the chloride of lead is an _electrolyte_, and when _electrolyzed_ evolves
the two _ions_, chlorine and lead, the former being an _anion_, and the
latter a _cation_.

[A] [Greek: anion] _that which goes up._ (Neuter participle.)

[B] [Greek: kation] _that which goes down._

666. These terms being once well-defined, will, I hope, in their use enable
me to avoid much periphrasis and ambiguity of expression. I do not mean to
press them into service more frequently than will be required, for I am
fully aware that names are one thing and science another.

667. It will be well understood that I am giving no opinion respecting the
nature of the electric current now, beyond what I have done on former
occasions (283. 517.); and that though I speak of the current as proceeding
from the parts which are positive to those which are negative (663.), it is
merely in accordance with the conventional, though in some degree tacit,
agreement entered into by scientific men, that they may have a constant,
certain, and definite means of referring to the direction of the forces of
that current.

[Since this paper was read, I have changed some of the terms which were
first proposed, that I might employ only such as were at the same time
simple in their nature, clear in their reference, and free from
hypothesis.

P iv. _On some general conditions of Electro-chemical Decomposition._

669. From the period when electro-chemical decomposition was first effected
to the present time, it has been a remark, that those elements which, in
the ordinary phenomena of chemical affinity, were the most directly opposed
to each other, and combined with the greatest attractive force, were those
which were the most readily evolved at the opposite extremities of the
decomposing bodies (549.).

670. If this result was evident when water was supposed to be essential to,
and was present in, almost every case of such decomposition (472.), it is
far more evident now that it has been shown and proved that water is not
necessarily concerned in the phenomena (474.), and that other bodies much
surpass it in some of the effects supposed to be peculiar to that
substance.

671. Water, from its constitution and the nature of its elements, and from
its frequent presence in cases of electrolytic action, has hitherto stood
foremost in this respect. Though a compound formed by very powerful
affinity, it yields up its elements under the influence of a very feeble
e