Use of the Exciting Acid 191

its decomposing effect in the same degree; as if acid instead of"
alkali had been used (603).

668. The proof, therefore, appears to me complete, that the
combination of the acid with the oxide, in the former experi-
ment, had nothing to do with the production of the electric
current; for the same current is here produced when the action
of the acid is absent, and the reverse action of an alkali is
present. I think it cannot be supposed for a moment that the
alkali acted chemically as an acid to the oxide formed; on the-
contrary, our general chemical knowledge leads to the conclu-
sion that the ordinary metallic oxides act rather as acids to
the alkalies; yet that kind of action would tend to give a reverse
current in the present case, if any were due to the union of
the oxide of the exciting metal with the body which combines
with it. But instead of any variation of this sort, the direction
of the electricity was constant, and its quantity also directly
proportional to the water decomposed, or the zinc oxidised.
There are reasons for believing that acids and alkalies, when
in contact with metals upon which they cannot act directly, still
have a power of influencing their attractions for oxygen (676);
but all the effects in these experiments prove, I think, that it
is the oxidation of the metal necessarily dependent upon, and
associated as it is with, the electrolysation of the water (656,
658) that produces the current; and that the acid or alkali
merely act as solvents, and by removing the oxidised zinc,
allow other portions to decompose fresh water, and so continue
the evolution or determination of the current.

669. The experiments were then varied by using solution of
ammonia instead of solution of potassa; and as it, when pure,
is like water, a bad conductor (290), it was occasionally improved
in that power by adding sulphate of ammonia to it. But in
all the cases the results were the same as before; decomposi-
tions of the same kind were effected, and the electric current
producing these was in the same direction as in the experiments
just described.

670. In order to put the equal and similar action of acid and
alkali to stronger proof, arrangements were made as in fig. 40;
the glass vessel A contained dilute sulphuric acid, the corre-
sponding glass vessel B solution of potassa, P P was a plate of
platina dipping into both solutions, and Z Z two plates of
amalgamated zinc connected with a delicate galvanometer.
When these were plunged at the same time into the two vessels,
there was generally a first feeble effect, and that in favour ofce any current of electricity. — Philosophical Tram actions,