sulphuric acid, whose type may be taken as magnesium sulphate , MgSO 4 . It occurs at Stassfurt as kieserite , MgSO 4 ,H 2 O, and generally separates from solutions as a heptahydrated salt, MgSO 4 ,7H 2 O, and from supersaturated solutions as a hexahydrated salt, MgSO 4 ,6H 2 O; at temperatures below 0° it crystallises out as a dodecahydrated salt, MgSO 4 ,12H 2 O, and a solution of the composition MgSO 4 ,2H 2 O solidifies completely at -5°. Thus between water and magnesium sulphate there may exist several definite and more or less stable degrees of equilibrium; the double salt MgSO 4 K 2 SO 4 ,6H 2 O may be regarded as one of these equilibrated systems, the more so since it contains 6H 2 O, whilst MgSO 4 forms its most stable system with 7H 2 O, and the double salt may be considered as this crystallo-hydrate in which one molecule of water is replaced by the molecule K 2 SO 4 .
The power of forming basic salts is a very remarkable peculiarity of magnesia and other feeble bases, and especially of those corresponding with polyvalent metals. The very powerful bases corresponding with univalent metals—like potassium and sodium—do not form basic salts, and, indeed, are more prone to give acid salts, whilst magnesium easily and frequently forms basic salts, especially with feeble acids, although there are some oxides—as, for example, copper and lead oxides—which still more frequently give basic salts. If a cold solution of magnesium sulphate be mixed with a solution of sodium 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