Part 26 (1/2)
EIGHTH SERIES.
--14. _On the Electricity of the Voltaic Pile; its source, quant.i.ty, intensity, and general characters._ -- i. _On simple Voltaic Circles._ -- ii.
_On the intensity necessary for Electrolyzation._ -- iii. _On a.s.sociated Voltaic Circles, or the Voltaic Battery._ -- iv. _On the resistance of an Electrolyte to Electrolytic action._ -- v. _General remarks on the active Voltaic Battery._
Received April 7,--Read June 5, 1831.
-- i. _On simple Voltaic Circles._
875. The great question of the source of electricity, in the voltaic pile has engaged the attention of so many eminent philosophers, that a man of liberal mind and able to appreciate their powers would probably conclude, although he might not have studied the question, that the truth was somewhere revealed. But if in pursuance of this impression he were induced to enter upon the work of collating results and conclusions, he would find such contradictory evidence, such equilibrium of opinion, such variation and combination of theory, as would leave him in complete doubt respecting what he should accept as the true interpretation of nature: he would be forced to take upon himself the labour of repeating and examining the facts, and then use his own judgement on them in preference to that of others.
876. This state of the subject must, to those who have made up their minds on the matter, be my apology for entering upon its investigation. The views I have taken of the definite action of electricity in decomposing bodies (783.), and the ident.i.ty of the power so used with the power to be overcome (855.), founded not on a mere opinion or general notion, but on facts which, being altogether new, were to my mind precise and conclusive, gave me, as I conceived, the power of examining the question with advantages not before possessed by any, and which might compensate, on my part, for the superior clearness and extent of intellect on theirs. Such are the considerations which have induced me to suppose I might help in deciding the question, and be able to render a.s.sistance in that great service of removing _doubtful knowledge_. Such knowledge is the early morning light of every advancing science, and is essential to its development; but the man who is engaged in dispelling that which is deceptive in it, and revealing more clearly that which is true, is as useful in his place, and as necessary to the general progress of the science, as he who first broke through the intellectual darkness, and opened a path into knowledge before unknown to man.
877. The ident.i.ty of the force const.i.tuting the voltaic current or electrolytic agent, with that which holds the elements of electrolytes together (855.), or in other words with chemical affinity, seemed to indicate that the electricity of the pile itself was merely a mode of exertion, or exhibition, or existence of _true chemical action_, or rather of its cause; and I have consequently already said that I agree with those who believe that the _supply_ of electricity is due to chemical powers (857.).
878. But the great question of whether it is originally due to metallic contact or to chemical action, i.e. whether it is the first or the second which _originates_ and determines the current, was to me still doubtful; and the beautiful and simple experiment with amalgamated zinc and platina, which I have described minutely as to its results (863, &c.), did not decide the point; for in that experiment the chemical action does not take place without the contact of the metals, and the metallic contact is inefficient without the chemical action. Hence either might be looked upon as the _determining_ cause of the current.
879. I thought it essential to decide this question by the simplest possible forms of apparatus and experiment, that no fallacy might be inadvertently admitted. The well-known difficulty of effecting decomposition by a single pair of plates, except in the fluid exciting them into action (863.), seemed to throw insurmountable obstruction in the way of such experiments; but I remembered the easy decomposability of the solution of iodide of pota.s.sium (316.), and seeing no theoretical reason, if metallic contact was not _essential_, why true electro-decomposition should not be obtained without it, even in a single circuit, I persevered and succeeded.
880. A plate of zinc, about eight inches long and half an inch wide, was cleaned and bent in the middle to a right angle, fig. 73 _a_, Plate VI. A plate of platina, about three inches long and half an inch wide, was fastened to a platina wire, and the latter bent as in the figure, _b_.
These two pieces of metal were arranged together as delineated, but as yet without the vessel _c_, and its contents, which consisted of dilute sulphuric acid mingled with a little nitric acid. At _x_ a piece of folded bibulous paper, moistened in a solution of iodide of pota.s.sium, was placed on the zinc, and was pressed upon by the end of the platina wire. When under these circ.u.mstances the plates were dipped into the acid of the vessel _c_, there was an immediate effect at _x_, the iodide being decomposed, and iodine appearing at the _anode_ (663.), i.e. against the end of the platina wire.
881. As long as the lower ends of the plates remained in the acid the electric current continued, and the decomposition proceeded at _x_. On removing the end of the wire from place to place on the paper, the effect was evidently very powerful; and on placing a piece of turmeric paper between the white paper and zinc, both papers being moistened with the solution of iodide of pota.s.sium, alkali was evolved at the _cathode_ (663.) against the zinc, in proportion to the evolution of iodine at the _anode_.
Hence the decomposition was perfectly polar, and decidedly dependent upon a current of electricity pa.s.sing from the zinc through the acid to the platina in the vessel _c_, and back from the platina through the solution to the zinc at the paper _x_.
