ALUMINA (Al200, the only oxide of the metal aluminum. As found native, crystallized, it is only second to the diamond in hardness. The transparent varieties arc the sapphire and ruby, the opaque are corundum and emery, only the corundum being pure. In combination with silica it is one of the most widely distributed of substances, ranking in this respect next to oxygen and silicon. It enters in large quantity into the composition of granites, traps,' slates, schists, clays, loams and other rocks. The hy drated oxide, A1,03.H.0, occurs as disapore, and with ferric oxide as bauxite. Various aluminates occur in certain gems, as in spinet and chrysoberyl. Alumina may be obtained by adding a solution of ammonium hydrate to purify alum dissolved in 20 parts of water, thoroughly washing the very gelatinous precip itate formed, and then drying it carefully. It may also be prepared by igniting powdered al uminum in air or oxygen. Alumina is a white powder, without taste or smell, and infusible except in the oxyhydrogen flame. It is the basis of porcelain, pottery, bricks and crucibles; and it has a strong affinity for oil and coloring matter, which causes it to be employed in the state of clays as a cleansing powder, and in a state of purity in the preparation of the colors called lakes, in dyeing and calico-printing. It
combines with the acids, and forms numerous salts, the most important of which are the sul phate (see ALUM) and the acetate. The latter salt is formed by digesting strong acetic acid (vinegar) with the newly precipitated earth; but for the use of the manufacturer, by decompos ing alum with acetate of lead (sugar of lead), or more economically with acetate of lime, a gallon of which, of the specific gravity, 1.050, is employed for every 234 pounds of alum. The sulphate of calcium formed falls to the bottom, and the acetate of aluminum remains in solu tion with an excess of alum, which is neces sary to prevent its decomposition. It is of ex tensive use in calico-printing and dyeing, as a mordant, and is employed in the place of alum, to which it is generally preferred.