Alum is a concrete salt, transparent, and of a very austere and astringent taste. It is in general a chemical preparation, being rarely found in a natural state, or freed from other ingredients. In Egypt, Sardinia, Spain, Bohemia, etc. it is said to be sometimes discovered in crystals.

There are various kinds, but that which is called the Roman alum, is preferable to any other. This is usually to be found in small crystals, and of a reddish colour, probably owing to a small quantity of calx of iron, which, however, does not in the least impair its qualities. The other kinds contain a proportion either of vitriolated tartar, or sal ammoniac.

In medicine, it has been considered as an astringent, i is of great service in restraining hemorrhages, and other immoderate secretions. It is likewise externally used in lotions and eye-waters : and one scruple of burnt alum has been found beneficial in removing violent colic-pains arising from flatulency, bile, or great relaxation of the bowels; but in other cases it may prove hurtful.

It is used for various purposes by dyers, to fix different colours upon cloth; in the making of candles, to give them a gloss and firm consistence ; wood soaked in a solution of alum, does not readily take fire ; and paper impregnated with it, is the most proper for the preservation of gunpowder, as it also excludes the moisture of the air. Tanners employ it to restore the cohesion of those skins which have been almost entirely destroyed by lime; and vintners in fining their wines, etc. Fishermen dry their cod-fish by means of it; and it is asserted, that bakers generally use it as an ingredient in bread: the truth of this assertion, however, has been much questioned, and the sole reason ascribed for its use, is, that corrupt flour, being mixed with good, thus acquires a proper degree of cohesion, as the aluminous particles equally pervade the whole mass, and render it of a due consistence. Although some writers have maintained, that this styptic salt " is entirely innocent, and now seldom used" in the process of making bread, yet we have but too much reason to believe the contrary. The English translator of Tissot's excellent "Advice to the People in general, " etc. very pertinently remarks, that the abuse of alum, and other pernicious materials, introduced by our bakers, may too justly be considered as one lamentable source of the numerous diseases of children. The Monthly Reviewer of that book, for July 1765, adds, with equal justice, the following commentary: ' Hence obstructions in the bowel and viscera, feebleness, slow-fevers, hectics, rickets, and other linger-in!;" and fatal diseases.'

To discover such unlawful practices, requires no chemical skill : on macerating a small piece of the Crumb of new-baked bread in cold water, sufficient to dissolve it, the taste of the latter, if alum has been used by the baker, will acquire a sweetish astringency. Another method of detecting this adulteration, consists in thrusting a heated knife into a loaf, before it has grown cold; and if it be free from that ingredient, scarce any alteration will be visible on the blade; but, in the contrary case, its surface, after being allowed to cool, will appear slightly covered with an aluminous incrustation. This method, we understand, is generally preferred in the experiments made by country-justices. It deserves, however, to be remarked, that a very small proportion of alum, such as a few grains to a quartern-loaf, cannot be productive of any serious effects. In relaxed and scorbutic-habits, or to those persons who are troubled' with flatulency, bilious colic, and jaundice, such medicated bread may be conducive to the recovery of health; while in others, of a plethoric constitution, and a rigid fibre, it cannot fail to aggravate their complaints. In short, such addition to a common article of subsistence is, to say the least of it, highly improper, and ought not to be intrusted to the hands of a mechanic.

One of the most important purposes, to which this concrete salt may be readily applied, is that of purifying and sweetening water that has become fetid and unfit for use. On long voyages, or at a distance from clear rivers and wells, each gallon requires, according to its impurity, only from live to ten grains of calcined alum, and double or triple that proportion of powdered charcoal, in order to render the most offensive water perfectly sweet and pellucid: both ingredients, however, ought to be preserved in close vessels, or otherwise their efficacy will be considerably diminished.

Alum has also been tried in the boiling of salt, to render it of a firm consistence, but the good which was supposed to be derived from it, is now solely attributed to the effects of the slow and gentle heat, so that in this process it has of late been discontinued.

The manufacture of alum was first invented in the year 1008, and greatly encouraged in England, by Lord Sheffield and other Gentlemen of the county of York. King James the 1st assumed a monopoly of that article, and prohibited its importation.

Alum. - Beside the methods of detecting alum in bread, already stated, there is a chemical process, that consists in combining a little chalk with a small portion of aquafortis, and pouring the mixture on water, in which the suspected bread has been immersed for some time.

If there be any aluminous acid, its presence will become evident, by a gypseous or chalky mass deposited at the bottom of the vessel: in the contrary case, no sediment will be formed.

In October, 1794, a patent was granted to the Earl of DUndonald, for his method of preparing alum, vitriol of argil, and other saline substances. He directs aluminous, vitriolic, or pyrituous schist, to be mixed with sea-water, or with solutions of sea-salt, kelp, sandiver, soap-boilers ashes, or any saline matter, containing muriat of soda. The liquor, resulting from such mixture, is then boiled till it be sufficiently concentrated for crystallization ; after which it is mixed with a due proportion of alum-schist, clay, or other argillaceous ingredient. The materials are next dried, pulverized, and submitted to the action of heat, till the muriatic acid be expelled : the result of these various processes, is alum. The substance remaining may, by repeated washing and drying, be used as a pigment; and, by collecting the muriatic acid in proper vessels, and combining it with volatile alkali, Lord D. procures sal ammoniac. - A more diffuse account of his inventions, is inserted in the 4th volume of the " Reper-tory of Arts, " etc.