t one of the electrodes, be ascertained, the electro-chemical
equivalent of that body may be deduced from the known constant composition
of the substance evolved. Thus, when fused protiodide of tin is decomposed
by the voltaic current (804.), the conclusion may be drawn, that both the
iodine and tin are _ions_, and that the proportions in which they combine
in the fused compound express their electro-chemical equivalents. Again,
with respect to the fused iodide of potassium (805.), it is an electrolyte;
and the chemical equivalents will also be the electro-chemical equivalents.

841. If proposition viii. sustain extensive experimental investigation,
then it will not only help to confirm the results obtained by the use of
the other propositions, but will give abundant original information of its
own.

842. In many instances, the _secondary results_ obtained by the action of
the evolved _ion_ on the substances present in the surrounding liquid or
solution, will give the electro-chemical equivalent. Thus, in the solution
of acetate of lead, and, as far as I have gone, in other proto-salts
subjected to the reducing action of the nascent hydrogen at the _cathode_,
the metal precipitated has been in the same quantity as if it had been a
primary product, (provided no free hydrogen escaped there,) and therefore
gave accurately the number representing its electro-chemical equivalent.

843. Upon this principle it is that secondary results may occasionally be
used as measurers of the volta-electric current (706. 740.); but there are
not many metallic solutions that answer this purpose well: for unless the
metal is easily precipitated, hydrogen will be evolved at the _cathode_ and
vitiate the result. If a soluble peroxide is formed at the _anode_, or if
the precipitated metal crystallize across the solution and touch the
positive electrode, similar vitiated results are obtained. I expect to find
in some salts, as the acetates of mercury and zinc, solutions favourable
for this use.

844. After the first experimental investigations to establish the definite
chemical action of electricity, I have not hesitated to apply the more
strict results of chemical analysis to correct the numbers obtained as
electrolytic results. This, it is evident, may be done in a great number of
cases, without using too much liberty towards the due severity of
scientific research. The series of numbers representing electro-chemical
equivalents must, like t