[28] Graham even distinguished the last equivalent of the water of crystallisation of the heptahydrated salt as that which is replaced by other salts, pointing out that double salts like MgK2(SO4)2,6H2O lose all their water at 135°, whilst MgSO4,7H2O only parts with 6H2O.
[29] The crystalline form of the anhydrous salt obtained in this manner is not the same as that of the natural salt. The former gives rhombohedra, like those in which calcium carbonate appears as calc spar, whilst the natural salt appears as rhombic prisms, like those sometimes presented by the same carbonate as aragonite, which will shortly be described.
[30] Magnesium sulphate enters into certain reactions which are proper to sulphuric acid itself. Thus, for instance, if a carefully prepared mixture of equivalent quantities of hydrated magnesium sulphate and sodium chloride be heated to redness, the evolution of hydrochloric acid is observed just as in the action of sulphuric acid on common salt, MgSO4 + 2NaCl + H2O = Na2SO4 + MgO + 2HCl. Magnesium sulphate acts in a similar manner on nitrates, with the evolution of nitric acid. A mixture of it with common salt and manganese peroxide gives chlorine. Sulphuric acid is sometimes replaced by magnesium sulphate in galvanic batteries—for example, in the well-known Meidinger battery. In the above-mentioned reactions we see a striking example of the similarity of the reactions of acids and salts, especially of salts which contain such feeble bases as magnesia.
[31] As sea-water contains many salts, MCl and MgX2, it follows, according to Berthollet's teaching, that MgCl2 is also present.
[32] As the crystallo-hydrates of the salts of sodium often contain 10H2O, so many of the salts of magnesium contain 6H2O.
[33] This decomposition is most simply defined as the result of the two reverse reactions, MgCl2 ÷ H2O = MgO + 2HCl and MgO + 2HCl = MgCl2 + H2O, or as a distribution between O and Cl2 on the one hand and H2 and Mg on the other. (With O, MgCl2 gives chlorine, see Chapter X., Note [33], and Chapter II., Note 3 bis and others, where the reactions and applications of MgCl2 are given.) It is then clear that, according to Berthollet's doctrine, the mass of the hydrochloric acid converts the magnesium oxide into chloride, and the mass of the water converts the magnesium chloride into oxide. The crystallo-hydrate, MgCl2,6H2O, forms the limit of the reversibility. But an intermediate state of equilibrium may exist in the form of basic salts. On mixing ignited magnesia with a solution of magnesium chloride of specific gravity about 1·2, a solid mass is obtained which is scarcely decomposed by water at the ordinary temperature (see Chapter XVI., Note 4). A similar means is employed for cementing sawdust into a solid mass, called cylolite, used for flooring, &c.
We may remark that MgBr2 crystallises not only with 6H2O (temperature of fusion 152°), but also with 10H2O (temperature of fusion +12°, formed at -18°). (Panfiloff, 1894).
[34] According to Thomsen, the combination of MgCl2 with 6H2O evolves 33,000 calories, and its solution in an excess of water 36,000.
[34 bis] Hence MgCl2 may be employed for the preparation of chlorine and hydrochloric acid (Chapters [X]. and [XI].). In general magnesium chloride, which is obtained in large quantities from sea water and Stassfurt carnallite, may find numerous practical uses.
[35] There are many other methods of separating calcium from magnesium besides that mentioned above (Note [22]). Among them it will be sufficient to mention the behaviour of these bases towards a solution of sugar; hydrated lime is exceedingly soluble in an aqueous solution of sugar, whilst magnesia is but little soluble. All the lime may be extracted from dolomite by burning it, slaking the mixture of oxides thus obtained, and adding a 10 p.c. solution of sugar. Carbonic anhydride precipitates calcium carbonate from this solution. The addition of sugar (molasses) to the lime used for building purposes powerfully increases the binding power of the mortar, as I have myself found. I have been told that in the East (India, Japan) the addition of sugar to cement has long been practised.