ALUMINA

Aluminous earth; earth of alum; argil. When a solution of ammonia is dropped into a solution of Blum, a white precipitate falls, which, thoroughly washed, dried, and heated, is pure aluminous earth. There are lwo properties of tins 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 the 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 52-91 aluminum, and 47-0(3 oxygen; like the other earths, as they are usually called, alumina, therefore,isa 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 nil ofa sweetish astringent taste. Exclusive of alum, the acetate of alumina is the most important of these ling 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. 52]