ing power, its miscibility with most of
the substances acted upon, and its general relation to them in respect to
chemical affinity. It consists of elements, which in their electrical and
chemical relations are directly and powerfully opposed, yet combining to
produce a body more neutral in its character than any other. So that there
are but few substances which do not come into relation, by chemical
affinity, with water or one of its elements; and therefore either the water
or its elements are transferred and assist in transferring the infinite
variety of bodies which, in association with it, can be placed in the
course of the electric current. Hence the reason why it so rarely happens
that the evolved substances rest at the first surface of the water, and why
it therefore does not exhibit the ordinary action of a pole.

559. Air, however, and some gases are free from the latter objection, and
may be used as poles in many cases (461, &c.); but, in consequence of the
extremely low degree of conducting power belonging to them, they cannot be
employed with the voltaic apparatus. This limits their use; for the voltaic
apparatus is the only one as yet discovered which supplies sufficient
quantity of electricity (371. 376.) to effect electro-chemical
decomposition with facility.

560. When the poles are liable to the chemical action of the substances
evolved, either simply in consequence of their natural relation to them, or
of that relation aided by the influence of the current (518.), then they
suffer corrosion, and the parts dissolved are subject to transference, in
the same manner as the particles of the body originally under
decomposition. An immense series of phenomena of this kind might be quoted
in support of the view I have taken of the cause of electro-chemical
decomposition, and the transfer and evolution of the elements. Thus platina
being made the positive and negative poles in a solution of sulphate of
soda, has no affinity or attraction for the oxygen, hydrogen, acid, or
alkali evolved, and refuses to combine with or retain them. Zinc can
combine with the oxygen and acid; at the positive pole it does combine, and
immediately begins to travel as oxide towards the negative pole. Charcoal,
which cannot combine with the metals, if made the negative pole in a
metallic solution, refuses to unite to the bodies which are ejected from
the solution upon its surface; but if made the positive pole in a dilute
solution of