Experimental Researches in Electricity Part 14

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 pa.s.sing 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 const.i.tuting 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 gla.s.s 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 pa.s.sed off from the string into the air, and diminished the quant.i.ty 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 gla.s.s 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 pa.s.s in succession through them, entering in at the litmus points _b, b_, and pa.s.sing 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 pota.s.sium, 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 circ.u.mstances 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 pota.s.sa_, 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 Inst.i.tution, 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 cla.s.s of substances, instead of being the _only one_ and _essential_; and it is of that cla.s.s 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 cla.s.s 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 a.n.a.logy 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 pota.s.sa 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 pota.s.sa; carbonate of pota.s.sa; sulphate of soda; chloride of lead, of sodium, of bis.m.u.th, of calcium; iodide of lead; oxide of bis.m.u.th; 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 surpa.s.sing 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 pota.s.sa 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 Chompre experimented on this subject in 1807.

They came to the conclusion that the voltaic current caused decompositions throughout its whole course in the humid conductor, not merely as preliminary to the recompositions spoken of by Grotthuss and Davy, but producing final separation of the elements in the _course_ of the current, and elsewhere than at the poles. They considered the _negative_ current as collecting and carrying the acids, &c. to the _positive_ pole, and the _positive_ current as doing the same duty with the bases, and collecting them at the _negative_ pole. They likewise consider the currents as _more powerful_ the nearer they are to their respective poles, and state that the positive current is _superior_ in power to the negative current[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] Precis Elementaire de Physique, 3me edition, 1824, tom. i. p. 641.