Prep. Usually made by burning alum schist, which contains metallic sulphurets as well as alumina, and subsequent exposure to air, by which means sulphuric acid is formed; this unites with the alumina, and the after-addition of sulphate of potash to the solution, causes the formation and crystallization of the alum.

Prop. & Comp. Alum is a double sulphate of alumina and potassa (Al2 O3, 3 So3 +KO, So3 + 24 HO); it forms transparent, white, regular octahedral crystals, having an acid sweet astringent taste; it is slightly efflorescent in dry air, from a loss of some of its water of crystallization; soluble in 18 parts of water at 60° Fah., in less than its own weight of boiling water. Alumina is precipitated from a solution of alum by the addition of alkalies and their carbonates, but re-dissolved by excess of the former. A solution of alum gives also an immediate precipitate with chloride of barium (sulphate of baryta), and, after some hours, a crystalline precipitate with tartaric acid (bitartrate of potash). It should not be coloured blue by a mixture of ferrocyanide and ferricyanide of potassium (indicating that neither protoxide nor peroxide of iron is present). Alum should be entirely soluble in hot solution of soda, without the evolution of ammonia, showing that it is a true potash salt; as occasionally sulphate of ammonia replaces, wholly 6r in part, the sulphate of potash.

Off. Prep. Alumen Exsiccatum. Dried Alum. It is simply alum deprived of its water by heat, which first fuses the salt, and then drives off the water of crystallization; this forms 45.43 per cent. of its weight. Dried, or burnt alum, as it is commonly termed, occurs as a white or light spongy mass, which unites with water with some intensity. It is usually reduced to powder before being employed as a medicinal agent.

Therapeutics. Alum acts as an astringent, and if applied as alumen exsiccatum, or burnt alum, it is a slight escharotic. Internally it first acts upon the mucous membrane of the stomach and intestines; it is afterwards absorbed, and produces remote astringent effects on the various secreting and other organs. In large doses it is a purgative. It is employed topically as a gargle or injection in sore throat, leucorrhcea, etc.; internally in haemorrhages and passive discharges; sometimes in colica pictorum as a purgative. Alum has also gained repute in the treatment of hooping-cough.

Dose. Of alum 10 gr. to 20 gr. as an astringent, alone or combined with kino, etc.; from 30 gr. to 60 gr. may be given as a purgative. [In doses of 1 or 2 drachms, it operates as an emetic] Dried alum is for external use only.

Incompatibles. Alkalies and their carbonates, tannic acid, or infusions and decoctions containing it; tartrates, salts of lead, baryta, lime, cause precipitates in solutions of alum.