the same _quantity_ of electricity were passed
in both cases (377.).
457. The negative point of the discharging train, the turmeric paper, and
the string were then removed; the positive point was left resting upon the
litmus paper, and the latter touched by a piece of moistened string held in
the hand. A few turns of the machine evolved acid at the positive point as
freely as before.
458. The end of the moistened string, instead of being held in the hand,
was suspended by glass in the air. On working the machine the electricity
proceeded from the conductor through the wire point to the litmus paper,
and thence away by the intervention of the string to the air, so that there
was (as in the last experiment) but one metallic pole; still acid was
evolved there as freely as in any former case.
459. When any of these experiments were repeated with electricity from the
negative conductor, corresponding effects were produced whether one or two
decomposing wires were used. The results were always constant, considered
in relation to the _direction_ of the electric current.
460. These experiments were varied so as to include the action of only one
metallic pole, but that not the pole connected with the machine. Turmeric
paper was moistened in solution of sulphate of soda, placed upon glass, and
connected with the discharging train (292.) by a decomposing wire (312.); a
piece of wet string was hung from it, the lower extremity of which was
brought opposite a point connected with the positive prime conductor of the
machine. The machine was then worked for a few turns, and alkali
immediately appeared at the point of the discharging train which rested on
the turmeric paper. Corresponding effects took place at the negative
conductor of a machine.
461. These cases are abundantly sufficient to show that electrochemical
decomposition does not depend upon the simultaneous action of two metallic
poles, since a single pole might be used, decomposition ensue, and one or
other of the elements liberated, pass to the pole, according as it was
positive or negative. In considering the course taken by, and the final
arrangement of, the other element, I had little doubt that I should find it
had receded towards the other extremity, and that the air itself had acted
as a pole, an expectation which was fully confirmed in the following
manner.
462. A piece of turmeric paper, not more than 0.4 of an inch in length and
0.5 of an inch in width, was moistened with sulphate of soda and placed
upon the edge of a glass plate opposite to, and about two inches from, a
point connected with the discharging train (Plate IV. fig. 47.); a piece of
tinfoil, resting upon the same glass plate, was connected with the machine,
and also with the turmeric paper, by a decomposing wire _a_ (312.). The
machine was then worked, the positive electricity passing into the turmeric
paper at the point _p_, and out at the extremity _n_. After forty or fifty
turns of the machine, the extremity _n_ was examined, and the two points or
angles found deeply coloured by the presence of free alkali (fig. 48.).
463. A similar piece of litmus paper, dipped in solution of sulphate of
soda _n_, fig. 49, was now supported upon the end of the discharging train
_a_, and its extremity brought opposite to a point _p_, connected with the
conductor of the machine. After working the machine for a short time, acid
was developed at both the corners towards the point, i.e. at both the
corners receiving the electricities from the air. Every precaution was
taken to prevent this acid from being formed by sparks or brushes passing
through the air (322.); and these, with the accompanying general facts, are
sufficient to show that the acid was really the result of electro-chemical
decomposition (466.).
464. Then a long piece of turmeric paper, large at one end and pointed at
the other, was moistened in the saline solution, and immediately connected
with the conductor of the machine, so that its pointed extremity was
opposite a point upon the discharging train. When the machine was worked,
alkali was evolved at that point; and even when the discharging train was
removed, and the electricity left to be diffused and carried off altogether
by the air, still alkali was evolved where the electricity left the
turmeric paper.
465. Arrangements were then made in which no metallic communication with
the decomposing matter was allowed, but both poles (if they might now be
called by that name) formed of air only. A piece of turmeric paper _a_ fig.
50, and a piece of litmus paper _b_, were dipped in solution of sulphate of
soda, put together so as to form one moist pointed conductor, and supported
on wax between two needle points, one, _p_, connected by a wire with the
conductor of the machine, and the other, _n_, with the discharging train.
The interval in each case between the points was about half an inch; the
positive point _p_ was opposite the litmus paper; the negative point _n_
opposite the turmeric. The machine was then worked for a time, upon which
evidence of decomposition quickly appeared, for the point of the litmus _b_
became reddened from acid evolved there, and the point of the turmeric _a_
red from a similar and simultaneous evolution of alkali.
466. Upon turning the paper conductor round, so that the litmus point
should now give off the positive electricity, and the turmeric point
receive it, and working the machine for a short time, both the red spots
disappeared, and as on continuing the action of the machine no red spot was
re-formed at the litmus extremity, it proved that in the first instance
(463.) the effect was not due to the action of brushes or mere electric
discharges causing the formation of nitric acid from the air (322.).
467. If the combined litmus and turmeric paper in this experiment be
considered as constituting a conductor independent of the machine or the
discharging train, and the final places of the elements evolved be
considered in relation to this conductor, then it will be found that the
acid collects at the _negative_ or receiving end or pole of the
arrangement, and the alkali at the _positive_ or delivering extremity.
468. Similar litmus and turmeric paper points were now placed upon glass
plates, and connected by a string six feet long, both string and paper
being moistened in solution of sulphate of soda; a needle point connected
with the machine was brought opposite the litmus paper point, and another
needle point connected with the discharging train brought opposite the
turmeric paper. On working the machine, acid appeared on the litmus, and
alkali on the turmeric paper; but the latter was not so abundant as in
former cases, for much of the electricity passed off from the string into
the air, and diminished the quantity discharged at the turmeric point.
469. Finally, a series of four small compound conductors, consisting of
litmus and turmeric paper (fig. 51.) moistened in solution of sulphate of
soda, were supported on glass rods, in a line at a little distance from
each other, between the points _p_ and _n_ of the machine and discharging
train, so that the electricity might pass in succession through them,
entering in at the litmus points _b, b_, and passing out at the turmeric
points _a, a_. On working the machine carefully, so as to avoid sparks and
brushes (322.), I soon obtained evidence of decomposition in each of the
moist conductors, for all the litmus points exhibited free acid, and the
turmeric points equally showed free alkali.
470. On using solutions of iodide of potassium, acetate of lead, &c.,
similar effects were obtained; but as they were all consistent with the
results above described, I refrain from describing the appearances
minutely.
471. These cases of electro-chemical decomposition are in their nature
exactly of the same kind as those affected under ordinary circumstances by
the voltaic battery, notwithstanding the great differences as to the
presence or absence, or at least as to the nature of the parts usually
called poles; and also of the final situation of the elements eliminated at
the electrified boundary surfaces (467.). They indicate at once an internal
action of the parts suffering decomposition, and appear to show that the
power which is effectual in separating the elements is exerted there, and
not at the poles. But I shall defer the consideration of this point for a
short time (493. 518.), that I may previously consider another supposed
condition of electro-chemical decomposition[A].
[A] I find (since making and describing these results,) from a note to
Sir Humphry Davy's paper in the Philosophical Transactions, 1807, p.
31, that that philosopher, in repeating Wollaston's experiment of the
decomposition of water by common electricity (327. 330.) used an
arrangement somewhat like some of those I have described. He immersed
a guarded platina point connected with the machine in distilled water,
and dissipated the electricity from the water into the air by
moistened filaments of cotton. In this way he states that he obtained
oxygen and hydrogen _separately_ from each other. This experiment, had
I known of it, ought to have been quoted in an earlier series of these
Researches (342.); but it does not remove any of the objections I have
made to the use of Wollaston's apparatus as a test of true chemical
action (331.).
¶ ii. _Influence of Water in Electro-chemical Decomposition._
472. It is the opinion of several philosophers, that the presence of water
is essential in electro-chemical decomposition, and also for the evolution
of electricity in the voltaic battery itself. As the decomposing cell is
merely one of the cells of the battery, into which particular substances
are introduced for the purpose of experiment, it is probable that what is
an essential condition in the one case is more or less so in the other. The
opinion, therefore, that water is necessary to decomposition, may have been
founded on the statement made by Sir Humphry Davy, that "there are no
fluids known, except such as contain water, which are capable of being made
the medium of connexion between the metals or metal of the voltaic
apparatus[A]:" and again, "when any substance rendered fluid by heat,
consisting of _water_, oxygen, and inflammable or metallic matter, is
exposed to those wires, similar phenomena (of decomposition) occur[B]."
[A] Elements of Chemical Philosophy, p. 160, &c.
[B] Ibid. pp. 144, 145.
473. This opinion has, I think, been shown by other philosophers not to be
accurate, though I do not know where to refer for a contradiction of it.
