Ferric oxide gives a yellow colour, but requires the presence of an oxidizing agent to prevent reduction to the ferrous state.
It combines directly with many elements and compounds and frequently acts as energetic oxidizing agent.
The oxidizing agent in commonest use is copper oxide, which must be freshly ignited before use on account of its hygroscopic nature.
It is an energetic oxidizing agent.
Constant cells may be divided into two groups, according as their action is chemical (as in the bichromate cell, where the hydrogen is converted into water by an oxidizing agent placed in a porous pot round the carbon plate) or electrochemical (as in Daniell's cell, where a copper plate is surrounded by a solution of copper sulphate, and the hydrogen, instead of being liberated, replaces copper, which is deposited on the plate from the solution).