CHAPTER XIV THE VALENCY AND SPECIFIC HEAT OF THE METALS. MAGNESIUM. CALCIUM, STRONTIUM, BARIUM, AND BERYLLIUM

carbonate there is formed a gelatinous precipitate of a basic salt, Mg(HO) 2 ,4MgCO 3 ,9H 2 O; but all the magnesia is not precipitated in this case, as a portion of it remains in solution as an acid double salt. If sodium carbonate be added to a boiling solution of magnesium sulphate a precipitate of a still more basic salt is formed, 4MgSO 4 + 4Na 2 CO 3 + 4H 2 O = 4Na 2 SO 4 + CO 2 + Mg(OH) 2 ,3MgCO 3 ,3H 2 O. This basic salt forms the ordinary drug magnesia ( magnesia alba ), in the form of light porous lumps. Other basic salts are formed under certain modifications of temperature and conditions of decomposition. But the normal salt , MgCO 3 , which occurs in nature as magnesite in the form of rhombohedra of specific gravity 3·056, cannot be obtained by such a method of precipitation. In fact, the formation of the different basic salts shows the power of water to decompose the normal salt. It is possible, however, to obtain this salt both in an anhydrous and hydrated state. A solution of magnesium carbonate in water containing carbonic acid is taken for this purpose. The reason for this is easily understood—carbonic anhydride is one of the products of the decomposition of magnesium carbonate in the presence of water. If this solution be left to evaporate spontaneously the normal salt separates in a hydrated form, but in the evaporation of a heated solution, through which a stream of carbonic anhydride is passed, the anhydrous salt is formed as a crystalline mass, which remains unaltered in the air, like the natural mineral. The decomposing influence of water on the salts of magnesium, which is directly dependent on the feeble basic properties of magnesia, is most clearly seen in magnesium chloride , MgCl 2 . This salt is contained in the last mother-liquors of the evaporation of sea-water. On cooling a sufficiently concentrated solution, the crystallo-hydrate,