Part 28 (2/2)
[A] Philosophical Transactions, 1807.
[B] Ibid. 1826, p. 383.
[C] Ibid. 1826, p. 389.
[D] I at one time intended to introduce here, in the form of a note, a table of reference to the papers of the different philosophers who have referred the origin of the electricity in the voltaic pile to contact, or to chemical action, or to both; but on the publication of the first volume of M. Becquerel's highly important and valuable Traite de l'Electricite et du Magnetisme, I thought it far better to refer to that work for these references, and the views held by the authors quoted. See pages 86, 91, 104, 110, 112, 117, 118, 120, 151, 152, 224, 227, 228, 232, 233, 252, 255, 257, 258, 290, &c.--July 3rd, 1834.
-- ii. _On the Intensity necessary for Electrolyzation._
966. It became requisite, for the comprehension of many of the conditions attending voltaic action, to determine positively, if possible, whether electrolytes could resist the action of an electric current when beneath a certain intensity? whether the intensity at which the current ceased to act would be the same for all bodies? and also whether the electrolytes thus resisting decomposition would conduct the electric current as a metal does, after they ceased to conduct as electrolytes, or would act as perfect insulators?
967. It was evident from the experiments described (904. 906.) that different bodies were decomposed with very different facilities, and apparently that they required for their decomposition currents of different intensities, resisting some, but giving way to others. But it was needful, by very careful and express experiments, to determine whether a current could really pa.s.s through, and yet not decompose an electrolyte (910.).
968. An arrangement (fig. 84.) was made, in which two gla.s.s vessels contained the same dilute sulphuric acid, sp. gr. 1.25. The plate _z_ was amalgamated zinc, in connexion, by a platina wire _a_, with the platina plate _e_; _b_ was a platina wire connecting the two platina plates PP'; _c_ was a platina wire connected with the platina plate P”. On the plate _e_ was placed a piece of paper moistened in solution of iodide of pota.s.sium: the wire _c_ was so curved that its end could be made to rest at pleasure on this paper, and show, by the evolution of iodine there, whether a current was pa.s.sing; or, being placed in the dotted position, it formed a direct communication with the platina plate _e_, and the electricity could pa.s.s without causing decomposition. The object was to produce a current by the action of the acid on the amalgamated zinc in the first vessel A; to pa.s.s it through the acid in the second vessel B by platina electrodes, that its power of decomposing water might, if existing, be observed; and to verify the existence of the current at pleasure, by decomposition at _e_, without involving the continual obstruction to the current which would arise from making the decomposition there constant. The experiment, being arranged, was examined and the existence of a current ascertained by the decomposition at _e_; the whole was then left with the end of the wire _c_ resting on the plate _e_, so as to form a constant metallic communication there.
969. After several hours, the end of the wire _c_ was replaced on the test-paper at _e_: decomposition occurred, and _the proof_ of a pa.s.sing current was therefore complete. The current was very feeble compared to what it had been at the beginning of the experiment, because of a peculiar state acquired by the metal surfaces in the second vessel, which caused them to oppose the pa.s.sing current by a force which they possess under these circ.u.mstances (1040.). Still it was proved, by the decomposition, that this state of the plates in the second vessel was not able entirely to stop the current determined in the first, and that was all that was needful to be ascertained in the present inquiry.
970. This apparatus was examined from time to time, and an electric current always found circulating through it, until twelve days had elapsed, during which the water in the second vessel had been constantly subject to its action. Notwithstanding this lengthened period, not the slightest appearance of a bubble upon either of the plates in that vessel occurred.
From the results of the experiment, I conclude that a current _had_ pa.s.sed, but of so low an intensity as to fall beneath that degree at which the elements of water, unaided by any secondary force resulting from the capability of combination with the matter of the electrodes, or of the liquid surrounding them, separated from each other.
971. It may be supposed, that the oxygen and hydrogen had been evolved in such small quant.i.ties as to have entirely dissolved in the water, and finally to have escaped at the surface, or to have reunited into water.
That the hydrogen can be so dissolved was shown in the first vessel; for after several days minute bubbles of gas gradually appeared upon a gla.s.s rod, inserted to retain the zinc and platina apart, and also upon the platina plate itself, and these were hydrogen. They resulted princ.i.p.ally in this way:--notwithstanding the amalgamation of the zinc, the acid exerted a little direct action upon it, so that a small stream of hydrogen bubbles was continually rising from its surface; a little of this hydrogen gradually dissolved in the dilute acid, and was in part set free against the surfaces of the rod and the plate, according to the well-known action of such solid bodies in solutions of gases (623. &c.).
972. But if the gases had been evolved in the second vessel by the decomposition of water, and had tended to dissolve, still there would have been every reason to expect that a few bubbles should have appeared on the electrodes, especially on the negative one, if it were only because of its action as a nucleus on the solution supposed to be formed; but none appeared even after twelve days.
973. When a few drops only of nitric acid were added to the vessel A, fig.
84, then the results were altogether different. In less than five minutes bubbles of gas appeared on the plates P' and P” in the second vessel. To prove that this was the effect of the electric current (which by trial at _c_ was found at the same time to be pa.s.sing,) the connexion at _c_ was broken, the plates P'P” cleared from bubbles and left in the acid of the vessel B, for fifteen minutes: during that time no bubbles appeared upon them; but on restoring the communication at _c_, a minute did not elapse before gas appeared in bubbles upon the plates. The proof, therefore, is most full and complete, that the current excited by dilute sulphuric acid with a little nitric acid in vessel A, has intensity enough to overcome the chemical affinity exerted between the oxygen and hydrogen of the water in the vessel B, whilst that excited by dilute sulphuric acid alone has _not_ sufficient intensity.
