Alum (alumen, aluminis) of the Pharmacopoeia is potassium alum, the double sulphate of aluminium and potassium, KAl(So4)2.12H2O, or ammonium alum, Nh4al(So4)2.12H20. It is soluble in about 9 parts of water and insoluble in alcohol. Its taste is sour, and it is decidedly astringent by coagulation of the proteins of the superficial cells, but it is not very irritant. It is a constituent of some baking-powders, but is, without much doubt, harmful to digestion.

It is employed, usually in 5 per cent, solution, as a gargle or spray in relaxed sore throat, as a vaginal douche, and as a wash for the skin to stop local sweating of the hands and feet or the night-sweats of tuberculosis. The crystals may be used to shrink canker sores in the mouth, or to check hemorrhage from scratches or small cuts. The powdered alum has been used in 60-grain (4 gm.) dose as an emetic, but is not at all reliable.

Burnt alum (alumen exsiccatum) is alum with the water of crystallization driven off by heat. It has a great affinity for water, is powerfully astringent, and is slightly caustic. Its chief employment is as an application to sluggish ulcers.

The solution of aluminium subacetate, N. F., is made by acting on calcium acetate with aluminium sulphate in solution, the insoluble calcium sulphate being removed by filtration. The solution of aluminium acetate, N. F. (Burow's solution) is prepared by mixing solutions of alum, 12.6 gm., and lead acetate, 15 gm., and adding water enough to make 100 c.c. The precipitate of lead sulphate is filtered off, as poisoning has occurred from failure to remove this. Either formula makes a slightly astringent, slightly antiseptic liquid, the chief use of which is as a wet dressing for infected wounds. Koll (1912) reports great success with it also in 42 cases of colon bacillus infection of the urinary tract.

A 25 per cent. solution of aluminium chloride may be applied every two or three days for sweating of hands, feet, or axillae. It is irritant and may cause a dermatitis.