This is prepared by exposing crystallized alum (i. 136) to a moderate beat, until the water of crystallization is driven off, and then rubbing it to powder. it is necessary not to urge the heat too far, as otherwise a part of the sulphuric acid may also be expelled.

Dried alum is a white powder, of a very strongly astringent taste, and slowly soluble in water. it differs from the crystallized salt simply by the want of water of crystallization, and some modification in its molecular condition.

It is a very mild escharotic, scarcely affecting parts covered with the cuticle, and only moderately the living healthy tissue; but acting with considerable energy on newly formed fungous growths. it is used chiefly to repress fungous granulations, or proud flesh in ulcers. in these cases, however, it does something more than destroy the exuberant granulations. By its astringency it counteracts the relaxation which occasions this sort of growth, and thus favours the healing of the ulcers. it is simply sprinkled on the diseased surface, so as to cover it with a thin layer of the powder. it is particularly useful in the fungous ulcers following burns. A recent application has been made of it to chronic catarrh and inflammation of the meatus and tympanum of the ear. it may be introduced by means of a moistened hair-pencil, or may be blown into the ear through a tube. The ear should be cleansed by gently injecting warm water, before each application of the alum. (Med. and Surg. Rep., Sept. 24, 1864, p. 107; from Med. Times and Gaz.)