numerous possible sources of error. The decomposition of his
acidulated water is certainly a direct result of the current; but as
the varied and important researches of MM. Becquerel, De la Rive,
and others had shown, there are also secondary actions which may
materially interfere with and complicate the pure action of the
current. These actions may occur in two ways: either the liberated
ion may seize upon the electrode against which it is set free,
forming a chemical compound with that electrode; or it may seize
upon the substance of the electrolyte itself, and thus introduce
into the circuit chemical actions over and above those due to the
current. Faraday subjected these secondary actions to an exhaustive
examination. Instructed by his experiments, and rendered competent
by them to distinguish between primary and secondary results, he
proceeds to establish the doctrine of 'Definite Electro-chemical
Decomposition.'

Into the same circuit he introduced his voltameter, which consisted
of a graduated tube filled with acidulated water and provided with
platinum plates for the decomposition of the water, and also a cell
containing chloride of tin. Experiments already referred to had
taught him that this substance, though an insulator when solid, is a
conductor when fused, the passage of the current being always
accompanied by the decomposition of the chloride. He wished to
ascertain what relation this decomposition bore to that of the water
in his voltameter.

Completing his circuit, he permitted the current to continue until
'a reasonable quantity of gas' was collected in the voltameter. The
circuit was then broken, and the quantity of tin liberated compared
with the quantity of gas. The weight of the former was 3.2 grains,