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 potassium, was placed on the zinc, and was pressed upon by the end of the platina wire.  When under these circumstances 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 potassium, 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 passing 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 passed 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].