Yellow and red prussiate, which have been already mentioned, also belong to the iron compounds. They have been separately mentioned because the iron is contained in them in a peculiar form as a portion of an organic radical. =Ferrous Chloride=, FeCl₂, may sometimes be used instead of green vitriol. When iron is dissolved in hydrochloric acid hydrogen is given off and a solution of ferrous chloride is obtained; but when rouge is dissolved in the same acid, ferric chloride is formed. When iron is dissolved in nitric acid, in consequence of the oxidising properties of this acid a ferric salt is obtained. Iron forms two series of salts: in the ferrous compounds the iron is in the same form as in green vitriol and the corresponding salts; in the ferric compounds the iron is contained in a higher state of oxidation. By powerful oxidising agents, as nitric acid or chlorine, ferrous compounds are converted into ferric. =Manganese Compounds.=—Manganese (Mn) is a metal whose compounds show great similarity with those of iron, like which it forms two oxides (also others), manganous oxide (MnO) and manganic oxide (Mn₂O₃). The salts of manganous oxide are not oxidised in the air like those of ferrous oxide. The raw material used in the preparation of manganese compounds is the mineral pyrolusite, which is manganese dioxide (MnO₂). Manganese sulphate (MnSO₄) forms rose-red crystals containing varying quantities of water. The residues from the preparation of chlorine can be used as the material for the preparation of colours. According as pyrolusite and hydrochloric acid or pyrolusite, salt and sulphuric acid are used for this purpose a solution of manganous chloride or sulphate is obtained. Manganese compounds have but a restricted use in colour making. =Nickel Compounds= are generally coloured green, but they are not used as pigments. =Cobalt Compounds.=—Among these are found many important pigments. All cobalt compounds are coloured; in beauty and variety of shade they can only be compared with those of chromium. In properties cobalt is very similar to iron and nickel.