'freezing point depression' of solutions (Chapter I., Note 49) found that the molecules Al_{2}(C_{2}H_{5})_{6} and Al_{2}(C_{5}H_{11})_{6}, &c., correspond to the type Al_{2}X_{6}. Thus it may now be accepted that the molecular composition of the compounds of aluminium in their simplest form is AlX_{3}, but that they may polymerise and give Al_{2}X_{6} or, in general, Al_{2}X_{3_n_}. [31] In the case of gallium, as a close analogue of aluminium, Lecoq de Boisbaudran (1880) showed that probably the molecule gallium chloride contains Ga_{2}Cl_{6} at low temperatures and high pressures, and that it dissociates into GaCl_{3} at high temperatures and low pressures. The molecule of indium chloride seems to exist only in the simplest form, InCl_{3}. _Aluminium sulphate_, Al_{2}(SO_{4})_{3}, which is obtained by treating clay or the hydrates of alumina with sulphuric acid, crystallises in the cold with 27H_{2}O, or at the ordinary temperature in pearly crystals, which are greasy to the touch and contain 16H_{2}O.[32] Its solutions act like sulphuric acid--for instance, they evolve hydrogen with zinc, forming basic salts, which are sometimes met with in nature (_aluminite_, Al_{2}O_{3},SO_{3},9H_{2}O, _alumiane_, Al_{2}O_{3},2SO_{3}, and others), and may be obtained by the decomposition of normal salts and by the direct solution of the hydroxide in normal salts: these exhibit a varying composition, (Al_{2}O_{3})_{_n_}(SO_{3})_{_m_}(H_{2}O)_{_q_}, where _m/n_ is less than 3. Aluminium sulphate is now prepared (from the pure hydrate obtained from bauxite, Note 21) in large quantities for dyeing purposes (instead of alums) as a mordant. With solutions of the alkali sulphates (potassium, sodium, ammonium, rubidium, and cæsium sulphates), the normal salt easily forms double salts, termed _alums_--for example, the ordinary crystalline alum contains KAl(SO_{4})_{2},12H_{2}O, or K_{2}SO_{4},Al_{2}(SO_{4})_{3},24H_{2}O. In the ammonium alums (which leave a residue of alumina when ignited) the potassium is replaced by ammonium (NH_{4}). Alums are used in large quantities, because there is scarcely any other salt which crystallises so easily. In this respect the alums formed by potassium and ammonium are equally convenient to purify, because they present a considerable difference in their solubility at the ordinary and higher temperatures. If the crystallisation be conducted rapidly, the salt separates in minute crystals, but if it be slowly deposited, especially in large masses, as in factories, then crystals several centimetres long are sometimes obtained. At a higher temperature alums are very much more soluble, and crystallise with greater difficulty, and are therefore less easily freed from impurities; at 0° 100 parts of water dissolve 3 parts, at 30° 22 parts, at 70° 90 parts, and at 100° 357 parts of potassium alum.[33] The solubility of ammonium alum is slightly less.