In a vacuum, or at 100°, it loses 5H2O, at 132° 6H2O, and at 210° all the 7H2O (Graham). If crystals of ferrous or cobaltic sulphate be placed in a saturated solution, hexagonal crystals of the heptahydrated salt are formed (Lecoq de Boisbaudran); they present an unstable state of equilibrium, and soon become cloudy, probably owing to their transformation into the more stable common form. Fritzsche, by cooling saturated solutions below 0°, obtained a mixture of crystals of ice and of a dodecahydrated salt, which easily split up at temperatures above 0°. Guthrie showed that dilute solutions of magnesium sulphate, when refrigerated, separate ice until the solution attains a composition MgSO4,24H2O, which will completely freeze into a crystallo-hydrate at -5·3°. According to Coppet and Rüdorff, the temperature of the formation of ice falls by 0·073° for every part by weight of the heptahydrated salt per 100 of water. This figure gives (Chapter I., Note 96) i = 1 for both the heptahydrated and the anhydrous salt, from which it is evident that it is impossible to judge the state of combination in which a dissolved substance occurs by the temperature of the formation of ice.
The solubility of the different crystallo-hydrates of magnesium sulphate, according to Loewel, also varies, like those of sodium sulphate or carbonate (see Chapter XII., Notes 733 and 746). At 0° 100 parts of water dissolves 40·75 MgSO4 in the presence of the hexahydrated salt, 34·67 MgSO4 in the presence of the hexagonal heptahydrated salt, and only 26 parts of MgSO4 in the presence of the ordinary heptahydrated salt—that is, solutions giving the remaining crystallo-hydrates will be supersaturated for the ordinary heptahydrated salt. All this shows how many diverse aspects of more or less stable equilibria may exist between water and a substance dissolved in it; this has already been enlarged on in Chapter I. Carefully purified magnesium sulphate in its aqueous solution gives, according to Stcherbakoff, an alkaline reaction with litmus, and an acid reaction with phenolphthalein. The specific gravity of solutions of certain salts of magnesium and calcium reduced to 15°/4° (see my work cited, Chapter I., Note 166), are, if water at 4° = 10,000, MgSO4: s = 9,992 + 99·89p + 0·553p2 MgCl2: s = 9,992 + 81·31p + 0·372p2 CaCl2: s = 9,992 + 80·24p + 0·476p2