Sir Humphry Davy himself said in 1801[A], that dry nitre, caustic potash
and soda are conductors of galvanism when rendered fluid by a high degree
of heat, but he must have considered them, or the nitre at least, as not
suffering decomposition, for the statements above were made by him eleven
years subsequently. In 1826 he also pointed out, that bodies not containing
water, as _fused litharge_ and _chlorate of potassa_, were sufficient to
form, with platina and zinc, powerful electromotive circles[B]; but he is
here speaking of the _production_ of electricity in the pile, and not of
its effects when evolved; nor do his words at all imply that any correction
of his former distinct statements relative to _decomposition_ was required.
[A] Journal of the Royal Institution, 1802, p. 53.
[B] Philosophical Transactions, 1826, p. 406.
474. I may refer to the last series of these Experimental Researches (380.
402.) as setting the matter at rest, by proving that there are hundreds of
bodies equally influential with water in this respect; that amongst binary
compounds, oxides, chlorides, iodides, and even sulphurets (402.) were
effective; and that amongst more complicated compounds, cyanides and salts,
of equal efficacy, occurred in great numbers (402.).
475. Water, therefore, is in this respect merely one of a very numerous
class of substances, instead of being the _only one_ and _essential_; and
it is of that class one of the _worst_ as to its capability of facilitating
conduction and suffering decomposition. The reasons why it obtained for a
time an exclusive character which it so little deserved are evident, and
consist, in the general necessity of a fluid condition (394.); in its being
the _only one_ of this class of bodies existing in the fluid state at
common temperatures; its abundant supply as the great natural solvent; and
its constant use in that character in philosophical investigations, because
of its having a smaller interfering, injurious, or complicating action upon
the bodies, either dissolved or evolved, than any other substance.
476. The analogy of the decomposing or experimental cell to the other cells
of the voltaic battery renders it nearly certain that any of those
substances which are decomposable when fluid, as described in my last paper
(402.), would, if they could be introduced between the metallic plates of
the pile, be equally effectual with water, if not more so. Sir Humphry Davy
found that litharge and chlorate of potassa were thus effectual[A]. I have
constructed various voltaic arrangements, and found the above conclusion to
hold good. When any of the following substances in a fused state were
interposed between copper and platina, voltaic action more or less powerful
was produced. Nitre; chlorate of potassa; carbonate of potassa; sulphate of
soda; chloride of lead, of sodium, of bismuth, of calcium; iodide of lead;
oxide of bismuth; oxide of lead: the electric current was in the same
direction as if acids had acted upon the metals. When any of the same
substances, or phosphate of soda, were made to act on platina and iron,
still more powerful voltaic combinations of the same kind were produced.
When either nitrate of silver or chloride of silver was the fluid substance
interposed, there was voltaic action, but the electric current was in the
reverse direction.
[A] Philosophical Transactions, 1826, p. 406.
iii. _Theory of Electro-chemical Decomposition._
477. The extreme beauty and value of electro-chemical decompositions have
given to that power which the voltaic pile possesses of causing their
occurrence an interest surpassing that of any other of its properties; for
the power is not only intimately connected with the continuance, if not
with the production, of the electrical phenomena, but it has furnished us
with the most beautiful demonstrations of the nature of many compound
bodies; has in the hands of Becquerel been employed in compounding
substances; has given us several new combinations, and sustains us with the
hope that when thoroughly understood it will produce many more.
478. What may be considered as the general facts of electrochemical
decomposition are agreed to by nearly all who have written on the subject.
They consist in the separation of the decomposable substance acted upon
into its proximate or sometimes ultimate principles, whenever both poles of
the pile are in contact with that substance in a proper condition; in the
evolution of these principles at distant points, i.e. at the poles of the
pile, where they are either finally set free or enter into union with the
substance of the poles; and in the constant determination of the evolved
elements or principles to particular poles according to certain
well-ascertained laws.
479. But the views of men of science vary much as to the nature of the
action by which these effects are produced; and as it is certain that we
shall be better able to apply the power when we really understand the
manner in which it operates, this difference of opinion is a strong
inducement to further inquiry. I have been led to hope that the following
investigations might be considered, not as an increase of that which is
doubtful, but a real addition to this branch of knowledge.
480. It will be needful that I briefly state the views of electro-chemical
decomposition already put forth, that their present contradictory and
unsatisfactory state may be seen before I give that which seems to me more
accurately to agree with facts; and I have ventured to discuss them freely,
trusting that I should give no offence to their high-minded authors; for I
felt convinced that if I were right, they would be pleased that their views
should serve as stepping-stones for the advance of science; and that if I
were wrong, they would excuse the zeal which misled me, since it was
exerted for the service of that great cause whose prosperity and progress
they have desired.
481. Grotthuss, in the year 1805, wrote expressly on the decomposition of
liquids by voltaic electricity[A]. He considers the pile as an electric
magnet, i.e. as an attractive and repulsive agent; the poles having
_attractive_ and _repelling_ powers. The pole from whence resinous
electricity issues attracts hydrogen and repels oxygen, whilst that from
which vitreous electricity proceeds attracts oxygen and repels hydrogen; so
that each of the elements of a particle of water, for instance, is subject
to an attractive and a repulsive force, acting in contrary directions, the
centres of action of which are reciprocally opposed. The action of each
force in relation to a molecule of water situated in the course of the
electric current is in the inverse ratio of the square of the distance at
which it is exerted, thus giving (it is stated) for such a molecule a
_constant force_[B]. He explains the appearance of the elements at a
distance from each other by referring to a succession of decompositions and
recompositions occurring amongst the intervening particles[C], and he
thinks it probable that those which are about to separate at the poles
unite to the two electricities there, and in consequence become gases[D].
[A] Annales de Chimie, 1806, tom, lviii. p. 64.
[B] Ibid. pp. 66, 67, also tom. lxiii. p. 20.
[C] Ibid. tom. lviii. p. 68, tom, lxiii. p. 20.
[D] Ibid. tom. lxiii. p. 34.
482. Sir Humphry Davy's celebrated Bakerian Lecture on some chemical
agencies of electricity was read in November 1806, and is almost entirely
occupied in the consideration of _electro-chemical decompositions_. The
facts are of the utmost value, and, with the general points established,
are universally known. The _mode of action_ by which the effects take place
is stated very generally, so generally, indeed, that probably a dozen
precise schemes of electro-chemical action might be drawn up, differing
essentially from each other, yet all agreeing with the statement there
given.
483. When Sir Humphry Davy uses more particular expressions, he seems to
refer the decomposing effects to the attractions of the poles. This is the
case in the "general expression of facts" given at pp. 28 and 29 of the
Philosophical Transactions for 1807, also at p. 30. Again at p. 160 of the
Elements of Chemical Philosophy, he speaks of the great attracting powers
of the surfaces of the poles. He mentions the probability of a succession
of decompositions and recompositions throughout the fluid,--agreeing in
that respect with Grotthuss[A]; and supposes that the attractive and
repellent agencies may be communicated from the metallic surfaces
throughout the whole of the menstruum[B], being communicated from _one
particle to another particle of the same kind_[C], and diminishing in
strength from the place of the poles to the middle point, which is
necessarily neutral[D]. In reference to this diminution of power at
increased distances from the poles, he states that in a circuit of ten
inches of water, solution of sulphate of potassa placed four inches from
the positive pole, did not decompose; whereas when only two inches from
that pole, it did render up its elements[E].
[A] Philosophical Transactions, 1807, pp. 29, 30.
[B] Ibid. p. 39.
[C] Ibid. p. 29.
[D] Ibid. p. 42.
[E] Ibid. p. 42.
484. When in 1826 Sir Humphry Davy wrote again on this subject, he stated
that he found nothing to alter in the fundamental theory laid down in the
original communication[A], and uses the terms attraction and repulsion
apparently in the same sense as before[B].
[A] Philosophical Transactions, 1826, p. 383.
[B] Ibid. pp. 389, 407, 115.
485. Messrs. Riffault and Chompré experimented on this subject in 1807.
They came to the conclusion that the voltaic current caused decompositions
throughout its whole course in the humid conductor, not merely as
preliminary to the recompositions spoken of by Grotthuss and Davy, but
producing final separation of the elements in the _course_ of the current,
and elsewhere than at the poles. They considered the _negative_ current as
collecting and carrying the acids, &c. to the _positive_ pole, and the
_positive_ current as doing the same duty with the bases, and collecting
them at the _negative_ pole. They likewise consider the currents as _more
powerful_ the nearer they are to their respective poles, and state that the
positive current is _superior_ in power to the negative current[A].
[A] Annales de Chimie, 1807, tom. lxiii. p. 83, &c.
