As already remarked, the raising of the valency, or, what is the same thing, the removal of electrons from an element, is often spoken of as " oxidation" ; the reducing of the valency, or the addition of electrons, as " reduction." The tendency of chromous halides to transform into chromic compounds is so great, that it is not possible to expose them to air without the change taking place, and consequently the reduction of chromic compounds to chromous is a difficult operation. But with iron, both classes of compounds have nearly equal stability ; hence oxidation and reduction play a great part in their formation. The action of nascent hydrogen from any source reduces ferric halide into ferrous: FeCl3.Aq 4- H = FeCl2.Aq 4 HCl.Aq. Similarly, a ferrous halide, in presence of halogen acid and either free or nascent oxygen, is oxidised to a ferric : 2FeCl2. Aq + 2HCI. Aq + O = 2FeCl3. Aq 4- H2O. Or the halogen itself may be used to effect the change: 2FeCl2.Aq4-Cl2 = 2FeCl3.Aq. On evaporating these solutions, hydrolysis takes place partially; thus ferric chloride yields compounds of a basic character, such as (OH)FeCl2, (OH)2FeCl, which are partly hydroxide, partly chloride. This statement applies to the halides of all these metals.