On the other hand, we see here how essential a rôle water plays in the formation of compounds, and how the affinity for water of crystallisation is essentially analogous to the affinity between salts, and hence also to the affinity of acids for bases, because the formation of double salts does not differ in any essential point (except the degree of affinity—that is, from a quantitative aspect) from the formation of salts themselves. When sodium hydroxide with nitric acid gives sodium nitrate and water the phenomenon is essentially the same as in the formation of astrakhanite from the salts Na2SO4,10H2O and MgSO4,7H2O. Water is disengaged in both cases, and hence the volumes are altered.
This salt, and especially its crystallo-hydrate with 7H2O, is generally known as Epsom salts. It has long been used as a purgative. It is easily obtained from magnesia and sulphuric acid, and it separates on the evaporation of sea water and of many saline springs. When carbonic anhydride is obtained by the action of sulphuric acid on magnesite, magnesium sulphate remains in solution. When dolomite—that is, a mixture of magnesium and calcium carbonates—is subjected to the action of a solution of hydrochloric acid until about half of the salt remains, the calcium carbonate is mostly dissolved and magnesium carbonate is left, which by treatment with sulphuric acid gives a solution of magnesium sulphate.
The anhydrous salt, MgSO4 (sp. gr. 2·61), attracts moisture (7 mol. H2O) from moist air; when heated in steam or hydrogen chloride it gives sulphuric acid, and when heated with carbon it is decomposed according to the equation 2MgSO4 + C = 2SO2 + CO2 + 2MgO. The monohydrated salt (kieserite), MgSO4,H2O (sp. gr. 2·56), dissolves in water with difficulty; it is formed by heating the other crystallo-hydrates to 135°. The hexahydrated salt is dimorphous. If a solution, saturated at the boiling-point, be prepared, and cooled without access of crystals of the heptahydrated salt, then MgSO4,6H2O crystallises out in monoclinic prisms (Loewel, Marignac), which are quite as unstable as the salt, Na2SO4,7H2O; but if prismatic crystals of the cubic system of the copper-nickel salts of the composition MSO4,6H2O be added, then crystals of MgSO4,6H2O are deposited on them as prisms of the cubic system (Lecoq de Boisbaudran). The common crystallo-hydrate, MgSO4,7H2O, Epsom salts, belongs to the rhombic system, and is obtained by crystallisation below 30°. Its specific gravity is 1·69.