974. On using a strong solution of caustic pota.s.sa in the vessel A, to excite the current, it was found by the decomposing effects at _e_, that the current pa.s.sed. But it had not intensity enough to decompose the water in the vessel B; for though left for fourteen days, during the whole of which time the current was found to be pa.s.sing, still not the slightest appearance of gas appeared on the plates P'P”, nor any other signs of the water having suffered decomposition.
975. Sulphate of soda in solution was then experimented with, for the purpose of ascertaining with respect to it, whether a certain electrolytic intensity was also required for its decomposition in this state, in a.n.a.logy with the result established with regard to water (974). The apparatus was arranged as in fig. 85; P and Z are the platina and zinc plates dipping into a solution of common salt; _a_ and _b_ are platina plates connected by wires of platina (except in the galvanometer _g_) with P and Z; _c_ is a connecting wire of platina, the ends of which can be made to rest either on the plates _a, b_, or on the papers moistened in solutions which are placed upon them; so that the pa.s.sage of the current without decomposition, or with one or two decompositions, was under ready command, as far as arrangement was concerned. In order to change the _anodes_ and _cathodes_ at the places of decomposition, the form of apparatus fig. 86, was occasionally adopted. Here only one platina plate, _c_, was used; both pieces of paper on which decomposition was to be effected were placed upon it, the wires from P and Z resting upon these pieces of paper, or upon the plate _c_, according as the current with or without decomposition of the solutions was required.
976. On placing solution of iodide of pota.s.sium in paper at one of the decomposing localities, and solution of sulphate of soda at the other, so that the electric current should pa.s.s through both at once, the solution of iodide was slowly decomposed, yielding iodine at the _anode_ and alkali at the _cathode_; but the solution of sulphate of soda exhibited no signs of decomposition, neither acid nor alkali being evolved from it. On placing the wires so that the iodide alone was subject to the action of the current (900.), it was quickly and powerfully decomposed; but on arranging them so that the sulphate of soda alone was subject to action, it still refused to yield up its elements. Finally, the apparatus was so arranged under a wet bell-gla.s.s, that it could be left for twelve hours, the current pa.s.sing during the whole time through a solution of sulphate of soda, retained in its place by only two thicknesses of bibulous litmus and turmeric paper. At the end of that time it was ascertained by the decomposition of iodide of pota.s.sium at the second place of action, that the current was pa.s.sing and had pa.s.sed for the twelve hours, and yet no trace of acid or alkali from the sulphate of soda appeared.
977. From these experiments it may, I think, be concluded, that a solution of sulphate of soda can conduct a current of electricity, which is unable to decompose the neutral salt present; that this salt in the state of solution, like water, requires a certain electrolytic intensity for its decomposition; and that the necessary intensity is much higher for this substance than for the iodide of pota.s.sium in a similar state of solution.
978. I then experimented on bodies rendered decomposable by fusion, and first on _chloride of lead_. The current was excited by dilute sulphuric acid without any nitric acid between zinc and platina plates, fig. 87, and was then made to traverse a little chloride of lead fused upon gla.s.s at _a_, a paper moistened in solution of iodide of pota.s.sium at _b_, and a galvanometer at _g_. The metallic terminations at _a_ and _b_ were of platina. Being thus arranged, the decomposition at _b_ and the deflection at _g_ showed that an electric current was pa.s.sing, but there was no appearance of decomposition at _a_, not even after a _metallic_ communication at _b_ was established. The experiment was repeated several times, and I am led to conclude that in this case the current has not intensity sufficient to cause the decomposition of the chloride of lead; and further, that, like water (974.), fused chloride of lead can conduct an electric current having an intensity below that required to effect decomposition.
979. _Chloride of silver_ was then placed at _a_, fig. 87, instead of chloride of lead. There was a very ready decomposition of the solution of iodide of pota.s.sium at _b_, and when metallic contact was made there, very considerable deflection of the galvanometer needle at _g_. Platina also appeared to be dissolved at the anode of the fused chloride at _a_, and there was every appearance of a decomposition having been effected there.
980. A further proof of decomposition was obtained in the following manner.
The platina wires in the fused chloride at _a_ were brought very near together (metallic contact having been established at _b_), and left so; the deflection at the galvanometer indicated the pa.s.sage of a current, feeble in its force, but constant. After a minute or two, however, the needle would suddenly be violently affected, and indicate a current as strong as if metallic contact had taken place at _a_. This I actually found to be the case, for the silver reduced by the action of the current crystallized in long delicate spiculae, and these at last completed the metallic communication; and at the same time that they transmitted a more powerful current than the fused chloride, they proved that electro-chemical decomposition of that chloride had been going on. Hence it appears, that the current excited by dilute sulphuric acid between zinc and platina, has an intensity above that required to electrolyze the fused chloride of silver when placed between platina electrodes, although it has not intensity enough to decompose chloride of lead under the same circ.u.mstances.
981. A drop of _water_ placed at _a_ instead of the fused chlorides, showed as in the former case (970.), that it could conduct a current unable to decompose it, for decomposition of the solution of iodide at _b_ occurred after some time. But its conducting power was much below that of the fused chloride of lead (978.).
982. Fused _nitre_ at _a_ conducted much better than water: I was unable to decide with certainty whether it was electrolyzed, but I incline to think not, for there was no discoloration against the platina at the _cathode_.
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