and only its resistance
to decomposition remains as the obstacle to be overcome by the affinities
exerted in the dilute sulphuric acid.

896. This becomes the condition of a single pair of active plates where
_metallic contact_ is allowed. In such cases, only one set of opposing
affinities are to be overcome by those which are dominant in the vessel
_c_; whereas, when metallic contact is not allowed, two sets of opposing
affinities must be conquered (894.).

897. It has been considered a difficult, and by some an impossible thing,
to decompose bodies by the current from a single pair of plates, even when
it was so powerful as to heat bars of metal red-hot, as in the case of
Hare's calorimeter, arranged as a single voltaic circuit, or of Wollaston's
powerful single pair of metals. This difficulty has arisen altogether from
the antagonism of the chemical affinity engaged in producing the current
with the chemical affinity to be overcome, and depends entirely upon their
relative intensity; for when the sum of forces in one has a certain degree
of superiority over the sum of forces in the other, the former gain the
ascendency, determine the current, and overcome the latter so as to make
the substance exerting them yield up its elements in perfect accordance,
both as to direction and quantity, with the course of those which are
exerting the most intense and dominant action.

898. Water has generally been the substance, the decomposition of which has
been sought for as a chemical test of the passage of an electric current.
But I now began to perceive a reason for its failure, and for a fact which
I had observed long before (315. 316.) with regard to the iodide of
potassium, namely, that bodies would differ in facility of decomposition by
a given electric current, according to the condition and intensity of their
ordinary chemical affinities. This reason appeared in their _re-action upon
the affinities_ tending to cause the current; and it appeared probable,
that many substances might be found which could be decomposed by the
current of a single pair of zinc and platina plates immersed in dilute
sulphuric acid, although water resisted its action. I soon found this to be
the case, and as the experiments offer new and beautiful proofs of the
direct relation and opposition of the chemical affinities concerned in
producing and in resisting the stream of electricity, I shall briefly
describe them.

899. The arrangement of the a