486. M. Biot is very cautious in expressing an opinion as to the cause of
the separation of the elements of a compound body[A]. But as far as the
effects can be understood, he refers them to the opposite electrical states
of the portions of the decomposing substance in the neighbourhood of the
two poles. The fluid is most positive at the positive pole; that state
gradually diminishes to the middle distance, where the fluid is neutral or
not electrical; but from thence to the negative pole it becomes more and
more negative[B]. When a particle of salt is decomposed at the negative
pole, the acid particle is considered as acquiring a negative electrical
state from the pole, stronger than that of the surrounding _undecomposed_
particles, and is therefore repelled from amongst them, and from out of
that portion of the liquid towards the positive pole, towards which also it
is drawn by the attraction of the pole itself and the particles of positive
_undecomposed_ fluid around it[C].
[A] Précis Elémentaire de Physique, 3me édition, 1824, tom. i. p. 641.
[B] Ibid. p. 637.
[C] Ibid. pp. 641, 642.
487. M. Biot does not appear to admit the successive decompositions and
recompositions spoken of by Grotthuss, Davy, &c. &c.; but seems to consider
the substance whilst in transit as combined with, or rather attached to,
the electricity for the time[A], and though it communicates this
electricity to the surrounding undecomposed matter with which it is in
contact, yet it retains during the transit a little superiority with
respect to that kind which it first received from the pole, and is, by
virtue of that difference, carried forward through the fluid to the
opposite pole[B].
[A] Précis Elémentaire de Physique, 3me édition, 1824, tom. i. p. 636.
[B] Ibid. p, 642.
488. This theory implies that decomposition takes place at both poles upon
distinct portions of fluid, and not at all in the intervening parts. The
latter serve merely as imperfect conductors, which, assuming an electric
state, urge particles electrified more highly at the poles through them in
opposite directions, by virtue of a series of ordinary electrical
attractions and repulsions[A].
[A] Précis Elémentaire de Physique, 3me édition, 1824, tom. i. pp.
638, 642.
489. M.A. de la Rive investigated this subject particularly, and published
a paper on it in 1825[A]. He thinks those who have referred the phenomena
to the attractive powers of the poles, rather express the general fact than
give any explication of it. He considers the results as due to an actual
combination of the elements, or rather of half of them, with the
electricities passing from the poles in consequence of a kind of play of
affinities between the matter and electricity[B]. The current from the
positive pole combining with the hydrogen, or the bases it finds there,
leaves the oxygen and acids at liberty, but carries the substances it is
united with across to the negative pole, where, because of the peculiar
character of the metal as a conductor[C], it is separated from them,
entering the metal and leaving the hydrogen or bases upon its surface. In
the same manner the electricity from the negative pole sets the hydrogen
and bases which it finds there, free, but combines with the oxygen and
acids, carries them across to the positive pole, and there deposits
them[D]. In this respect M. de la Rive's hypothesis accords in part with
that of MM. Riffault and Chompré (485.).
[A] Annales de Chimie, tom, xxviii. p. 190.
[B] Ibid. pp. 200, 202.
[C] Ibid. p. 202.
[D] Ibid. p. 201.
490. M. de la Rive considers the portions of matter which are decomposed to
be those contiguous to _both_ poles[A]. He does not admit with others the
successive decompositions and recompositions in the whole course of the
electricity through the humid conductor[B], but thinks the middle parts are
in themselves unaltered, or at least serve only to conduct the two contrary
currents of electricity and matter which set off from the opposite
poles[C]. The decomposition, therefore, of a particle of water, or a
particle of salt, may take place at either pole, and when once effected, it
is final for the time, no recombination taking place, except the momentary
union of the transferred particle with the electricity be so considered.
[A] Annales de Chimie, tom, xxviii. pp. 197, 198.
[B] Ibid. pp. 192, 199.
[C] Ibid. p. 200.
491. The latest communication that I am aware of on the subject is by M.
Hachette: its date is October 1832[A]. It is incidental to the description
of the decomposition of water by the magneto-electric currents (346.). One
of the results of the experiment is, that "it is not necessary, as has been
supposed, that for the chemical decomposition of water, the action of the
two electricities, positive and negative, should be simultaneous."
[A] Annales de Chimie, tom, xxviii. tom. li. p. 73.
492. It is more than probable that many other views of electro-chemical
decomposition may have been published, and perhaps amongst them some which,
differing from those above, might, even in my own opinion, were I
acquainted with them, obviate the necessity for the publication of my
views. If such be the case, I have to regret my ignorance of them, and
apologize to the authors.
* * * * *
493. That electro-chemical decomposition does not depend upon any direct
attraction and repulsion of the poles (meaning thereby the metallic
terminations either of the voltaic battery, or ordinary electrical machine
arrangements (312.),) upon the elements in contact with or near to them,
appeared very evident from the experiments made in air (462, 465, &c.),
when the substances evolved did not collect about any poles, but, in
obedience to the direction of the current, were evolved, and I would say
ejected, at the extremities of the decomposing substance. But
notwithstanding the extreme dissimilarity in the character of air and
metals, and the almost total difference existing between them as to their
mode of conducting electricity, and becoming charged with it, it might
perhaps still be contended, although quite hypothetically, that the
bounding portions of air were now the surfaces or places of attraction, as
the metals had been supposed to be before. In illustration of this and
other points, I endeavoured to devise an arrangement by which I could
decompose a body against a surface of water, as well as against air or
metal, and succeeded in doing so unexceptionably in the following manner.
As the experiment for very natural reasons requires many precautions, to be
successful, and will be referred to hereafter in illustration of the views
I shall venture to give, I must describe it minutely.
494. A glass basin (fig. 52.), four inches in diameter and four inches
deep, had a division of mica _a_, fixed across the upper part so as to
descend one inch and a half below the edge, and be perfectly water-tight at
the sides: a plate of platina _b_, three inches wide, was put into the
basin on one side of the division _a_, and retained there by a glass block
below, so that any gas produced by it in a future stage of the experiment
should not ascend beyond the mica, and cause currents in the liquid on that
side. A strong solution of sulphate of magnesia was carefully poured
without splashing into the basin, until it rose a little above the lower
edge of the mica division _a_, great care being taken that the glass or
mica on the unoccupied or _c_ side of the division in the figure, should
not be moistened by agitation of the solution above the level to which it
rose. A thin piece of clean cork, well-wetted in distilled water, was then
carefully and lightly placed on the solution at the _c_ side, and distilled
water poured gently on to it until a stratum the eighth of an inch in
thickness appeared over the sulphate of magnesia; all was then left for a
few minutes, that any solution adhering to the cork might sink away from
it, or be removed by the water on which it now floated; and then more
distilled water was added in a similar manner, until it reached nearly to
the top of the glass. In this way solution of the sulphate occupied the
lower part of the glass, and also the upper on the right-hand side of the
mica; but on the left-hand side of the division a stratum of water from _c_
to _d_, one inch and a half in depth, reposed upon it, the two presenting,
when looked through horizontally, a comparatively definite plane of
contact. A second platina pole _e_, was arranged so as to be just under the
surface of the water, in a position nearly horizontal, a little inclination
being given to it, that gas evolved during decomposition might escape: the
part immersed was three inches and a half long by one inch wide, and about
seven-eighths of an inch of water intervened between it and the solution of
sulphate of magnesia.
495. The latter pole _e_ was now connected with the negative end of a
voltaic battery, of forty pairs of plates four inches square, whilst the
former pole _b_ was connected with the positive end. There was action and
gas evolved at both poles; but from the intervention of the pure water, the
decomposition was very feeble compared to what the battery would have
effected in a uniform solution. After a little while (less than a minute,)
magnesia also appeared at the negative side: _it did not make its
appearance at the negative metallic pole, but in the water_, at the plane
where the solution and the water met; and on looking at it horizontally, it
could be there perceived lying in the water upon the solution, not rising
more than the fourth of an inch above the latter, whilst the water between
it and the negative pole was perfectly clear. On continuing the action, the
bubbles of hydrogen rising upwards from the negative pole impressed a
circulatory movement on the stratum of water, upwards in the middle, and
downwards at the side, which gradually gave an ascending form to the cloud
of magnesia in the part just under the pole, having an appearance as if it
were there attracted to it; but this was altogether an effect of the
currents, and did not occur until long after the phenomena looked for were
satisfactorily ascertained.
496. After a little while the voltaic communication was broken, and the
platina poles removed with as little agitation as possible from the water
and solution, for the purpose of examining the liquid adhering to them. The
pole _c_, when touched by turmeric paper, gave no traces of alkali, nor
could anything but pure water be found upon it. The pole _b_, though drawn
through a much greater depth and quantity of fluid, was found so acid as to
give abundant evidence to litmus paper, the tongue, and other tests. Hence
there had been no interference of alkaline salts in any way, undergoing
first decomposition, and then causing the separation of the magnesia at a
distance from the pole by mere chemical agencies. This experiment was
repeated again and again, and always successfully.
