ALUM

A salt composed of alumina, potash, and sulphuric acid, and in its usual form containing a large quantity of water of crystallisation. Its octohedral crystals consist of Alumina Pota a Sulphuric acid Water - 3 atoms = 54 - 1 =48 - 4 =160 - 24 = 21G 10-76 9-95 33-74 45-55 Crystallised alum - I 478 100-00 Alum di olves in about five parts of water at 60*^. The solution has a sweet and astringent taste, and is a powerful styptic. When crystallised alum is heated, it melts, aad, gradually losing water of crystallisation, becomes a white spongy ma , called burned alum. Alum is largely manufactured for the uses of the arts, especially dying and calico printing. What is termed alum ore, is an aluminous slate containing sulphuret of iron; it is calcined, exposed to air, lixiviated, and the solution so obtained mixed with sulphate of potash, and crystallised. The alum-works near Paisley, and at Wiiitby, in Yorkshire, are the largest in this country. Milk, curdled by stirring it with a lump of alum, furnishes alum whey, which is sometimes taken as a remedy for relaxed bowels. Alum curd is made by beating the white of egg with a piece of alum till it coagulates. ALUMIN A. Aluminous earth; earth of alum; argil When a solution of ammonia is dropped into a solution of alum, a white precipitate falls, which, thoroughly washed, dried, and heated, is pure aluminous earth. There are two properties of this earth which render it of great importance in the arts: one is, that it forms a plastic mixture with water, and, though it is not the predominant ingredient in, yet it confers the valuable property of plasticity upon, all natural clays, which enables them to be moulded into the various forms of pottery and earthenware; the other is the remarkable affinity of alumina for colouring and extractive matter, whence its use in the arts of dyeing and calico printing. In a pure and crystalline form, alumina constitutes the sapphire, one of the hardest and most valuable of the gems. In its common state, aluminous earth is a soft white powder, strongly attractive of moisture; hence, aluminous fo ils are often recognised by adhering to the tongue, and many of them exhale an earthy smell when breathed upon, as we observe in common slate. Alumina consists of 52*94 aluminum, and 4706 oxygen; like the other earths, as they are usually called, alumina, therefore, is a metallic oxide. Aluminum is with difficulty obtained, and in small quantities, by heating chloride of aluminum with pota ium; it is a grey, difficultly fusible metal, not easily acted on by water, and which, when heated in the air, burns with great brilliancy, and forms alumina by the absorption of oxygen. Alumina has but a feeble attraction for acids, and does not fully neutralise them; and when it has been heated red hot, or is in an indurated state, as it exists in the sapphire, in corundum, and some other minerals, it is absolutely insoluble. The aluminous salts are mostly colourle , soluble in water, and of a sweetish astringent taste. Exclusive of alum, the acetate of alumina is the most important of these salts, being used as a base or mordant by the dyers. (See Dyeing.) It is usually prepared by mixing a solution of 190 parts of acetate of lead with one of 487 parts of alum: a white precipitate of sulphate of lead falls, and acetate of alumina remains in solution. [s. 51]