Part 30 (1/2)

1020. The effects of r.e.t.a.r.dation just described were altered altogether when changes were made in the _nature of the liquid_ used between the plates, either in what may be called the _exciting_ or the _r.e.t.a.r.ding_ cells. Thus, retaining the exciting force the same, by still using pure dilute sulphuric acid for that purpose, if a little nitric acid were added to the liquid in the _r.e.t.a.r.ding_ cells, then the transmission of the current was very much facilitated. For instance, in the experiment with one pair of exciting plates and one intervening plate (1011.), fig. 92, when a few drops of nitric acid were added to the contents of cell ii, then the current of electricity pa.s.sed with considerable strength (though it soon fell from other causes (1036; 1040.),) and the same increased effect was produced by the nitric acid when many interposed plates were used.

1021. This seems to be a consequence of the diminution of the difficulty of decomposing water when its hydrogen, instead of being absolutely expelled, as in the former cases, is transferred to the oxygen of the nitric acid, producing a secondary result at the _cathode_ (752.); for in accordance with the chemical views of the electric current and its action already advanced (913.), the water, instead of opposing a resistance to decomposition equal to the full amount of the force of mutual attraction between its oxygen and hydrogen, has that force counteracted in part, and therefore diminished by the attraction of the hydrogen at the _cathode_ for the oxygen of the nitric acid which surrounds it, and with which it ultimately combines instead of being evolved in its free state.

1022. When a little nitric acid was put into the exciting cells, then again the circ.u.mstances favouring the transmission of the current were strengthened, for the _intensity_ of the current itself was increased by the addition (906.). When therefore a little nitric acid was added to both the _exciting_ and the _r.e.t.a.r.ding_ cells, the current of electricity pa.s.sed with very considerable freedom.

1023. When dilute muriatic acid was used, it produced and transmitted a current more easily than pure dilute sulphuric acid, but not so readily as dilute nitric acid. As muriatic acid appears to be decomposed more freely than water (765.), and as the affinity of zinc for chlorine is very powerful, it might be expected to produce a current more intense than that from the use of dilute sulphuric acid; and also to transmit it more freely by undergoing decomposition at a lower intensity (912.).

1024. In relation to the effect of these interpositions, it is necessary to state that they do not appear to be at all dependent upon the size of the electrodes, or their distance from each other in the acid, except that when a current _can pa.s.s_, changes in these facilitate or r.e.t.a.r.d its pa.s.sage.

For on repeating the experiment with one intervening and one pair of exciting plates (1011.), fig. 92, and in place of the interposed plate P using sometimes a mere wire, and sometimes very large plates (1008.), and also changing the terminal exciting plates Z and P, so that they were sometimes wires only and at others of great size, still the results were the same as those already obtained.

1025. In ill.u.s.tration of the effect of distance, an experiment like that described with two exciting pairs and one intervening plate (1012.), fig.

93, was arranged so that the distance between the plates in the third cell could be increased to six or eight inches, or diminished to the thickness of a piece of intervening bibulous paper. Still the result was the same in both cases, the effect not being sensibly greater, when the plates were merely separated by the paper, than when a great way apart; so that the princ.i.p.al opposition to the current in this case does not depend upon the _quant.i.ty_ of intervening electrolytic conductor, but on the _relation of its elements to the intensity of the current_, or to the chemical nature of the electrodes and the surrounding fluids.

1026. When the acid was sulphuric acid, _increasing its strength_ in any of the cells, caused no change in the effects; it did not produce a more intense current in the exciting cells (908.), or cause the current produced to traverse the decomposing cells more freely. But if to very weak sulphuric acid a few drops of nitric acid were added, then either one or other of those effects could be produced; and, as might be expected in a case like this, where the exciting or conducting action bore a _direct_ reference to the acid itself, increasing the strength of this (the nitric acid), also increased its powers.

1027. The _nature of the interposed plate_ was now varied to show its relation to the phenomena either of excitation or r.e.t.a.r.dation, and amalgamated zinc was first subst.i.tuted for platina. On employing one voltaic pair and one interposed zinc plate, fig. 100, there was as powerful a current, apparently, as if the interposed zinc plate was away. Hydrogen was evolved against P in cell ii, and against the side of the second zinc in cell i; but no gas appeared against the side of the zinc in cell ii, nor against the zinc in cell i.

1028. On interposing two amalgamated zinc plates, fig. 101, instead of one, there was still a powerful current, but interference had taken place. On using three intermediate zinc plates, fig. 102, there was still further r.e.t.a.r.dation, though a good current of electricity pa.s.sed.

