eficient in force it may
be helped by calling in chemical aid, the want in the former being made up
by an equivalent of the latter; that, in other words, _the forces termed
chemical affinity and electricity are one and the same._

919. When the circumstances connected with the production of electricity in
the ordinary voltaic circuit are examined and compared, it appears that the
source of that agent, always meaning the electricity which circulates and
completes the current in the voltaic apparatus, and gives that apparatus
power and character (947. 996.), exists in the chemical action which takes
place directly between the metal and the body with which it combines, and
not at all in the subsequent action of the substance so produced with the
acid present[A]. Thus, when zinc, platina, and dilute sulphuric acid are
used, it is the union of the zinc with the oxygen of the water which
determines the current; and though the acid is essential to the removal of
the oxide so formed, in order that another portion of zinc may act on
another portion of water, it does not, by combination with that oxide,
produce any sensible portion of the current of electricity which
circulates; for the quantity of electricity is dependent upon the quantity
of zinc oxidized, and in definite proportion to it: its intensity is in
proportion to the intensity of the chemical affinity of the zinc for the
oxygen under the circumstances, and is scarcely, if at all, affected by the
use of either strong or weak acid (908.).

[A] Wollaston, Philosophical Transactions, 1801, p. 427.

920. Again, if zinc, platina, and muriatic acid are used, the electricity
appears to be dependent upon the affinity of the zinc for the chlorine, and
to be circulated in exact proportion to the number of particles of zinc and
chlorine which unite, being in fact an equivalent to them.

921. But in considering this oxidation, or other direct action upon the
METAL itself, as the cause and source of the electric current, it is of the
utmost importance to observe that the oxygen or other body must be in a
peculiar condition, namely, in the state of _combination_; and not only so,
but limited still further to such a state of combination and in such
proportions as will constitute an _electrolyte_ (823.). A pair of zinc and
platina plates cannot be so arranged in oxygen gas as to produce a current
of electricity, or act as a voltaic circle, even though the temperature may
be raised