Alum is a double salt, composed, in the crystalline state, of one equivalent of sulphate of alumina, one of sulphate of potassa, and twenty-four of water; and denominated, chemically, sulphate of alumina and potassa. It is prepared either by the direct combination of its constituents, or by various processes from certain minerals called alum ores, containing ingredients, by the mutual reaction of which, under favourable circumstances, and with necessary additions, the salt is generated.

Sensible and Chemical Properties. The salt is in octohedral or more rarely cubic crystals, or, as it is usually found in commerce, in irregular crystalline masses or fragments, whitish and translucent, slightly efflorescent, inodorous, and of a strongly astringent, sweet, and acidulous taste. Exposed to heat, it first melts, then boils up, loses its water of crystallization, and becomes white and opaque, and readily reducible to powder. In this state it is called burnt alum, or, officinally, Dried Alum (Alumen Exsiccatum, U. S.). By a strong heat it is quite decomposed. In the crystalline state, it is soluble in about eighteen parts of cold, and three-quarters of its own weight of boiling water; is entirely insoluble in absolute alcohol, and very nearly so in proof spirit. It has an acid reaction with vegetable colours.

Incompatibles. Alum is incompatible with alkalies and their carbonates, lime-water, magnesia and its carbonate, tartrate of potassa, phosphate of soda, and acetate and subacetate of lead, with all of which it produces precipitates. Those thrown down by the alkalies are dissolved by an excess of alkali. It also precipitates solutions of albumen, gelatin, and many of the vegetable astringents, and coagulates milk.

Effects on the System

When applied externally, or taken internally in such manner and quantity as not to excite irritation, alum acts, so far as can be observed, purely as an astringent, contracting the tissues, diminishing the caliber of the blood-vessels, and thus lessening the colour of the part, and diminishing secretion and exhalation. On the mucous membrane of the mouth and fauces its astringent effect is strong, and the impression which it leaves behind it durable. I have often observed that, when used as a gargle at bedtime, it so affects the tongue and palate that the sense of taste remains much blunted in the morning. Acting directly on the alimentary mucous membrane, it lessens the number and quantity of the stools.

There can be no doubt that, when taken internally, it exerts its peculiar action also on the whole system, though its general is much less powerful than its local operation. The probability is that it is absorbed, as alumina has been found in the urine and viscera of animals to which it has been administered; but in what state precisely it enters the circulation has not been determined. Its effects upon the system at large are more observable in disease than in health; but dryness of the throat and fauces, with thirst, has been noticed as one of the results of its internal use.

It is believed by some, and not, I think, without reason, to be somewhat refrigerant or sedative in its influence on the circulation.

Some ascribe the astringent effects of alum to its chemical reaction with the tissues. Considering how instantaneous and considerable is the shrinking of the mucous membrane of the mouth, when a strong solution is applied to it, I cannot conceive that the result is owing to a mere chemical change. Not only in this case, but in every other, I believe that it operates by calling the vital property of contractility into action.

When used either outwardly or inwardly in large quantities, though primarily astringent, it becomes irritant after a time, and at length, if continued, may excite inflammation. This effect will follow the application even of small quantities to a very delicate or unprotected surface, as to the conjunctiva of the eye, or to the skin recently denuded of the epidermis. In such cases, its peculiar astringent effect is overwhelmed by the inflammatory action. Thus, when swallowed in the quantity of a drachm or more, it not unfrequently causes nausea and vomiting, and sometimes produces griping pains and purging. Devergie found about six drachms of dried alum, given to a dog, to produce death when the oesophagus was tied, so as to prevent vomiting. Under such circumstances, the mucous membrane of the stomach and bowels has been observed to be much inflamed. The same quantity, when the oesophagus is not tied, is discharged by vomiting without any permanent evil effects. Orfila found that seven drachms of powdered crystallized alum produced vomiting in dogs in from ten to thirty minutes. Dried alum, applied to a denuded surface, acts as a mild caustic, and is sometimes used with reference to this effect.

When used for a considerable time, in doses insufficient to nauseate, alum not unfrequently produces a sense of stricture in the epigastrium, precordial oppression, and other dyspeptic feelings, probably by interfering with the secretion of the gastric juice, and thus impairing digestion. Therapeutic Application. Alum is useful, as an internal remedy, in those forms and states of diarrhoea and chronic dysentery to which astringents are applicable, and in hemorrhage from the bowels under similar circumstances. It is not so well adapted to hemorrhage of the stomach, in consequence of its liability to produce nausea and vomiting; but might nevertheless be employed in this affection, in considerable doses, should other remedies fail, and the case be urgent. In the treatment of the bowel affections, it has not unfrequently been associated with some of the vegetable astringents, such as tannic acid, kino, extract of rhatany, etc.; and, though it undergoes chemical change through reaction with these substances, yet it does not follow that the resulting products are inert; and experience has shown that the combination is often effectual. In the dose of ten or twelve grains three or four times a day, with an equal quantity of bitartrate of potassa, it has been found by Sir James Murray very useful in the chronic gastric affection, characterized by vomiting of glairy mucus.