497. As, therefore, the substances evolved in cases of electrochemical
decomposition may be made to appear against air (465. 469.),--which,
according to common language, is not a conductor, nor is decomposed, or
against water (495.), which is a conductor, and can be decomposed,--as well
as against the metal poles, which are excellent conductors, but
undecomposable, there appears but little reason to consider the phenomena
generally, as due to the _attraction_ or attractive powers of the latter,
when used in the ordinary way, since similar attractions can hardly be
imagined in the former instances.
498. It may be said that the surfaces of air or of water in these cases
become the poles, and exert attractive powers; but what proof is there of
that, except the fact that the matters evolved collect there, which is the
point to be explained, and cannot be justly quoted as its own explanation?
Or it may be said, that any section of the humid conductor, as that in the
present case, where the solution and the water meet, may be considered as
representing the pole. But such does not appear to me to be the view of
those who have written on the subject, certainly not of some of them, and
is inconsistent with the supposed laws which they have assumed, as
governing the diminution of power at increased distances from the poles.
499. Grotthuss, for instance, describes the poles as centres of attractive
and repulsive forces (481.), these forces varying inversely as the squares
of the distances, and says, therefore, that a particle placed anywhere
between the poles will be acted upon by a constant force. But the compound
force, resulting from such a combination as he supposes, would be anything
but a constant force; it would evidently be a force greatest at the poles,
and diminishing to the middle distance. Grotthuss is right, however, _in
the fact_, according to my experiments (502. 505.), that the particles are
acted upon by equal force everywhere in the circuit, when the conditions of
the experiment are the simplest possible; but the fact is against his
theory, and is also, I think, against all theories that place the
decomposing effect in the attractive power of the poles.
500. Sir Humphry Davy, who also speaks of the _diminution_ of power with
increase of distance from the poles[A] (483.), supposes, that when both
poles are acting on substances to decompose them, still the power of
decomposition _diminishes_ to the middle distance. In this statement of
fact he is opposed to Grotthuss, and quotes an experiment in which sulphate
of potassa, placed at different distances from the poles in a humid
conductor of constant length, decomposed when near the pole, but not when
at a distance. Such a consequence would necessarily result theoretically
from considering the poles as centres of attraction and repulsion; but I
have not found the statement borne out by other experiments (505.); and in
the one quoted by him the effect was doubtless due to some of the many
interfering causes of variation which attend such investigations.
[A] Philosophical Transactions, 1807, p. 42.
501. A glass vessel had a platina plate fixed perpendicularly across it, so
as to divide it into two cells: a head of mica was fixed over it, so as to
collect the gas it might evolve during experiments; then each cell, and the
space beneath the mica, was filled with dilute sulphuric acid. Two poles
were provided, consisting each of a platina wire terminated by a plate of
the same metal; each was fixed into a tube passing through its upper end by
an air-tight joint, that it might be moveable, and yet that the gas evolved
at it might be collected. The tubes were filled with the acid, and one
immersed in each cell. Each platina pole was equal in surface to one side
of the dividing plate in the middle glass vessel, and the whole might be
considered as an arrangement between the poles of the battery of a humid
decomposable conductor divided in the middle by the interposed platina
diaphragm. It was easy, when required, to draw one of the poles further up
the tube, and then the platina diaphragm was no longer in the middle of the
humid conductor. But whether it were thus arranged at the middle, or
towards one side, it always evolved a quantity of oxygen and hydrogen equal
to that evolved by both the extreme plates[A].
[A] There are certain precautions, in this and such experiments, which
can only be understood and guarded against by a knowledge of the
phenomena to be described in the first part of the Sixth Series of
these Researches.
502. If the wires of a galvanometer be terminated by plates, and these be
immersed in dilute acid, contained in a regularly formed rectangular glass
trough, connected at each end with a voltaic battery by poles equal to the
section of the fluid, a part of the electricity will pass through the
instrument and cause a certain deflection. And if the plates are always
retained at the _same distance from each other_ and from the sides of the
trough, are always parallel to each other, and uniformly placed relative to
the fluid, then, whether they are immersed near the middle of the
decomposing solution, or at one end, still the instrument will indicate the
same deflection, and consequently the same electric influence.
503. It is very evident, that when the width of the decomposing conductor
varies, as is always the case when mere wires or plates, as poles, are
dipped into or are surrounded by solution, no constant expression can be
given as to the action upon a single particle placed in the course of the
current, nor any conclusion of use, relative to the supposed attractive or
repulsive force of the poles, be drawn. The force will vary as the distance
from the pole varies; as the particle is directly between the poles, or
more or less on one side; and even as it is nearer to or further from the
sides of the containing vessels, or as the shape of the vessel itself
varies; and, in fact, by making variations in the form of the arrangement,
the force upon any single particle may be made to increase, or diminish, or
remain constant, whilst the distance between the particle and the pole
shall remain the same; or the force may be made to increase, or diminish,
or remain constant, either as the distance increases or as it diminishes.
504. From numerous experiments, I am led to believe the following general
expression to be correct; but I purpose examining it much further, and
would therefore wish not to be considered at present as pledged to its
accuracy. The _sum of chemical decomposition is constant_ for any section
taken across a decomposing conductor, uniform in its nature, at whatever
distance the poles may be from each other or from the section; or however
that section may intersect the currents, whether directly across them, or
so oblique as to reach almost from pole to pole, or whether it be plane, or
curved, or irregular in the utmost degree; provided the current of
electricity be retained constant in quantity (377.), and that the section
passes through every part of the current through the decomposing conductor.
505. I have reason to believe that the statement might be made still more
general, and expressed thus: That _for a constant quantity of electricity,
whatever the decomposing conductor may be, whether water, saline solutions,
acids, fused bodies, &c., the amount of electro-chemical action is also a
constant quantity, i.e. would always be equivalent to a standard chemical
effect founded upon ordinary chemical affinity_. I have this investigation
in hand, with several others, and shall be prepared to give it in the next
series but one of these Researches.
506. Many other arguments might be adduced against the hypotheses of the
attraction of the poles being the cause of electro-chemical decomposition;
but I would rather pass on to the view I have thought more consistent with
facts, with this single remark; that if decomposition by the voltaic
battery depended upon the attraction of the poles, or the parts about them,
being stronger than the mutual attraction of the particles separated, it
would follow that the weakest _electrical_ attraction was stronger than, if
not the strongest, yet very strong _chemical_ attraction, namely, such as
exists between oxygen and hydrogen, potassium and oxygen, chlorine and
sodium, acid and alkali, &c., a consequence which, although perhaps not
impossible, seems in the present state of the subject very unlikely.
507. The view which M. de la Rive has taken (489.), and also MM. Riffault
and Chompré (485.), of the manner in which electro-chemical decomposition
is effected, is very different to that already considered, and is not
affected by either the arguments or facts urged against the latter.
Considering it as stated by the former philosopher, it appears to me to be
incompetent to account for the experiments of decomposition against
surfaces of air (462. 469.) and water (495.), which I have described; for
if the physical differences between metals and humid conductors, which M.
de la Rive supposes to account for the transmission of the compound of
matter and electricity in the latter, and the transmission of the
electricity only with the rejection of the matter in the former, be allowed
for a moment, still the analogy of air to metal is, electrically
considered, so small, that instead of the former replacing the latter
(462.), an effect the very reverse might have been expected. Or if even
that were allowed, the experiment with water (495.), at once sets the
matter at rest, the decomposing pole being now of a substance which is
admitted as competent to transmit the assumed compound of electricity and
matter.
508. With regard to the views of MM. Riffault and Chompré (485.), the
occurrence of decomposition alone in the _course_ of the current is so
contrary to the well-known effects obtained in the forms of experiment
adopted up to this time, that it must be proved before the hypothesis
depending on it need be considered.
509. The consideration of the various theories of electro-chemical
decomposition, whilst it has made me diffident, has also given me
confidence to add another to the number; for it is because the one I have
to propose appears, after the most attentive consideration, to explain and
agree with the immense collection of facts belonging to this branch of
science, and to remain uncontradicted by, or unopposed to, any of them,
that I have been encouraged to give it.
510. Electro-chemical decomposition is well known to depend essentially
upon the _current_ of electricity. I have shown that in certain cases
(375.) the decomposition is proportionate to the quantity of electricity
passing, whatever may be its intensity or its source, and that the same is
probably true for all cases (377.), even when the utmost generality is
taken on the one hand, and great precision of expression on the other
(505.).
