174 Faraday's Researches

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 (399), i.e. against the end of the platina wire.
616. As long as the lower ends of the plates
remained in the acid the electric current continued,
and the decomposition proceeded at x. On re-
moving the end of the wire from place to place
on the paper, the effect was evidently very power-
ful; and on placing a piece of turmeric paper
between the white paper and zinc, both papers

Fig. 33-

being moistened with the solution of iodide of potassium,
alkali was evolved at the cathode (399) 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.

617. 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 decom-
position 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.

618. When this experiment was made with pieces of zinc
amalgamated over the whole surface (598), the results were
obtained with equal facility and in the same direction, even
when only dilute surphuric acid was contained in the vessel c
(fig. 33). Whichsoever end of the zinc was immersed in the about three inches long and half