double salt, K 2 Mg(SO 4 ) 2 ,6H 2 O, which closely resembles the above-mentioned ammonium salt, crystallises from these solutions. The nearest analogues of magnesium are able to give exactly similar double salts, both in crystalline form (monoclinic system) and composition; they, like this salt ( see Chapter XV.), are easily able (at 140°) to part with all their water of crystallisation, and correspond with the salts of 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