Note 33 — Chapter XIV · THE VALENCY AND SPECIFIC HEAT OF THE METALS. MAGNESIUM. CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM

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).