511. In speaking of the current, I find myself obliged to be still more
particular than on a former occasion (283.), in consequence of the variety
of views taken by philosophers, all agreeing in the effect of the current
itself. Some philosophers, with Franklin, assume but one electric fluid;
and such must agree together in the general uniformity and character of the
electric current. Others assume two electric fluids; and here singular
differences have arisen.
512. MM. Riffault and Chompré, for instance, consider the positive and
negative currents each as causing decomposition, and state that the
positive current is _more powerful_ than the negative current[A], the
nitrate of soda being, under similar circumstances, decomposed by the
former, but not by the latter.
[A] Annales de Chimie, 1807, tom, lxiii. p. 84.
513. M. Hachette states[A] that "it is not necessary, as has been believed,
that the action of the two electricities, positive and negative, should be
simultaneous for the decomposition of water." The passage implying, if I
have caught the meaning aright, that one electricity can be obtained, and
can be applied in effecting decompositions, independent of the other.
[A] Annales de Chimie, 1832, tom. li. p. 73.
514. The view of M. de la Rive to a certain extent agrees with that of M.
Hachette, for he considers that the two electricities decompose separate
portions of water (490.)[A]. In one passage he speaks of the two
electricities as two influences, wishing perhaps to avoid offering a
decided opinion upon the independent existence of electric fluids; but as
these influences are considered as combining with the elements set free as
by a species of chemical affinity, and for the time entirely masking their
character, great vagueness of idea is thus introduced, inasmuch as such a
species of combination can only be conceived to take place between things
having independent existences. The two elementary electric currents, moving
in opposite directions, from pole to pole, constitute the ordinary _voltaic
current._
[A] Annales de Chimie, 1825, tom, xxviii. pp. 197, 201.
515. M. Grotthuss is inclined to believe that the elements of water, when
about to separate at the poles, combine with the electricities, and so
become gases. M. de la Rive's view is the exact reverse of this: whilst
passing through the fluid, they are, according to him, compounds with the
electricities; when evolved at the poles, they are de-electrified.
516. I have sought amongst the various experiments quoted in support of
these views, or connected with electro-chemical decompositions or electric
currents, for any which might be considered as sustaining the theory of two
electricities rather than that of one, but have not been able to perceive a
single fact which could be brought forward for such a purpose: or,
admitting the hypothesis of two electricities, much less have I been able
to perceive the slightest grounds for believing that one electricity in a
current can be more powerful than the other, or that it can be present
without the other, or that one can be varied or in the slightest degree
affected, without a corresponding variation in the other[A]. If, upon the
supposition of two electricities, a current of one can be obtained without
the other, or the current of one be exalted or diminished more than the
other, we might surely expect some variation either of the chemical or
magnetical effects, or of both; but no such variations have been observed.
If a current be so directed that it may act chemically in one part of its
course, and magnetically in another, the two actions are always found to
take place together. A current has not, to my knowledge, been produced
which could act chemically and not magnetically, nor any which can act on
the magnet, and not _at the same time_ chemically[B].
[A] See now in relation to this subject, 1627-1645.--_Dec. 1838._
[B] Thermo-electric currents are of course no exception, because when
they fail to act chemically they also fail to be currents.
517. _Judging from facts only_, there is not as yet the slightest reason
for considering the influence which is present in what we call the electric
current,--whether in metals or fused bodies or humid conductors, or even in
air, flame, and rarefied elastic media,--as a compound or complicated
influence. It has never been resolved into simpler or elementary
influences, and may perhaps best be conceived of as _an axis of power
having contrary forces, exactly equal in amount, in contrary directions_.
* * * * *
518. Passing to the consideration of electro-chemical decomposition, it
appears to me that the effect is produced by an _internal corpuscular
action_, exerted according to the direction of the electric current, and
that it is due to a force either _super to_, or _giving direction to the
ordinary chemical affinity_ of the bodies present. The body under
decomposition may be considered as a mass of acting particles, all those
which are included in the course of the electric current contributing to
the final effect; and it is because the ordinary chemical affinity is
relieved, weakened, or partly neutralized by the influence of the electric
current in one direction parallel to the course of the latter, and
strengthened or added to in the opposite direction, that the combining
particles have a tendency to pass in opposite courses.
519. In this view the effect is considered as _essentially dependent_ upon
the _mutual chemical affinity_ of the particles of opposite kinds.
Particles _aa_, fig. 53, could not be transferred or travel from one pole N
towards the other P, unless they found particles of the opposite kind _bb_,
ready to pass in the contrary direction: for it is by virtue of their
increased affinity for those particles, combined with their diminished
affinity for such as are behind them in their course, that they are urged
forward: and when any one particle _a_, fig. 54, arrives at the pole, it is
excluded or set free, because the particle _b_ of the opposite kind, with
which it was the moment before in combination, has, under the superinducing
influence of the current, a greater attraction for the particle _a'_, which
is before it in its course, than for the particle _a_, towards which its
affinity has been weakened.
520. As far as regards any single compound particle, the case may be
considered as analogous to one of ordinary decomposition, for in fig. 54,
_a_ may be conceived to be expelled from the compound _ab_ by the superior
attraction of _a'_ for _b_, that superior attraction belonging to it in
consequence of the relative position of _a'b_ and _a_ to the direction of
the axis of electric power (517.) superinduced by the current. But as all
the compound particles in the course of the current, except those actually
in contact with the poles, act conjointly, and consist of elementary
particles, which, whilst they are in one direction expelling, are in the
other being expelled, the case becomes more complicated, but not more
difficult of comprehension.
521. It is not here assumed that the acting particles must be in a right
line between the poles. The lines of action which may be supposed to
represent the electric currents passing through a decomposing liquid, have
in many experiments very irregular forms; and even in the simplest case of
two wires or points immersed as poles in a drop or larger single portion of
fluid, these lines must diverge rapidly from the poles; and the direction
in which the chemical affinity between particles is most powerfully
modified (519. 520.) will vary with the direction of these lines, according
constantly with them. But even in reference to these lines or currents, it
is not supposed that the particles which mutually affect each other must of
necessity be parallel to them, but only that they shall accord generally
with their direction. Two particles, placed in a line perpendicular to the
electric current passing in any particular place, are not supposed to have
their ordinary chemical relations towards each other affected; but as the
line joining them is inclined one way to the current their mutual affinity
is increased; as it is inclined in the other direction it is diminished;
and the effect is a maximum, when that line is parallel to the current[A].
[A] In reference to this subject see now electrolytic induction and
discharge, Series XII. ¶ viii. 1343-1351, &c.--_Dec. 1838._
522. That the actions, of whatever kind they may be, take place frequently
in oblique directions is evident from the circumstance of those particles
being included which in numerous cases are not in a line between the poles.
Thus, when wires are used as poles in a glass of solution, the
decompositions and recompositions occur to the right or left of the direct
line between the poles, and indeed in every part to which the currents
extend, as is proved by many experiments, and must therefore often occur
between particles obliquely placed as respects the current itself; and when
a metallic vessel containing the solution is made one pole, whilst a mere
point or wire is used for the other, the decompositions and recompositions
must frequently be still more oblique to the course of the currents.
523. The theory which I have ventured to put forth (almost) requires an
admission, that in a compound body capable of electro-chemical
decomposition the elementary particles have a mutual relation to, and
influence upon each other, extending beyond those with which they are
immediately combined. Thus in water, a particle of hydrogen in combination
with oxygen is considered as not altogether indifferent to other particles
of oxygen, although they are combined with other particles of hydrogen; but
to have an affinity or attraction towards them, which, though it does not
at all approach in force, under ordinary circumstances, to that by which it
is combined with its own particle, can, under the electric influence,
exerted in a definite direction, be made even to surpass it. This general
relation of particles already in combination to other particles with which
they are not combined, is sufficiently distinct in numerous results of a
purely chemical character; especially in those where partial decompositions
only take place, and in Berthollet's experiments on the effects of quantity
upon affinity: and it probably has a direct relation to, and connexion
with, attraction of aggregation, both in solids and fluids. It is a
remarkable circumstance, that in gases and vapours, where the attraction of
aggregation ceases, there likewise the decomposing powers of electricity
apparently cease, and there also the chemical action of quantity is no
longer evident. It seems not unlikely, that the inability to suffer
decomposition in these cases may be dependent upon the absence of that
mutual attractive relation of the particles which is the cause of
aggregation.
