At the end of the above letter as published in Nicholson's
Journal, the editor, William Nicholson, comments at length on
Volta's theory of the source of current in the cell and calls
attention to the fact that Davy had already made cells by the
use of a single metal and two different liquids. At the
conclusion of his comments he calls attention to the fact that
Bennett and Cavallo had performed experiments with contact
electrification prior to Volta's experiments, and says in
conclusion, after referring to Bennett,

'This last philosopher, as well as Cavallo, appears to think
that different bodies have different attractions or capacities
for electricity; but the singular hypothesis of electromotion,
or a perpetual current of electricity being produced, by the
contact of two metals is, I apprehend, peculiar to Volta.'

This peculiar theory of Volta's probably never gained many
adherents and was necessarily abandoned as soon as the energy
relations of the current were considered, but the controversy
as to whether the electrical current or the accompanying
chemical changes was the primary phenomenon soon became
transferred to a quite different field, viz., to the origin of
the electrical charges which Bennett had shown resulted from
the contact of different metals. Bennett attempted to account
for the phenomena which he had observed on the hypothesis that
different substances "have a greater or less affinity with the
electric fluid," and Cavallo says:

'I am inclined to suspect that different bodies have different
capacities for holding the electric fluid.'