A mineral called a salt by chemists. It contains either Ammonium or Potassium with Aluminium and Sulpuric acid in combination. (There is also an Iron Alum, in which, likewise, Ammonium is present.) It is crystalline, and has a peculiar taste, easily recognized after making its acquaintance. Alum is not often given as a medi-cine for the stomach, except as an emetic in bad cases of croup. For that purpose, its dose, in powder, is half a teaspoonful, with the same amount of the powder, or a tea-spoonful of Syrup of Ipecacuanha. In small dose, it is an astringent; that is, it tends to make the tissues which it touches shrink or contract together. Thus it helps to lessen the swelling of the mucous membrane, which is inflamed in sore throat, and it is much used for that, either in powder or in solution as a gargle. The powder may be blown into the throat through a quill, or, sometimes, put on the sore place with the end of one's finger. A gargle is made by dissolving a piece as large as a thumb in half a tumblerful of water. It is used by taking a mouthful of it and throwing the head back without swallowing it, letting it go as far down into the throat as it can without being swallowed.

Alum should not be employed in mouth-washes, because, when left long in contact with the teeth, the Sulphuric Acid in it acts somewhat upon their enamel. A solution of alum in pure water makes a good astringent eye-water, for inflammation of the eyes an even teaspoonful of alum in a tumblerful of water will be strong enough.