524. I hope I have now distinctly stated, although in general terms, the
view I entertain of the cause of electro-chemical decomposition, _as far as
that cause can at present be traced and understood_. I conceive the effects
to arise from forces which are _internal_, relative to the matter under
decomposition--and _not external_, as they might be considered, if directly
dependent upon the poles. I suppose that the effects are due to a
modification, by the electric current, of the chemical affinity of the
particles through or by which that current is passing, giving them the
power of acting more forcibly in one direction than in another, and
consequently making them travel by a series of successive decompositions
and recompositions in opposite directions, and finally causing their
expulsion or exclusion at the boundaries of the body under decomposition,
in the direction of the current, _and that_ in larger or smaller
quantities, according as the current is more or less powerful (377.). I
think, therefore, it would be more philosophical, and more directly
expressive of the facts, to speak of such a body, in relation to the
current passing through it, rather than to the poles, as they are usually
called, in contact with it; and say that whilst under decomposition,
oxygen, chlorine, iodine, acids, &c., are rendered at its negative
extremity, and combustibles, metals, alkalies, bases, &c., at its positive
extremity (467.), I do not believe that a substance can be transferred in
the electric current beyond the point where it ceases to find particles
with which it can combine; and I may refer to the experiments made in air
(465.) and in water (495.), already quoted, for facts illustrating these
views in the first instance; to which I will now add others.
525. In order to show the dependence of the decomposition and transfer of
elements upon the chemical affinity of the substances present, experiments
were made upon sulphuric acid in the following manner. Dilute sulphuric
acid was prepared: its specific gravity was 1.0212. A solution of sulphate
of soda was also prepared, of such strength that a measure of it contained
exactly as much sulphuric acid as an equal measure of the diluted acid just
referred to. A solution of pure soda, and another of pure ammonia, were
likewise prepared, of such strengths that a measure of either should be
exactly neutralized by a measure of the prepared sulphuric acid.
526. Four glass cups were then arranged, as in fig. 55; seventeen measures
of the free sulphuric acid (525.) were put into each of the vessels _a_ and
_b_, and seventeen measures of the solution of sulphate of soda into each
of the vessels A and B. Asbestus, which had been well-washed in acid, acted
upon by the voltaic pile, well-washed in water, and dried by pressure, was
used to connect _a_ with _b_ and A with B, the portions being as equal as
they could be made in quantity, and cut as short as was consistent with
their performing the part of effectual communications, _b_ and A were
connected by two platina plates or poles soldered to the extremities of one
wire, and the cups _a_ and B were by similar platina plates connected with
a voltaic battery of forty pairs of plates four inches square, that in _a_
being connected with the negative, and that in B with the positive pole.
The battery, which was not powerfully charged, was retained in
communication above half an hour. In this manner it was certain that the
same electric current had passed through _a b_ and A B, and that in each
instance the same quantity and strength of acid had been submitted to its
action, but in one case merely dissolved in water, and in the other
dissolved and also combined with an alkali.
527. On breaking the connexion with the battery, the portions of asbestus
were lifted out, and the drops hanging at the ends allowed to fall each
into its respective vessel. The acids in _a_ and _b_ were then first
compared, for which purpose two evaporating dishes were balanced, and the
acid from _a_ put into one, and that from _b_ into the other; but as one
was a little heavier than the other, a small drop was transferred from the
heavier to the lighter, and the two rendered equal in weight. Being
neutralized by the addition of the soda solution (525.), that from _a_, or
the negative vessel, required 15 parts of the soda solution, and that from
_b_, or the positive vessel, required 16.3 parts. That the sum of these is
not 34 parts is principally due to the acid removed with the asbestus; but
taking the mean of 15.65 parts, it would appear that a twenty-fourth part
of the acid originally in the vessel _a_ had passed, through the influence
of the electric current, from _a_ into _b_.
528. In comparing the difference of acid in A and B, the necessary equality
of weight was considered as of no consequence, because the solution was at
first neutral, and would not, therefore, affect the test liquids, and all
the evolved acid would be in B, and the free alkali in A. The solution in A
required 3.2 measures of the prepared acid (525.) to neutralize it, and the
solution in B required also 3.2 measures of the soda solution (525.) to
neutralize it. As the asbestus must have removed a little acid and alkali
from the glasses, these quantities are by so much too small; and therefore
it would appear that about a tenth of the acid originally in the vessel A
had been transferred into B during the continuance of the electric action.
529. In another similar experiment, whilst a thirty-fifth part of the acid
passed from _a_ to _b_; in the free acid vessels, between a tenth and an
eleventh passed from A to B in the combined acid vessels. Other experiments
of the same kind gave similar results.
530. The variation of electro-chemical decomposition, the transfer of
elements and their accumulation at the poles, according as the substance
submitted to action consists of particles opposed more or less in their
chemical affinity, together with the consequent influence of the latter
circumstances, are sufficiently obvious in these cases, where sulphuric
acid is acted upon in the _same quantity_ by the _same_ electric current,
but in one case opposed to the comparatively weak affinity of water for it,
and in the other to the stronger one of soda. In the latter case the
quantity transferred is from two and a half to three times what it is in
the former; and it appears therefore very evident that the transfer is
greatly dependent upon the mutual action of the particles of the
decomposing bodies[A].
[A] See the note to (675.),--_Dec. 1838._
531. In some of the experiments the acid from the vessels _a_ and _b_ was
neutralized by ammonia, then evaporated to dryness, heated to redness, and
the residue examined for sulphates. In these cases more sulphate was always
obtained from _a_ than from _b_; showing that it had been impossible to
exclude saline bases (derived from the asbestus, the glass, or perhaps
impurities originally in the acid,) and that they had helped in
transferring the acid into _b_. But the quantity was small, and the acid
was principally transferred by relation to the water present.
532. I endeavoured to arrange certain experiments by which saline solutions
should be decomposed against surfaces of water; and at first worked with
the electric machine upon a piece of bibulous paper, or asbestus moistened
in the solution, and in contact at its two extremities with pointed pieces
of paper moistened in pure water, which served to carry the electric
current to and from the solution in the middle piece. But I found numerous
interfering difficulties. Thus, the water and solutions in the pieces of
paper could not be prevented from mingling at the point where they touched.
Again, sufficient acid could be derived from the paper connected with the
discharging train, or it may be even from the air itself, under the
influence of electric action, to neutralize the alkali developed at the
positive extremity of the decomposing solution, and so not merely prevent
its appearance, but actually transfer it on to the metal termination: and,
in fact, when the paper points were not allowed to touch there, and the
machine was worked until alkali was evolved at the delivering or positive
end of the turmeric paper, containing the sulphate of soda solution, it was
merely necessary to place the opposite receiving point of the paper
connected with the discharging train, which had been moistened by distilled
water, upon the brown turmeric point and press them together, when the
alkaline effect immediately disappeared.
533. The experiment with sulphate of magnesia already described (495.) is a
case in point, however, and shows most clearly that the sulphuric acid and
magnesia contributed to each other's transfer and final evolution, exactly
as the same acid and soda affected each other in the results just given
(527, &c.); and that so soon as the magnesia advanced beyond the reach of
the acid, and found no other substance with which it could combine, it
appeared in its proper character, and was no longer able to continue its
progress towards the negative pole.
* * * * *
534. The theory I have ventured to put forth appears to me to explain all
the prominent features of electro-chemical decomposition in a satisfactory
manner.
535. In the first place, it explains why, in all ordinary cases, the
evolved substances _appear only at the poles_; for the poles are the
limiting surfaces of the decomposing substance, and except at them, every
particle finds other particles having a contrary tendency with which it can
combine.
536. Then it explains why, in numerous cases, the elements or evolved
substances are not _retained_ by the poles; and this is no small difficulty
in those theories which refer the decomposing effect directly to the
attractive power of the poles. If, in accordance with the usual theory, a
piece of platina be supposed to have sufficient power to attract a particle
of hydrogen from the particle of oxygen with which it was the instant
before combined, there seems no sufficient reason, nor any fact, except
those to be explained, which show why it should not, according to analogy
with all ordinary attractive forces, as those of gravitation, magnetism,
cohesion, chemical affinity, &c. _retain_ that particle which it had just
before taken from a distance and from previous combination. Yet it does not
do so, but allows it to escape freely. Nor does this depend upon its
assuming the gaseous state, for acids and alkalies, &c. are left equally at
liberty to diffuse themselves through the fluid surrounding the pole, and
show no particular tendency to combine with or adhere to the latter. And
though there are plenty of cases where combination with the pole does take
place, they do not at all explain the instances of non-combination, and do
not therefore in their particular action reveal the general principle of
decomposition.
