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]. [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 potassium, 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 glass _c_ could produce all these effects.