al'um

A name given to a cla of double sulphates, resembling each other in chemical structure and in crystalline form. a. of the U. S. Ph. should contain not le than 99.5 per cent, of pure aluminum and pota ium sulphate, A1K(S04)2+ 12H2O, known also as pota a alum. [U. S. Ph., Br. Ph.] ammonia a. The double sulphate of aluminum and ammonium; its uses are those of the ordinary alum. burnt a. See alumen exsiccation, under alumen. calcined a. See alumen exsiccation, under alumen. iron a. A double sulphate of iron and another element or a radicle, especially the double sulphate of iron and pota ium. [U. S. Ph.] iron ammonium a., iron pota ium a. See iron and ammonium sulphate and iron and pota ium sulphate, under iron. [Mid. Eng., alum, alom, from Lat., alumen. 1