by other acid. From the serious influence of this cause in
experiments with single pairs of plates of different metals, in which I was
at one time engaged, and the extreme care required to avoid it, I cannot
help feeling a strong suspicion that it interferes more frequently and
extensively than experimenters are aware of, and therefore direct their
attention to it.

1038. In considering the effect in delicate experiments of this source of
irregularity of action, in the voltaic apparatus, it must be remembered
that it is only that very small portion of matter which is directly in
contact with the oxidizable metal which has to be considered with reference
to the change of its nature; and this portion is not very readily displaced
from its position upon the surface of the metal (582. 605.), especially if
that metal be rough and irregular. In illustration of this effect, I will
quote a remarkable experiment. A burnished platina plate (569.) was put
into hot strong sulphuric acid for an instant only: it was then put into
distilled water, moved about in it, taken out, and wiped dry: it was put
into a second portion of distilled water, moved about in it, and again
wiped: it was put into a third portion of distilled water, in which it was
moved about for nearly eight seconds; it was then, without wiping, put into
a fourth portion of distilled water, where it was allowed to remain five
minutes. The two latter portions of water were then tested for sulphuric
acid; the third gave no sensible appearance of that substance, but the
fourth gave indications which were not merely evident, but abundant for the
circumstances under which it had been introduced. The result sufficiently
shows with what difficulty that portion of the substance which is in
_contact_ with the metal leaves it; and as the contact of the fluid formed
against the plate in the voltaic circuit must be as intimate and as perfect
as possible, it is easy to see how quickly and greatly it must vary from
the general fluid in the cells, and how influential in diminishing the
force of the battery this effect must be.

1039. In the ordinary voltaic pile, the influence of this effect will occur
in all variety of degrees. The extremities of a trough of twenty pairs of
plates of Wollaston's construction were connected with the
volta-electrometer, fig. 66. (711.), of the Seventh Series of these
Researches, and after five minutes the number of bubbles of gas issuing
from the ext