QUALITIES. _Form_, octohedral crystals, whose sides are equilateral triangles; they are slightly efflorescent. _Taste_, sweet, rough, and acidulous. CHEMICAL COMPOSITION. It is a triple, or sometimes a quadruple salt, with excess of acid, consisting of sulphuric acid and alumina, with potass, or ammonia, or frequently both of them; the nature of the alkali however does not in the least appear to affect the properties of alum, although it produces a crystallographic modification; for where potass is present the summit of the crystal will exhibit a truncation. Dr. Ure has lately produced alum with soda, and the combination differs from common alum only in its greater degree of solubility, a property which at once recommends it to the attention of the pharmaceutist and physician. SOLUBILITY. A fluid-ounce of cold water dissolves 30 grains, but if boiling four drachms; it is insoluble in alcohol. INCOMPATIBLE SUBSTANCES. _Alkalies_ and _alkaline salts_, after neutralizing the excess of acid, precipitate the alumine. It is also decomposed by _carbonate_ and _muriate of ammonia_, _carbonate of magnesia_, and _tartrate of potass_, by _lime-water_, _acetate of lead_, and _the salts of mercury_, as well as by many vegetable and animal substances, especially _galls_ and _kino_. It is on this account very injudicious to combine alum with any vegetable astringent with a view to encrease its virtues; thus the “_Pulvis Sulphatis Aluminæ compositus_” of the Edinburgh college, is less powerful than any of the ingredients of which it is composed; and the addition of alum to the decoction of bark, undoubtedly diminishes its efficacy as an astringent injection. MEDICINAL USES. Alum is internally a powerful astringent, in hæmorrhages and inordinate fluxes, and is externally useful for repellent and astringent lotions, gargles, and collyria. Dioscorides and Hippocrates praised its effects as a lotion in various kinds of ulcers, and particularly in sores of the mouth, and in spongy, swelled gums. Van-Helmont was the first person who employed alum in uterine hemorrhage, and the success of the practice very considerably enhanced his reputation. _Boerhaave’s_ astringent powder for the ague consisted of _Alum_ and _Nutmeg_ with the addition of _Armenian bole_. DOSE, gr. x. In large doses it is liable to excite nausea, and to act upon the bowels. Nutmeg or some aromatic should therefore be joined with it. FORMS OF EXHIBITION. In solution, or in substance made into pills with extract; (_Form: 53, 56_,) it is sometimes given with advantage in the form of whey (_Alum-whey—Serum Aluminosum_) made by boiling ʒij with a pint of milk, and then straining, the dose of which is a wine glass full; (_Form; 54._) By briskly agitating a drachm of alum with the white of an egg, a coagulum is formed, (_Alum curd of Riverius_; _Albumen. Aluminosum_) which is serviceable in some species of ophthalmia, when applied between two pieces of thin linen rag.[365] As alum is not