The taste of alum is rather sweet, but astringent. It is a super-sulphate, and reddens the vegetable blues. The spec. gray. is about 1.71. It is soluble in 16 parts of water at 600, and in 'the of its weight of boiling water. By long exposure to air, the surface effloresces, but the interior will remain a long time unchanged. A moderate degree of heat expels its water of crystallization, but an intense heat decomposes it, by separating a great part of its acid. The alum of commerce generally contains much impure matter, particularly if kelp or urine has been used in the manufacture. The sulphate of iron is, perhaps, the most injurious foreign ingredient in its composition ; this may be detected by ferro-prussiate of potash. Alum is of great use and importance in many pro cesses of the arts. It is very extensively employed in dyeing, as a mordant. Most of the vegetable colours employed in dyeing have no affinity for the material intended to be dyed. In most of these cases alum is employed as the intermediate medium by which the colouring matter may be fixed, as it has an affinity both for the material and the colouring matter. An acetate of alumina is frequently employed by the dyer, instead of the ordinary sulphate. The acetate does not act so corrosively on delicate articles as the common alum, and its affinity for some colours is greater. It is easily prepared by the double decomposition of a solution of the sulphate of alumina and potash, and a solu tion of the acetate of lead. When these two solutions are mixed together, the sulphuric acid of the alum combines with the lead, forming an insoluble preci pitate, which is sulphate of lead, while the liberated acetic acid unites with the alumina and potash, forming the required acetate, which is separated by filtration. from the solid substance. If paper, linen, or wood, be soaked in a solution of alum, it is rendered nearly incombustible, and less liable to be affected by mois ture. Paper thus prepared is advantageously employed in wrapping gunpowder and is also useful in whitening silver. Alum has the property of hardening
tallow, for which purpose it has been sometimes advantageously employed. It is used with much success in the preparation of skins for tanning, giving them firmness after they have become flaccid by immersion in the lime pits. Turbid water is rendered limpid and transparent by the addition of a small quantity of alum in solution ; its purity, however, is by no means promoted by the process. It is usefully combined with the salt by which cod fish are cured, as it prevents a deliquescence which would otherwise take place. It is used in the manu facture of London bread rather extensively, as it enables the baker to employ flour of inferior quality ; the alum promotes a white colour and closeness of texture in the bread,—qualities much admired, although they generally indicate impurity. It improves the colour of the beautiful pigment, Prussian blue, and is extensively employed in the preparation of that article. In medicine it is administered as an astringent and tonic, and also as a collyrium. Alum, deprived by heat of its water of crystallization, which, in medicine, is called alumen astern (burnt alum), has been greatly extolled as a remedy in cases of colic, and some other disorders. A remarkable substance, called Homberg's pyrophorus, is prepared from alum. Equal parts of alum and brown sugar are melted over a clear fire, and stirred constantly until they become dry. When cold the mixture is finely powdered, and placed in a phial coated with clay, having an open glass tube lured into its neck. This is to be intensely heated in a crucible con taining sand, when gas will issue from the tube, which may be inflamed. When this ceases to come over, the crucible is removed, and the tube stopped by moistened clay until the bottle is sufficiently cold to be corked. The substance thus formed is a black and light powder, which takes fire when poured from the bottle into the air. If poured into oxygen gas a more vivid combustion takes place.