(Alum, Arab.).,the Greeks called it Assos, azub, Aseb; and when extremely hard, as iron, Elanula. It is an earthy salt, consisting, in a great measure, of the vitriolic acid and a pure clay, changing the purple juices of vegetables into a red colour; and extracted from substances usually called alum ores, which either are, or probably were, originally composed of clay and sulphur.

The present practice employs only the last two of the following species; but all the four have been in use.

1. Alumen plumosum officinarum. Earth flax.

It is entirely rejected from medicine, being more dan-gcrous than useful. See Amianthus.

2. Alumen plumosum verum, also called scissile, jamenum, plumeum, trichites. The plumose, feathered, or Hairy Alum.

It sometimes shoots upon the surface of those minerals that afford the factitious alum, and is also found on other bodies in the form of fibrous efflorescences. It seems to be the native alum of the ancients; and is formed by the evaporation of water that hath passed over beds of alum stone.

3. Alumen commune, common alum; also called alumen crystallinum,rupeum,factitium. Factitious or rock alum; English alum.

4. Alumen Romanum, Roman alum; also called alumen rubrum, rutilum, rochi Gallis. Rock, red, or Roch alum, by the French.

These two latter agree in their general qualities. The greatest quantities of them are artificially produced from different minerals, such as a blue slate, which is found about Scarborough in Yorkshire, Preston in Lancashire; and a whitish stone at Tolfa near Rome: these stones are calcined and exposed to the air. They thus absorb oxygen and become an acid salt; for in all alum the acid is in excess, which occasions its changing the blue colour of vegetables to a red. In the alum of commerce, or rather in the form offered to us as a medicine, there is a proportion of potash or ammonia. Each of these is supplied in its preparation; the latter from urine; and the former, at least, is essential to its crystallisation, but the excess of either, above the other, does not seem to injure its medicinal properties. It generally contains, according to Vauquelin, 49 parts in 100 of sulphate of alumine; 7 of sulphate of potash; and 44 of water. This proportion of water occasions by heat the watery fusion; but, in a higher temperature, the water, and at last the acid, escape. Its crystals are regular octaedrons, representing an indented column, whose sides arc equilateral triangles. It is soluble in about 16 times its weight of cold water, in a temperate heat, and in about 3-fourths, at the boiling point. Its specific gravity is 1.71. As the acid is in excess, its proper name is super sulphas alumina el potassae. When more completely saturated with the aluminous earth, it crystallises in cubes, and is called cubic alum; but it then loses its medicinal powers.

The English or common alum is colourless, and commonly in large masses, into which it is east by melting the crystals after the alum is perfectly made; and then pouring the fused matter into vessels, whose cavities give the forms it appears in. The Roman alum is of a reddish colour, and in small crystallised masses; but its chief difference from the English is in its being less styptic, and less nauseous. The name of roch, or rock alum, is applied to our alum, on account of the hardness and size of the masses; but foreigners apply it to the Roman, on account of the hard stone, or rock, from which it is extracted.

Alum hath a peculiarly sharp, rough, astringent taste; it melts over a gentle fire, sending up in a vapour nearly one-third of its weight, and becomes a light, white, spongy substance, called alumen ustum, burnt alum; it is the only salt that, with other animal ingredients, or vegetable matters, will make the black pyrophorus, which is owing almost exclusively to its potash. With an infusion of galls it becomes turbid and whitish. Upon adding a fixed alkaline salt to a dissolution of alum, its earth is precipitated, and its acid uniting with the alkali forms a tartarum vitriolatum, sulphas potasssae.

It is used by dyers to strike, fix, clear, and brighten their colours: it serves as the mordant to all colours; and by dipping paper in it, ink is prevented from spreading: vintners fine their liquors with alum; fishermen use it to dry codfish; it preserves animal substances from putrefaction, and wood from burning; it is used in the manufacture of leather; by calico printers, engravers, and soap-boilers; and bakers mix it with flout-to make their bread white and compact.

Medicinally, it is employed as a powerful astringent; as such it is prescribed to preserve the gums, to restrain uterine haemorrhages, and check the fluor albus; but though in these diseases it is highly commended, it is rarely, and with great caution, to be admitted in dysenteries, particularly in the beginning. Though celebrated as an astringent in some cases, it is no less extolled in the colic and other painful disorders of the bowels, attended with obstinate constipation. See Percival's Essays, vol. ii. The doses in these cases are from five to twenty grains, and may be repeated every four, eight, or twelve hours; and when duly persisted in, prove gently laxative, mitigate the pain, abate flatulence, restore the appetite, and strengthen the organs of digestion. Alum is powerfully tonic, and is supposed to contribute to the relief of pain in the intestines, by blunting the morbid sensibility of their nerves. In robust habits, after due bleeding and purging, it cures agues; and Dr. Cullen thinks it ought to be employed with other astringents in diarrhoeas. In active haemorrhages it is not useful, though a powerful medicine in those which are passive. It should be given in small doses, and gradually increased. It has been tried in the diabetes without success: joined with nutmeg it has been more successful in intermittehts, given in a large dose, an hour, or a little longer, before the approach of the paroxysm. In gargles, in relaxations of the uvula, divested of acute inflammation, it has been used advantageously; as well as in every state of the cynanche tonsilaris. It is also preferable to white vitriol, or acetated cerusse, in the ophthalmia membrana-rum; from two to five grains are dissolved in an ounce of water, for this purpose. Cullen's Mat. Med.

The Roman alum is counterfeited with common alum

A L V 85 A 31 A coloured; but break it, and the counterfeit will be found pale within, while the true is of a deeper red.

Its pharmaceutical preparations are well known. The aqua aluminosa contains a drachm of the salt to six ounces of water. In the compound alum water, as much variolated zinc is added: in many hospitals, however, the proportions are greatly increased. The alum curd consists of the white of two or three eggs shaken with a little alum, recommended for chronic inflammation of the eyes; and alum whey is made by adding two drachms of alum to a pint of cow's milk. The purifi-cationof alum by chalk, as directed by the London College, seems to injure its virtues. See Stypticus hel-vetii pulvis.

In extemporaneous prescription, the greatest caution is requisite. Almost all the salts destroy the union of its ingredients, but fortunately selenite is not of that number; so that hard water may be employed in its solution. Yet carbonat, nitrat, and muriat of lime, will decompose it. Mild ammonia and magnesia with its various neutrals, and all salts whose bases are barytes, potash, soda, and perhaps strontian, will have the same effect. It is decomposed also by the gallic acid; probably by tanin; certainly by the gummy resin kino; by various colouring matters; by different animal and vegetable substances. A similar effect is produced by eau de luce, by the different mercurial salts, and by the sugar of lead.

Alumen catinum. A name of potash. See Clavellati Cineres.

Alumen glaciale. When transparent like ice.