537. But in the theory that I have just given, the effect appears to be a
natural consequence of the action: the evolved substances are _expelled_
from the decomposing mass (518. 519.), not _drawn out by an attraction_
which ceases to act on one particle without any assignable reason, while it
continues to act on another of the same kind: and whether the poles be
metal, water, or air, still the substances are evolved, and are sometimes
set free, whilst at others they unite to the matter of the poles, according
to the chemical nature of the latter, i.e. their chemical relation to those
particles which are leaving the substance under operation.
538. The theory accounts for the _transfer of elements_ in a manner which
seems to me at present to leave nothing unexplained; and it was, indeed,
the phenomena of transfer in the numerous cases of decomposition of bodies
rendered fluid by heat (380. 402.), which, in conjunction with the
experiments in air, led to its construction. Such cases as the former where
binary compounds of easy decomposability are acted upon, are perhaps the
best to illustrate the theory.
539. Chloride of lead, for instance, fused in a bent tube (400.), and
decomposed by platina wires, evolves lead, passing to what is usually
called the negative pole, and chlorine, which being evolved at the positive
pole, is in part set free, and in part combines with the platina. The
chloride of platina formed, being soluble in the chloride of lead, is
subject to decomposition, and the platina itself is gradually transferred
across the decomposing matter, and found with the lead at the negative
pole.
540. Iodide of lead evolves abundance of lead at the negative pole, and
abundance of iodine at the positive pole.
541. Chloride of silver furnishes a beautiful instance, especially when
decomposed by silver wire poles. Upon fusing a portion of it on a piece of
glass, and bringing the poles into contact with it, there is abundance of
silver evolved at the negative pole, and an equal abundance absorbed at the
positive pole, for no chlorine is set free: and by careful management, the
negative wire may be withdrawn from the fused globule as the silver is
reduced there, the latter serving as the continuation of the pole, until a
wire or thread of revived silver, five or six inches in length, is
produced; at the same time the silver at the positive pole is as rapidly
dissolved by the chlorine, which seizes upon it, so that the wire has to be
continually advanced as it is melted away. The whole experiment includes
the action of only two elements, silver and chlorine, and illustrates in a
beautiful manner their progress in opposite directions, parallel to the
electric current, which is for the time giving a uniform general direction
to their mutual affinities (524.).
542. According to my theory, an element or a substance not decomposable
under the circumstances of the experiment, (as for instance, a dilute acid
or alkali,) should not be transferred, or pass from pole to pole, unless it
be in chemical relation to some other element or substance tending to pass
in the opposite direction, for the effect is considered as essentially due
to the mutual relation of such particles. But the theories attributing the
determination of the elements to the attractions and repulsions of the
poles require no such condition, i.e. there is no reason apparent why the
attraction of the positive pole, and the repulsion of the negative pole,
upon a particle of free acid, placed in water between them, should not
(with equal currents of electricity) be as strong as if that particle were
previously combined with alkali; but, on the contrary, as they have not a
powerful chemical affinity to overcome, there is every reason to suppose
they would be stronger, and would sooner bring the acid to rest at the
positive pole[A]. Yet such is not the case, as has been shown by the
experiments on free and combined acid (526. 528.).
[A] Even Sir Humphry Davy considered the attraction of the pole as
being communicated from one particle to another of the _same_ kind
(483.).
543. Neither does M. de la Rive's theory, as I understand it, _require_
that the particles should be in combination: it does not even admit, where
there are two sets of particles capable of combining with and passing by
each other, that they do combine, but supposes that they travel as separate
compounds of matter and electricity. Yet in fact the free substance
_cannot_ travel, the combined one _can_.
544. It is very difficult to find cases amongst solutions or fluids which
shall illustrate this point, because of the difficulty of finding two
fluids which shall conduct, shall not mingle, and in which an element
evolved from one shall not find a combinable element in the other.
_Solutions_ of acids or alkalies will not answer, because they exist by
virtue of an attraction; and increasing the solubility of a body in one
direction, and diminishing it in the opposite, is just as good a reason for
transfer, as modifying the affinity between the acids and alkalies
themselves[A]. Nevertheless the case of sulphate of magnesia is in point
(494. 495.), and shows that _one element or principle only_ has no power of
transference or of passing towards either pole.
[A] See the note to (670.).--_Dec. 1838._
545. Many of the metals, however, in their solid state, offer very fair
instances of the kind required. Thus, if a plate of platina be used as the
positive pole in a solution of sulphuric acid, oxygen will pass towards it,
and so will acid; but these are not substances having such chemical
relation to the platina as, even under the favourable condition
superinduced by the current (518. 524.), to combine with it; the platina
therefore remains where it was first placed, and has no tendency to pass
towards the negative pole. But if a plate of iron, zinc or copper, be
substituted for the platina, then the oxygen and acid can combine with
these, and the metal immediately begins to travel (as an oxide) to the
opposite pole, and is finally deposited there. Or if, retaining the platina
pole, a fused chloride, as of lead, zinc, silver, &c., be substituted for
the sulphuric acid, then, as the platina finds an element it can combine
with, it enters into union, acts as other elements do in cases of voltaic
decomposition, is rapidly transferred across the melted matter, and
expelled at the negative pole.
546. I can see but little reason in the theories referring the
electro-chemical decomposition to the attractions and repulsions of the
poles, and I can perceive none in M. de la Rive's theory, why the metal of
the positive pole should not be transferred across the intervening
conductor, and deposited at the negative pole, even when it cannot act
chemically upon the element of the fluid surrounding it. It cannot be
referred to the attraction of cohesion preventing such an effect; for if
the pole be made of the lightest spongy platina, the effect is the same. Or
if gold precipitated by sulphate of iron be diffused through the solution,
still accumulation of it at the negative pole will not take place; and yet
the attraction of cohesion is almost perfectly overcome, the particles are
in it so small as to remain for hours in suspension, and are perfectly free
to move by the slightest impulse towards either pole; and _if in relation_
by chemical affinity to any substance present, are powerfully determined to
the negative pole[A].
[A] In making this experiment, care must be taken that no substance be
present that can act chemically on the gold. Although I used the metal
very carefully washed, and diffused through dilute sulphuric acid, yet
in the first instance I obtained gold at the negative pole, and the
effect was repeated when the platina poles were changed. But on
examining the clear liquor in the cell, after subsidence of the
metallic gold, I found a little of that metal in solution, and a
little chlorine was also present. I therefore well washed the gold
which had thus been subjected to voltaic action, diffused it through
other pure dilute sulphuric acid, and then found, that on subjecting
it to the action of the pile, not the slightest tendency to the
negative pole could be perceived.
547. In support of these arguments, it may be observed, that as yet no
determination of a substance to a pole, or tendency to obey the electric
current, has been observed (that I am aware of,) in cases of mere mixture;
i.e. a substance diffused through a fluid, but having no sensible chemical
affinity with it, or with substances that may be evolved from it during the
action, does not in any case seem to be affected by the electric current.
Pulverised charcoal was diffused through dilute sulphuric acid, and
subjected with the solution to the action of a voltaic battery, terminated
by platina poles; but not the slightest tendency of the charcoal to the
negative pole could be observed, Sublimed sulphur was diffused through
similar acid, and submitted to the same action, a silver plate being used
as the negative pole; but the sulphur had no tendency to pass to that pole,
the silver was not tarnished, nor did any sulphuretted hydrogen appear. The
case of magnesia and water (495. 533.), with those of comminuted metals in
certain solutions (546.), are also of this kind; and, in fact, substances
which have the instant before been powerfully determined towards the pole,
as magnesia from sulphate of magnesia, become entirely _indifferent to it_
the moment they assume their independent state, and pass away, diffusing
themselves through the surrounding fluid.
548. There are, it is true, many instances of insoluble bodies being acted
upon, as glass, sulphate of baryta, marble, slate, basalt, &c., but they
form no exception; for the substances they give up are in direct and strong
relation as to chemical affinity with those which they find in the
surrounding solution, so that these decompositions enter into the class of
ordinary effects.
549. It may be expressed as a general consequence, that the more directly
bodies are opposed to each other in chemical affinity, the more _ready_ is
their separation from each other in cases of electro-chemical
decomposition, i.e. provided other circumstances, as insolubility,
deficient conducting power, proportions, &c., do not interfere. This is
well known to be the case with water and saline solutions; and I have found
it to be equally true with _dry_ chlorides, iodides, salts, &c., rendered
subject to electro-chemical decomposition by fusion (402.). So that in
applying the voltaic battery for the purpose of decomposing bodies not yet
resolved into forms of matter simpler than their own, it must be
remembered, that success may depend not upon the weakness, or failure upon
the strength, of the affinity by which the elements sought for are held
together, but contrariwise; and then modes of application may be devised,
by which, in _association_ with ordinary chemical powers, and the
assistance of fusion (394. 417.), we may be able to penetrate much further
than at present into the constitution of our chemical elements.