1029. Considering the r.e.t.a.r.dation as due to the inaction of the amalgamated zinc upon the dilute acid, in consequence of the slight though general effect of diminished chemical power produced by the mercury on the surface, and viewing this inaction as the circ.u.mstance which rendered it necessary that each plate should have its tendency to decompose water a.s.sisted slightly by the electric current, it was expected that plates of the metal in the unamalgamated state would probably not require such a.s.sistance, and would offer no sensible impediment to the pa.s.sing of the current. This expectation was fully realized in the use of two and three interposed unamalgamated plates. The electric current pa.s.sed through them as freely as if there had been no such plates in the way. They offered no obstacle, because they could decompose water without the current; and the latter had only to give direction to a part of the forces, which would have been active whether it had pa.s.sed or not.

1030. Interposed plates of copper were then employed. These seemed at first to occasion no obstruction, but after a few minutes the current almost entirely ceased. This effect appears due to the surfaces taking up that peculiar condition (1010.) by which they tend to produce a reverse current; for when one or more of the plates were turned round, which could easily be effected with the couronne des ta.s.ses form of experiment, fig. 90, then the current was powerfully renewed for a few moments, and then again ceased.

Plates of platina and copper, arranged as a voltaic pile with dilute sulphuric acid, could not form a voltaic trough competent to act for more than a _few_ minutes, because of this peculiar counteracting effect.

1031. All these effects of r.e.t.a.r.dation, exhibited by decomposition against surfaces for which the evolved elements have more or less affinity, or are altogether deficient in attraction, show generally, though beautifully, the chemical relations and source of the current, and also the balanced state of the affinities at the places of excitation and decomposition. In this way they add to the ma.s.s of evidence in favour of the ident.i.ty of the two; for they demonstrate, as it were, the antagonism of the _chemical powers_ at the electromotive part with the _chemical powers_ at the interposed parts; they show that the first are _producing_ electric effects, and the second _opposing_ them; they bring the two into direct relation; they prove that either can determine the other, thus making what appears to be cause and effect convertible, and thereby demonstrating that both chemical and electrical action are merely two exhibitions of one single agent or power (916. &c.).

1032. It is quite evident, that as water and other electrolytes can conduct electricity without suffering decomposition (986.), when the electricity is of sufficiently low intensity, it may not be a.s.serted as absolutely true in all cases, that whenever electricity pa.s.ses through an electrolyte, it produces a definite effect of decomposition. But the quant.i.ty of electricity which can pa.s.s in a given time through an electrolyte without causing decomposition, is so small as to bear no comparison to that required in a case of very moderate decomposition, and with electricity above the intensity required for electrolyzation, I have found no sensible departure as yet from the law of _definite electrolytic action_ developed in the preceding series of these Researches (783. &c.).

1033. I cannot dismiss this division of the present Paper without making a reference to the important experiments of M. Aug. De la Rive on the effects of interposed plates[A]. As I have had occasion to consider such plates merely as giving rise to new decompositions, and in that way only causing obstruction to the pa.s.sage of the electric current, I was freed from the necessity of considering the peculiar effects described by that philosopher. I was the more willing to avoid for the present touching upon these, as I must at the same time have entered into the views of Sir Humphry Davy upon the same subject[B] and also those of Marianini[C] and Hitter[D], which are connected with it.

[A] Annales de Chimie, tom. xxviii. p 190; and Memoires de Geneve.

[B] Philosophical Transactions, 1826, p. 413.

[C] Annales de Chimie, tom. x.x.xiii. pp. 117, 119, &c.

[D] Journal de Physique, tom. lvii. pp. 319, 350.

-- v. _General Remarks on the active Voltaic Battery._

1034. When the ordinary voltaic battery is brought into action, its very activity produces certain effects, which re-act upon it, and cause serious deterioration of its power. These render it an exceedingly inconstant instrument as to the _quant.i.ty_ of effect which it is capable of producing.

They are already, in part, known and understood; but as their importance, and that of certain other coincident results, will be more evident by reference to the principles and experiments already stated and described, I have thought it would be useful, in this investigation of the voltaic pile, to notice them briefly here.

1035. When the battery is in action, it causes such substances to be formed and arranged in contact with the plates as very much weaken its power, or even tend to produce a counter current. They are considered by Sir Humphry Davy as sufficient to account for the phenomena of Ritter's secondary piles, and also for the effects observed by M.A. De la Rive with interposed platina plates[A].

[A] Philosophical Transactions, 1826, p. 113.