882. That the decomposition at _x_ was a true electrolytic action, due to a current determined by the state of things in the vessel _c_, and not dependent upon any mere direct chemical action of the zinc and platina on the iodide, or even upon any _current_ which the solution of iodide might by its action on those metals tend to form at _x_, was shown, in the first place, by removing the vessel _c_ and its acid from the plates, when all decomposition at _x_ ceased, and in the next by connecting the metals, either in or out of the acid, together, when decomposition of the iodide at _x_ occurred, but in a _reverse order_; for now alkali appeared against the end of the platina wire, and the iodine pa.s.sed to the zinc, the current being the contrary of what it was in the former instance, and produced directly by the difference of action of the solution in the paper on the two metals. The iodine of course _combined_ with the zinc.
883. When this experiment was made with pieces of zinc amalgamated over the whole surface (863.), the results were obtained with equal facility and in the same direction, even when only dilute sulphuric acid was contained in the vessel _c_ (fig. 73.). Whichsoever end of the zinc was immersed in the acid, still the effects were the same: so that if, for a moment, the mercury might be supposed to supply the metallic contact, the inversion of the amalgamated piece destroys that objection. The use of _unamalgamated zinc_ (880.) removes all possibility of doubt[A].
[A] The following is a more striking mode of making the above elementary experiment. Prepare a plate of zinc, ten or twelve inches long and two inches wide, and clean it thoroughly: provide also two discs of clean platina, about one inch and a half in diameter:--dip three or four folds of bibulous paper into a strong solution of iodide of pota.s.sium, place them on the clean zinc at one end of the plate, and put on them one of the platina discs: finally dip similar folds of paper or a piece of linen cloth into a mixture of equal parts nitric acid and water, and place it at the other end of the zinc plate with the second platina disc upon it. In this state of things no change at the solution of the iodide will be perceptible; but if the two discs be connected by a platina (or any other) wire for a second or two, and then that over the iodide be raised, it will be found that the _whole_ of the surface beneath is deeply stained with _evolved iodine_.--_Dec.
1838._
884 When, in pursuance of other views (930.), the vessel _c_ was made to contain a solution of caustic potash in place of acid, still the same results occurred. Decomposition of the iodide was effected freely, though there was no metallic contact of dissimilar metals, and the current of electricity was in the _same direction_ as when acid was used at the place of excitement.
885. Even a solution of common salt in the gla.s.s _c_ could produce all these effects.
886. Having made a galvanometer with platina wires, and introduced it into the course of the current between the platina plate and the place of decomposition _x_, it was affected, giving indications of currents in the same direction as those shown to exist by the chemical action.
887. If we consider these results generally, they lead to very important conclusions. In the first place, they prove, in the most decisive manner, that _metallic contact is not necessary for the production of the voltaic current._ In the next place, they show a most extraordinary mutual relation of the chemical affinities of the fluid which _excites_ the current, and the fluid which is _decomposed_ by it.
888. For the purpose of simplifying the consideration, let us take the experiment with amalgamated zinc. The metal so prepared exhibits no effect until the current can pa.s.s: it at the same time introduces no new action, but merely removes an influence which is extraneous to those belonging either to the production or the effect of the electric current under investigation (1000.); an influence also which, when present, tends only to confuse the results.
889. Let two plates, one of amalgamated zinc and the other of platina, be placed parallel to each other (fig. 74.), and introduce a drop of dilute sulphuric acid, _y_, between them at one end: there will be no sensible chemical action at that spot unless the two plates are connected somewhere else, as at PZ, by a body capable of conducting electricity. If that body be a metal or certain forms of carbon, then the current pa.s.ses, and, as it circulates through the fluid at _y_, decomposition ensues.
890. Then remove the acid from _y_, and introduce a drop of the solution of iodide of pota.s.sium at _x_ (fig. 75.). Exactly the same set of effects occur, except that when the metallic communication is made at PZ, the electric current is in the opposite direction to what it was before, as is indicated by the arrows, which show the courses of the currents (667.).
891. Now _both_ the solutions used are conductors, but the conduction in them is essentially connected with decomposition (858.) in a certain constant order, and therefore the appearance of the elements in certain places _shows_ in what direction a current has pa.s.sed when the solutions are thus employed. Moreover, we find that when they are used at opposite ends of the plates, as in the last two experiments (889. 890.), metallic contact being allowed at the other extremities, the currents are in opposite directions. We have evidently, therefore, the power of opposing the actions of the two fluids simultaneously to each other at the opposite ends of the plates, using each one as a conductor for the discharge of the current of electricity, which the other tends to generate; in fact, subst.i.tuting them for metallic contact, and combining both experiments into one (fig. 76.). Under these circ.u.mstances, there is an opposition of forces: the fluid, which brings into play the stronger set of chemical affinities for the zinc, (being the dilute acid,) overcomes the force of the other, and determines the formation and direction of the electric current; not merely making that current pa.s.s through the weaker liquid, but actually reversing the tendency which the elements of the latter have in relation to the zinc and platina if not thus counteracted, and forcing them in the contrary direction to that they are inclined to follow, that its own current may have free course. If the dominant action at _y_ be removed by making metallic contact there, then the liquid at _x_ resumes its power; or if the metals be not brought into contact at _y_ but the affinities of the solution there weakened, whilst those active _x_ are strengthened, then the latter gains the ascendency, and the decompositions are produced in a contrary order.