804. Protiodide of tin.—­This substance, when fused (402.), conducts and is decomposed by the electric current, tin is evolved at the anode, and periodide of tin as a secondary result (779. 790.) at the cathode.  The temperature required for its fusion is too high to allow of the production of any results fit for weighing.

805. Iodide of potassium was subjected to electrolytic action in a tube, like that in fig. 68. (789.).  The negative electrode was a globule of lead, and I hoped in this way to retain the potassium, and obtain results that could be weighed and compared with the volta-electrometer indication; but the difficulties dependent upon the high temperature required, the action upon the glass, the fusibility of the platina induced by the presence of the lead, and other circumstances, prevented me from procuring such results.  The iodide was decomposed with the evolution of iodine at the anode, and of potassium at the cathode, as in former cases.

806.  In some of these experiments several substances were placed in succession, and decomposed simultaneously by the same electric current:  thus, protochloride of tin, chloride of lead, and water, were thus acted on at once.  It is needless to say that the results were comparable, the tin, lead, chlorine, oxygen, and hydrogen evolved being definite in quantity and electro-chemical equivalents to each other.

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807.  Let us turn to another kind of proof of the definite chemical action of electricity.  If any circumstances could be supposed to exert an influence over the quantity of the matters evolved during electrolytic action, one would expect them to be present when electrodes of different substances, and possessing very different chemical affinities for such matters, were used.  Platina has no power in dilute sulphuric acid of combining with the oxygen at the anode, though the latter be evolved in the nascent state against it.  Copper, on the other hand, immediately unites with the oxygen, as the electric current sets it free from the hydrogen; and zinc is not only able to combine with it, but can, without any help from the electricity, abstract it directly from the water, at the same time setting torrents of hydrogen free.  Yet in cases where these three substances were used as the positive electrodes in three similar portions of the same dilute sulphuric acid, specific gravity 1.336, precisely the same quantity of water was decomposed by the electric current, and precisely the same quantity of hydrogen set free at the cathodes of the three solutions.