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.)