definite action_ in this and all similar cases
of electro-decomposition.

792. It is not often I have obtained an accordance in numbers so near as
that I have just quoted. Four experiments were made on the protochloride of
tin, the quantities of gas evolved in the volta-electrometer being from
2.05 to 10.29 cubic inches. The average of the four experiments gave 58.53
as the electro-chemical equivalent for tin.

793. The chloride remaining after the experiment was pure protochloride of
tin; and no one can doubt for a moment that the equivalent of chlorine had
been evolved at the _anode_, and, having formed bichloride of tin as a
secondary result, had passed away.

794. _Chloride of lead_ was experimented upon in a manner exactly similar,
except that a change was made in the nature of the positive electrode; for
as the chlorine evolved at the _anode_ forms no perchloride of lead, but
acts directly upon the platina, it produces, if that metal be used, a
solution of chloride of platina in the chloride of lead; in consequence of
which a portion of platina can pass to the _cathode_, and would then
produce a vitiated result. I therefore sought for, and found in plumbago,
another substance, which could be used safely as the positive electrode in
such bodies as chlorides, iodides, &c.

The chlorine or iodine does not act upon it, but is evolved in the free
state; and the plumbago has no re-action, under the circumstances, upon the
fused chloride or iodide in which it is plunged. Even if a few particles of
plumbago should separate by the heat or the mechanical action of the
evolved gas, they can do no harm in the chloride.

795. The mean of three experiments gave the number of 100.85 as the
equivalent for lead. The chemical equivalent is 103.5. The deficiency in my
experiments I attribute to the solution of part of the gas (716.) in the
volta-electrometer; but the results leave no doubt on my mind that both the
lead and the chlorine are, in this case, evolved in _definite quantities_
by the action of a given quantity of electricity (814. &c.).

796. _Chloride of antimony._--It was in endeavouring to obtain the
electro-chemical equivalent of antimony from the chloride, that I found
reasons for the statement I have made respecting the presence of water in
it in an earlier part of these Researches (690. 693. &c.).

797. I endeavoured to experiment upon the _oxide of lead_ obtained by
fusion and ignition of the nitrate in a pla