Properties. - Dry white powder with the taste and other properties of alum.

Preparation. - By heating potassium alum until the water of crystallisation is driven off.

Dose. - As an astringent, 10 to 40 grs.; as an emetic, 30 to 60 grs. For a lotion or gargle, 4 to 20 grs. to an ounce of water, or in the glycerinum aluminis, B.P.

Action. - Alum precipitates albumen and gelatin. It has no action on the unbroken skin, but when applied to parts from which the epidermis has been removed, it causes a film of coagulated albumen to form on the surface, and produces contraction of the tissues and vessels below. It thus lessens the supply of blood to the part, relieves congestion, diminishes the swelling, lessens the discharge from inflamed surfaces, and therefore acts as an astringent. By causing contraction of vessels and aiding the formation of coagula, it arrests haemorrhage, and is therefore used either as a strong solution, or, if this prove insufficient, in the form of powder mixed with starch as a styptic. Dried alum abstracts water from the tissues and acts as a slight caustic. When swallowed in large quantities alum produces gastro-enteritis. In smaller doses it acts as an emetic. It is not so powerful as a caustic, astringent, styptic, or emetic as the salts of zinc or copper.

Uses. - Dried alum is sometimes used to check exuberant granulation in ulcers. Bleeding from the nose may be stopped by sniffing up or injecting a solution of alum into the nostrils, and if the solution be ineffectual, powdered alum may be blown up by means of a paper funnel; it is also employed locally in bleeding from the mouth, throat, gums, haemorrhoids, and the uterus. As an astringent, alum is used in both purulent and simple ophthalmia, but on account of its solvent action on the cornea it may lead to perforation, and should therefore be avoided (p. 216). A 1 per cent. solution, with 1 per cent. borax, is useful in acute eczema. It is used as a lotion in otorrhoea; as a wash to the mouth in ptyalism, aphthae, and ulceration of the mouth and gums; as a gargle for sore-throat, congestion of the pharynx, and elongation of the uvula, as well as for the tickling, violent coughs which depend upon them, and are often accompanied by retching (p. 248). Dried alum has been applied in powder to remove the false membrane from the throat in croup and diphtheria.

Alum may be employed as a spray to the larynx in coughs and hoarseness depending upon chronic laryngeal catarrh. As a wash it may be used in inflammation of the vulva in children, to relieve itching in pruritis vulvae, and to prevent the recurrence of prolapsus ani. It is useful as an injection in gonorrhoea and leucorrhcea.

When swallowed it will act on the stomach as an astringent, and is useful in preventing the vomiting of phthisis. It is not improbable that the vomiting which occurs usually after paroxysms of coughing is due to the congestion produced in the stomach by the cough, and that the alum prevents the vomiting by lessening this congestion (p. 377). When given in larger quantities alum is an emetic, acting promptly, and producing little depression. A teaspoonful of powdered alum proves a very useful emetic in cases of croup, and may be given to children mixed with honey. In the intestines alum acts as an astringent also, and is useful in diarrhoea; but, curiously enough, in lead colic it will act as a purgative, relieving the pain and opening the bowels. Its utility in lead-poisoning probably depends, to a considerable extent, on its being a sulphate, and thus precipitating any lead salts it may meet in the intestine in the form of insoluble lead sulphate, and preventing absorption from the intestinal canal. In typhoid fever, and in chronic dysentery and diarrhoea, it is said to be useful in checking the discharges from the bowels.

After its absorption into the blood it is supposed to exercise an astringent action, and is given to check sweating.

Internally, as a styptic, it is employed to check bleeding from the stomach, intestines, lungs, uterus, or kidneys.

Antidote. - Give tepid water with small doses of carbonate of sodium to decompose the alum, and empty the stomach by the stomach-pump or emetics.