Ferric oxide gives a yellow colour, but requires the presence of an oxidizing agent to prevent reduction to the ferrous state.
To mix the substance with an oxidizing agent - mercuric oxide, lead dioxide, and afterwards copper oxide - and absorb the carbon dioxide in potash solution.
The native chrome-ironstone (Cr 2 O 3 FeO) may be used in this way as a source of such compounds, being fused in a reverberatory furnace, along with soda-ash and lime, the oxidizing agent in this case being atmospheric oxygen.
It is a monobasic acid, and a very powerful oxidizing agent (M.
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).
The oxidizing agent in commonest use is copper oxide, which must be freshly ignited before use on account of its hygroscopic nature.