separate the moral and emotional from the intellectual; and thus it
is that the discussion of a point of science may rise to the heat of
a battle-field. The fight between the rival optical theories of
Emission and Undulation was of this fierce character; and scarcely
less fierce for many years was the contest as to the origin and
maintenance of the power of the voltaic pile. Volta himself supposed
it to reside in the Contact of different metals. Here was exerted
his 'Electro-motive force,' which tore the combined electricities
asunder and drove them as currents in opposite directions.
To render the circulation of the current possible, it was necessary
to connect the metals by a moist conductor; for when any two metals
were connected by a third, their relation to each other was such
that a complete neutralisation of the electric motion was the result.
Volta's theory of metallic contact was so clear, so beautiful, and
apparently so complete, that the best intellects of Europe accepted
it as the expression of natural law.

Volta himself knew nothing of the chemical phenomena of the pile;
but as soon as these became known, suggestions and intimations
appeared that chemical action, and not metallic contact, might be
the real source of voltaic electricity. This idea was expressed by
Fabroni in Italy, and by Wollaston in England. It was developed and
maintained by those 'admirable electricians,' Becquerel, of Paris,
and De la Rive, of Geneva. The Contact Theory, on the other hand,
received its chief development and illustration in Germany.
It was long the scientific creed of the great chemists and natural
philosophers of that country, and to the present hour there may be
some of them unable to liberate themselves from the fascination of
their first-love.