Lond. Edinb. Dub. Alum.

Syn. Alum(F.), Alaun (G.), Alum (Dutch) Aluin (Dan. Swed.), Halun (Polish), Kevaszu (Russ.), Allume (I.), Alumbre (S.), Pedrathune (Portug.), Sp'hatica (San.), P'hiteari (H.), Shob (Arab.), Paddicarum (Tam.)

This salt is a compound of alumina, potassa, and sulphuric acid, a double sulphate of alumina and potassa. It is found native in some places, either effloresced on bituminous schistus, as at Gottwig in Austria; or united with the soil in volcanic regions, as at the Solfatara near Naples, where the only processes requisite for its extraction are lixiviation and evaporation.1 But the greater quantity of the alum of commerce is prepared by a peculiar management of schistose pyritic clays, usually denominated alum ores. At La Tolfa, near Civita Veechia, where the best Roman alum is made, the ore is alum stone or sulphureted clay, found in large stratified masses among compact iron-shot argillaceous limestone2; but at other places, both on the Continent and in Great Britain, it is manufactured from pyritaceous clay, which is in black, hard, brittle masses; volcanic aluminous ores, a white saline earth; shale alum slate, which occurs amorphous, or in concentric balls. At Hurlett and Nitshill near Paisley, the largest alum mine in this country, the schistus lies ten inches thick above coal.

But the alum is in every instance the result of the decomposition of what is termed its ore, and a subsequent synthetical union of its components.

To prepare the alum, the slate is roasted and properly exposed to the atmosphere, when the sulphur of the sulphuret of iron, present in it, is oxidized by the air and converted into sulphuric acid, which, combining with the alumina and potassa present in the slate, produces an aluminous efflorescence. In general, however, the slate is first calcined with a low heat, so as to destroy the bituminous matter, and partly convert the sulphur into sulphuric acid; the oxidizement is then completed by exposing the roasted slate to air and moisture, by which means a super-sulphate of alumina is formed, which is extracted by lixiviation. To the solution, concentrated until it acquire the spec. grav. 1.35, is added, when the slate contains no potassa, either impure subcarbonate of potassa of commerce, or super-sulphate of potassa, or putrid urine, or muriate of potassa, after which it is run into coolers to be crystallized. At the end of four days the mother waters are drained off, and the crystals, being washed, are re-dissolved in boiling water to saturation, and then roched, that is, run into casks.

On taking asunder the casks at the end of sixteen days, the alum exteriorly is found in a solid cake, but interiorly crystallized in large octahedral pyramids, inserted into one another.

1 These processes are performed in pans sunk in the ground, the heat of which is sufficient to carry on the evaporation.

2 An alum is found near Moscow which contains much sulphate of iron. Mem. de la Soc. Imp. de Moscow t. i. p. 22.

The ancients were probably unacquainted with alum, for the of the Greeks, and the alumen of the Romans, were merely vitriolic earths: and the first regular alum-works appear to have been established by the Asiatics, in the middle ages, particularly at Roccha, in Syria, whence probably was derived the name Roch alum; and from them Europe was supplied till the fifteenth century. After this period, works were begun at Tolfa and Volterra in Italy, at Oberkaufungen and several other places in Germany, and at Almacoran in Spain. In England, in the reign of Elizabeth, Sir Thomas Chaloner established the first alum-work at Gisborough, in Yorkshire. The largest alum-works at present in this country are those on the estate of Lord Glasgow at Hurlett, and those of Lords Dundas and Mulgrave at Whitby, in Yorkshire.1

The best alum is the Roman, which is in irregular octahedral, crystalline masses, powdery on the surface. The English is in large, irregular, semitransparent, colourless masses, having a glassy fracture, not efflorescent, and difficult to pulverize; and that from the Levant, or Roccha alum, is in small morsels, about the size of an almond, rather friable, and of a pale rose colour. The form of the regular crystal of alum is an octahedron.

Qualities.-Alum is inodorous, and has a sweetish, acidulous, astringent taste. Its specific gravity is about 1.71. It reddens litmus and tincture of cabbage1; is in a small degree efflorescent; and soluble in five parts of water at 60°, and in its weight of boiling water. When exposed to a gentle heat, it undergoes the watery fusion, and in a stronger heat swells, loses 50 per cent, of its weight, which is water, and becomes an opaque, white, friable, spongy mass. It is decomposed by the alkalies and alkaline earths, which attract the greater part of its acid, and precipitate the alumina united with a small portion of acid and potassa. According to Vauquelin, its constituents are, - acid 30.52, alumina 10.50, potassa 10.40, and water 48.58, in 100 parts: but by a more recent analysis, Berzelius makes them - sulphate of alumina 36.85, sulphate of potassa 18.15, and water 45. According to the analysis of Mr. R. Phillips, it is composed of sulphuric acid 34.94 parts, alumina 11.18, potassa 10.33, and water 43.55, in 100 parts. In the crystallized state its equivalent is 474.95, or 1 equiv. of alumina = 51.4; 1 of potassa = 47.15; 4 of sulphuric acid = 160.4; and 24 of water=216. It often contains ammonia, and none of the alum of commerce is wholly free from traces of iron.

Gallic acid precipitates the earth of alum: the alkalies and their carbonates, muriate of ammonia, magnesia, lime, salts of baryta, phosphates, carbonate of magnesia, chalk, tartrate of potassa, and infusions of galls and of cinchona, are incompatible in prescriptions with solutions of alum; as are also acetate of lead, and the salts of mercury. The alumina thrown down by ammonia or potassa is re-dissolved by an excess of the alkalies.

1 The alum works at Whitby were established in 1600. The ore is alum slate; the stratum is twenty-eight miles; and the cliffs are from 100 to 750 feet in height. One hundred and thirty parts of the calcined slate yield one part of alum.

Medical properties and uses.-Alum is a powerful astringent. It is used both as an internal and external remedy for restraining violent haemorrhages; and also in cases of obstinate diarrhoea, diabetes, and fluor albus; but we agree with Dr. Cullen, that it is not to be depended upon in the two latter diseases. It has been given as an auxiliary to cinchona in intermittents, and in confluent small-pox, when the pustules are bloody; and Dr. Percival regarded it as a prophylactic in colica pictonum, and a cure for slighter cases.2 It is used locally in gargles, in cases of cynanche, relaxation of the uvula, and aphthae; and as the basis of injections, in cases of gleet and leucorrhoea, and of eye-waters in chronic ophthalmia.

The dose in haemorrhages is from grs. v. to Эj., repeated every hour or two hours, till the bleeding abate: in other cases, smaller doses are more advisable, larger being apt to nauseate

1 Much of the English alum which we have examined strikes a green with syrup of violets, and most other vegetable colours, with the exception of the two above mentioned.

2 Observations on Lead, etc.

the stomach, and occasion violent constipation The addition, however, of an aromatic prevents it to a certain degree from exciting nausea. It is sometimes administered dissolved in the serum of milk, in the form of whey (serum lactis alumi-nosum), which is prepared by boiling 3ij. of powdered alum in a pint of milk, and straining. A small piece of alum, briskly agitated with the white of an egg, forms a coagulum, which, applied between two pieces of gauze or thin rag, proves very serviceable in ecchymosis of the eye, and in some species of ophthalmia. The dose of the whey is f ij. to f iij.

Officinal preparations.-Alumen siccatum, E. D. Liquor Alu-minis comp. L. Pulvis Aluminis comp. E.