different states of electricity whilst in contact, but only on
being separated after contact. Now in the Voltaic battery, those plates
that are in contact always continue so, being soldered together: and
they cannot therefore receive a succession of charges. Besides, if we
consider the mere disturbance of the balance of electricity by the
contact of the plates, as the sole cause of the production of Voltaic
electricity, it remains to be explained how this disturbed balance
becomes an inexhaustible source of electrical energy, capable of pouring
forth a constant and copious supply of electrical fluid, though without
any means of replenishing itself from other sources. This subject, it
must be owned, is involved in too much obscurity to enable us to speak
very decidedly in favour of any theory. But, in order to avoid
perplexing you with different explanations, I shall confine myself to
one which appears to me to be least encumbered with difficulties, and
most likely to accord with truth.*

This theory supposes the electricity to be excited by the chemical
action of the acid on the zinc; but you are yet such novices in
chemistry, that I think it will be necessary to give you some previous
explanation of the nature of this action.

All metals have a strong attraction for oxygen, and this element is
found in great abundance both in water and in acids. The action of the
diluted acid on the zinc consists therefore in its oxygen combining with
it, and dissolving its surface.

[Footnote *: This mode of explaining the phenomena of the Voltaic
pile is called the _chemical theory_ of electricity, because it
ascribes the cause of these phenomena to certain chemical changes
which take place during their appearance. In the preceding edition
of this work, the same theory was presented in a more elaborate,
but less easy form than it is in this. The mode of viewing the
subject which is here sketched was long since suggested by Dr.
Bostock, of whose theory, however, this is by no means to be
considered as a complete statement.]

CAROLINE.

In the same manner I suppose as we saw an acid dissolve copper?

MRS. B.

Yes; but in the Voltaic battery the diluted acid is not strong enough to
produce so complete an effect; it acts only on the surface of the zinc,
to which it yields its oxygen, forming upon it a film or crust, which is
a compound of the oxygen and the metal.

EMILY.

Since there is s