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 potassium (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 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_.