=Potassium Aluminium Alum=, KAl(SO₄)₂.12H₂O = 474.—This is the substance commonly called alum. Like all alums it crystallises in fine octahedral crystals, which at first are quite transparent, but slowly effloresce in the air and become covered by a white powder. It dissolves with difficulty in cold, but readily in hot water. The solution has at first a sweet taste, with an astringent after-taste. Alum comes into the market in different forms, of which the following are the most important: as crystallised alum, in the form of large crystals united together; as alum meal, a coarse crystalline powder obtained by rapidly cooling and stirring a hot alum solution. On account of the larger surface this form dissolves more quickly than the large crystals. Roman alum is the name of a variety chiefly imported from Italy; it owes its reputation to its great purity—it contains a very small quantity of iron. In order to prepare alum quite free from iron from the ordinary alum containing iron, it is recrystallised, that is, as much as possible is dissolved in boiling water and the solution quickly cooled with continual stirring; the small crystals so obtained are then washed with cold water. The residual saturated solution of alum, which contains the greater part of the iron, can be used for the preparation of those colours which are not injured by the presence of iron. The solubility of alum in water varies greatly at different temperatures. The table gives the weight of alum dissolved by 100 parts of water at different temperatures. +------------+------------------+---------------+ |Temperature.|Crystallised Alum.|Anhydrous Alum.| +------------+------------------+---------------+ | 0° C. | 3·90 | 2·10 | | 10° ” | 9·52 | 4·99 | | 20° ” | 15·13 | 7·74 | | 30° ” | 22·01 | 10·94 | | 40° ” | 30·92 | 14·88 | | 50° ” | 44·11 | 20·09 | | 60° ” | 66·65 | 26·76 | | 70° ” | 90·67 | 35·11 | | 80° ” | 134·47 | 45·66 | | 90° ” | 209·31 | 58·64 | | 100° ” | 357·48 | 74·53 | +------------+------------------+---------------+ When potash alum is heated it loses water, 75 per cent. of the total at 61° C.; at 92° C. it melts completely, and all the water is lost by continued heating at 100° C. The residue is known as burnt alum.