Part 15 (1/2)

[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] Precis Elementaire de Physique, 3me edition, 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, a.s.suming 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] Precis Elementaire de Physique, 3me edition, 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 pa.s.sing 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 Chompre (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 ill.u.s.tration 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 ill.u.s.tration of the views I shall venture to give, I must describe it minutely.

494. A gla.s.s 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 gla.s.s 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 splas.h.i.+ng into the basin, until it rose a little above the lower edge of the mica division _a_, great care being taken that the gla.s.s 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 gla.s.s. In this way solution of the sulphate occupied the lower part of the gla.s.s, 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 quant.i.ty 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.