nc, or tin, or lead
be used, the metal in the acid evolves hydrogen the moment it is placed in
communication with that in the alkali, not from any direct action of the
acid upon it, for if the contact be broken the action ceases, but because
it is powerfully negative with regard to the metal in the alkali.

[A] Elements of Chemical Philosophy, p. 149; or Philosophical
Transactions, 1826, p. 403.

942. The superiority of alkali is further proved by this, that if zinc and
tin be used, or tin and lead, whichsoever metal is put into the alkali
becomes positive, that in the acid being negative. Whichsoever is in the
alkali is oxidized, whilst that in the acid remains in the metallic state,
as far as the electric current is concerned.

943. When sulphuretted solutions are used (930.) in illustration of the
assertion, that it is the chemical action of the metal and one of the
_ions_ of the associated electrolyte that produces all the electricity of
the voltaic circuit, the proofs are still the same. Thus, as Sir Humphry
Davy[A] has shown, if iron and copper be plunged into dilute acid, the
current is from the iron through the liquid to the copper; in solution of
potassa it is in the same direction, but in solution of sulphuret of
potassa it is reversed. In the two first cases it is oxygen which combines
with the iron, in the latter sulphur which combines with the copper, that
produces the electric current; but both of these are _ions_, existing as
such in the electrolyte, which is at the same moment suffering
decomposition; and, what is more, both of these are _anions_, for they
leave the electrolytes at their _anodes_, and act just as chlorine, iodine,
or any other _anion_ would act which might have been previously chosen as
that which should be used to throw the voltaic circle into activity.

[A] Elements of Chemical Philosophy, p. 148.

944. The following experiments complete the series of proofs of the origin
of the electricity in the voltaic pile. A fluid amalgam of potassium,
containing not more than a hundredth of that metal, was put into pure
water, and connected, through the galvanometer with a plate of platina in
the same water. There was immediately an electric current from the amalgam
through the electrolyte to the platina. This must have been due to the
oxidation only of the metal, for there was neither acid nor alkali to
combine with, or in any way act on, the body produced.

945. Again, a plate of cle