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.