550. Some of the most beautiful and surprising cases of electro-chemical
decomposition and _transfer_ which Sir Humphry Davy described in his
celebrated paper[A], were those in which acids were passed through
alkalies, and alkalies or earths through acids[B]; and the way in which
substances having the most powerful attractions for each other were thus
prevented from combining, or, as it is said, had their natural affinity
destroyed or suspended throughout the whole of the circuit, excited the
utmost astonishment. But if I be right in the view I have taken of the
effects, it will appear, that that which made the _wonder_, is in fact the
_essential condition_ of transfer and decomposition, and that the more
alkali there is in the course of an acid, the more will the transfer of
that acid be facilitated from pole to pole; and perhaps a better
illustration of the difference between the theory I have ventured, and
those previously existing, cannot be offered than the views they
respectively give of such facts as these.
[A] Philosophical Transactions, 1807, p. 1.
[B] Ibid. p, 24, &c.
551. The instances in which sulphuric acid could not be passed though
baryta, or baryta through sulphuric acid[A], because of the precipitation
of sulphate of baryta, enter within the pale of the law already described
(380. 412.), by which liquidity is so generally required for conduction and
decomposition. In assuming the solid state of sulphate of baryta, these
bodies became virtually non-conductors to electricity of so low a tension
as that of the voltaic battery, and the power of the latter over them was
almost infinitely diminished.
[A] Philosophical Transactions, 1807, p. 25, &c.
552. The theory I have advanced accords in a most satisfactory manner with
the fact of an element or substance finding its place of rest, or rather of
evolution, sometimes at one pole and sometimes at the other. Sulphur
illustrates this effect very well[A]. When sulphuric acid is decomposed by
the pile, sulphur is evolved at the negative pole; but when sulphuret of
silver is decomposed in a similar way (436.), then the sulphur appears at
the positive pole; and if a hot platina pole be used so as to vaporize the
sulphur evolved in the latter case, then the relation of that pole to the
sulphur is exactly the same as the relation of the same pole to oxygen upon
its immersion in water. In both cases the element evolved is liberated at
the pole, but not retained by it; but by virtue of its elastic,
uncombinable, and immiscible condition passes away into the surrounding
medium. The sulphur is evidently determined in these opposite directions by
its opposite chemical relations to oxygen and silver; and it is to such
relations generally that I have referred all electro-chemical phenomena.
Where they do not exist, no electro-chemical action can take place. Where
they are strongest, it is most powerful; where they are reversed, the
direction of transfer of the substance is reversed with them.
[A] At 681 and 757 of Series VII, will be found corrections of the
statement here made respecting sulphur and sulphuric acid. At present
there is no well-ascertained fact which proves that the same body can
go directly to _either_ of the two poles at pleasure.--_Dec. 1838._
553. _Water_ may be considered as one of those substances which can be made
to pass to _either_ pole. When the poles are immersed in dilute sulphuric
acid (527.), acid passes towards the positive pole, and water towards the
negative pole; but when they are immersed in dilute alkali, the alkali
passes towards the negative pole, and water towards the positive pole.
554. Nitrogen is another substance which is considered as determinable to
either pole; but in consequence of the numerous compounds which it forms,
some of which pass to one pole, and some to the other, I have not always
found it easy to determine the true circumstances of its appearance. A pure
strong solution of ammonia is so bad a conductor of electricity that it is
scarcely more decomposable than pure water; but if sulphate of ammonia be
dissolved in it, then decomposition takes place very well; nitrogen almost
pure, and in some cases quite, is evolved at the positive pole, and
hydrogen at the negative pole.
555. On the other hand, if a strong solution of nitrate of ammonia be
decomposed, oxygen appears at the positive pole, and hydrogen, with
sometimes nitrogen, at the negative pole. If fused nitrate of ammonia be
employed, hydrogen appears at the negative pole, mingled with a little
nitrogen. Strong nitric acid yields plenty of oxygen at the positive pole,
but no gas (only nitrous acid) at the negative pole. Weak nitric acid
yields the oxygen and hydrogen of the water present, the acid apparently
remaining unchanged. Strong nitric acid with nitrate of ammonia dissolved
in it, yields a gas at the negative pole, of which the greater part is
hydrogen, but apparently a little nitrogen is present. I believe, that in
some of these cases a little nitrogen appeared at the negative pole. I
suspect, however, that in all these, and in all former cases, the
appearance of the nitrogen at the positive or negative pole is entirely a
secondary effect, and not an immediate consequence of the decomposing power
of the electric current[A].
[A] Refer for proof of the truth of this supposition to 748, 752,
&c.--_Dec. 1838._
556. A few observations on what are called the _poles_ of the voltaic
battery now seem necessary. The poles are merely the surfaces or doors by
which the electricity enters into or passes out of the substance suffering
decomposition. They limit the extent of that substance in the course of the
electric current, being its _terminations_ in that direction: Hence the
elements evolved pass so far and no further.
557. Metals make admirable poles, in consequence of their high conducting
power, their immiscibility with the substances generally acted upon, their
solid form, and the opportunity afforded of selecting such as are not
chemically acted upon by ordinary substances.
558. Water makes a pole of difficult application, except in a few cases
(494.), because of its small conducting power, its miscibility with most of
the substances acted upon, and its general relation to them in respect to
chemical affinity. It consists of elements, which in their electrical and
chemical relations are directly and powerfully opposed, yet combining to
produce a body more neutral in its character than any other. So that there
are but few substances which do not come into relation, by chemical
affinity, with water or one of its elements; and therefore either the water
or its elements are transferred and assist in transferring the infinite
variety of bodies which, in association with it, can be placed in the
course of the electric current. Hence the reason why it so rarely happens
that the evolved substances rest at the first surface of the water, and why
it therefore does not exhibit the ordinary action of a pole.
559. Air, however, and some gases are free from the latter objection, and
may be used as poles in many cases (461, &c.); but, in consequence of the
extremely low degree of conducting power belonging to them, they cannot be
employed with the voltaic apparatus. This limits their use; for the voltaic
apparatus is the only one as yet discovered which supplies sufficient
quantity of electricity (371. 376.) to effect electro-chemical
decomposition with facility.
560. When the poles are liable to the chemical action of the substances
evolved, either simply in consequence of their natural relation to them, or
of that relation aided by the influence of the current (518.), then they
suffer corrosion, and the parts dissolved are subject to transference, in
the same manner as the particles of the body originally under
decomposition. An immense series of phenomena of this kind might be quoted
in support of the view I have taken of the cause of electro-chemical
decomposition, and the transfer and evolution of the elements. Thus platina
being made the positive and negative poles in a solution of sulphate of
soda, has no affinity or attraction for the oxygen, hydrogen, acid, or
alkali evolved, and refuses to combine with or retain them. Zinc can
combine with the oxygen and acid; at the positive pole it does combine, and
immediately begins to travel as oxide towards the negative pole. Charcoal,
which cannot combine with the metals, if made the negative pole in a
metallic solution, refuses to unite to the bodies which are ejected from
the solution upon its surface; but if made the positive pole in a dilute
solution of sulphuric acid, it is capable of combining with the oxygen
evolved there, and consequently unites with it, producing both carbonic
acid and carbonic oxide in abundance.
561. A great advantage is frequently supplied, by the opportunity afforded
amongst the metals of selecting a substance for the pole, which shall or
shall not be acted upon by the elements to be evolved. The consequent use
of platina is notorious. In the decomposition of sulphuret of silver and
other sulphurets, a positive silver pole is superior to a platina one,
because in the former case the sulphur evolved there combines with the
silver, and the decomposition of the original sulphuret is rendered
evident; whereas in the latter case it is dissipated, and the assurance of
its separation at the pole not easily obtained.
562. The effects which take place when a succession of conducting
decomposable and undecomposable substances are placed in the electric
circuit, as, for instance, of wires and solutions, or of air and solutions
(465, 469.), are explained in the simplest possible manner by the
theoretical view I have given. In consequence of the reaction of the
constituents of each portion of decomposable matter, affected as they are
by the supervention of the electric current (524.), portions of the
proximate or ultimate elements proceed in the direction of the current as
far as they find matter of a contrary kind capable of effecting their
transfer, and being equally affected by them; and where they cease to find
such matter, they are evolved in their free state, i.e. upon the surfaces
of metal or air bounding the extent of decomposable matter in the direction
of the current.
563. Having thus given my theory of the mode in which electro-chemical
decomposition is effected, I will refrain for the present from entering
upon the numerous general considerations which it suggests, wishing first
to submit it to the test of publication and discussion.
_Royal Institution,
June 1833._