Alumina, and Alumine

[Fr. alumine.] (Chem.) One of the earths, the only known oxide of aluminium. It forms one of the materials that enter most largely into the composition of the superficial strata of the earth. It is the basis of all the varieties of clay, and is present in greater or le quantity in almost every soil. It occurs nearly pure, and crystallized in six-sided prisms, in Corundum, hard enough to cut gla . The sapphire and ruby are also composed of this earth, tinged with a small quantity of oxide of chromium. They are only inferior to the diamond in hardne . Emery is another form of alumina, colored with oxides of iron and manganese. It is procured from alum, or from aluminate of sodium. It po e es properties which approach somewhat to those of an acid, for it has a strong tendency to unite with basic oxides. When combined with silica, it forms clay, which is the basis of porcelain and of earthenware. To the dyer and the calico-printer, the compounds of A. are of high value; the hydrate of A has the property of combining intimately with certain kinds of organic matter, and when salts of aluminium are mingled with colored vegetable or animal solutions, and precipitated by the addition of an alkali, the alumina carries down the greater portion of the cole ring matter, forming a species of pigments termed lakes. By soaking the cloth with a preparation of aluminium, the earth attaches itself to the fibre; and if cloth thus prepared be plunged into a bath of the coloring matter, it becomes permanently dyed. Sp. gr. 3-95; form. (A1203).