9. Considering the retardation as due to the inaction of the amalgamated
zinc upon the dilute acid, in consequence of the slight though general
effect of diminished chemical power produced by the mercury on the surface,
and viewing this inaction as the circumstance which rendered it necessary
that each plate should have its tendency to decompose water assisted
slightly by the electric current, it was expected that plates of the metal
in the unamalgamated state would probably not require such assistance, and
would offer no sensible impediment to the passing of the current. This
expectation was fully realized in the use of two and three interposed
unamalgamated plates. The electric current passed through them as freely as
if there had been no such plates in the way. They offered no obstacle,
because they could decompose water without the current; and the latter had
only to give direction to a part of the forces, which would have been
active whether it had passed or not.

1030. Interposed plates of copper were then employed. These seemed at first
to occasion no obstruction, but after a few minutes the current almost
entirely ceased. This effect appears due to the surfaces taking up that
peculiar condition (1010.) by which they tend to produce a reverse current;
for when one or more of the plates were turned round, which could easily be
effected with the couronne des tasses form of experiment, fig. 90, then the
current was powerfully renewed for a few moments, and then again ceased.
Plates of platina and copper, arranged as a voltaic pile with dilute
sulphuric acid, could not form a voltaic trough competent to act for more
than a _few_ minutes, because of this peculiar counteracting effect.

1031. All these effects of retardation, exhibited by decomposition against
surfaces for which the evolved elements have more or less affinity, or are
altogether deficient in attraction, show generally, though beautifully, the
chemical relations and source of the current, and also the balanced state
of the affinities at the places of excitation and decomposition. In this
way they add to the mass of evidence in favour of the identity of the two;
for they demonstrate, as it were, the antagonism of the _chemical powers_
at the electromotive part with the _chemical powers_ at the interposed
parts; they show that the first are _producing_ electric effects, and the
second _opposing_ them; they bring the two into direct relation; they prove
that