=Ferrous Sulphate= (Green Vitriol, Copperas), FeSO₄.7H₂0.—This substance, which occurs commercially in a form of great purity at a very low price, is generally the starting point in the preparation of iron pigments. When pure, it forms fine sea-green crystals, with an astringent metallic taste, which are not poisonous and are easily soluble in water. After long exposure to the air, ferrous sulphate becomes covered with an ochre-coloured crust, consisting of basic ferric sulphate. The ferrous oxide contained in the green vitriol has united with oxygen and been converted into ferric oxide. The latter requires a larger quantity of acid than ferrous oxide for the formation of soluble salts, so that an insoluble basic salt is separated. The same thing occurs when a solution of ferrous sulphate is exposed to the air. When green vitriol, or any other ferrous salt, is exposed to the action of oxidising agents, such as chlorine or nitric acid, the iron is rapidly changed into the ferric state. This transformation is of particular importance in the manufacture of certain blue pigments. We give in a table the relation between the percentage of crystallised ferrous sulphate (FeSO₄.7H₂O) contained in a solution at 15° C. and its specific gravity:— +--------+-----------------+--------+-----------------+ |Specific| Percentage of |Specific| Percentage of | |Gravity.|Ferrous Sulphate.|Gravity.|Ferrous Sulphate.| +--------+-----------------+--------+-----------------+ | 1·000 | 0 | 1·118 | 21 | | 1·005 | 1 | 1·125 | 22 | | 1·011 | 2 | 1·131 | 23 | | 1·016 | 3 | 1·137 | 24 | | 1·021 | 4 | 1·143 | 25 | | 1·027 | 5 | 1·149 | 26 | | 1·032 | 6 | 1·155 | 27 | | 1·037 | 7 | 1·161 | 28 | | 1·043 | 8 | 1·168 | 29 | | 1·048 | 9 | 1·174 | 30 | | 1·054 | 10 | 1·180 | 31 | | 1·059 | 11 | 1·187 | 32 | | 1·065 | 12 | 1·193 | 33 | | 1·071 | 13 | 1·200 | 34 | | 1·077 | 14 | 1·206 | 35 | | 1·082 | 15 | 1·213 | 36 | | 1·088 | 16 | 1·219 | 37 | | 1·094 | 17 | 1·226 | 38 | | 1·100 | 18 | 1·232 | 39 | | 1·106 | 19 | 1·239 | 40 | | 1·112 | 20 | —— | —— | +--------+-----------------+--------+-----------------+