Alumina, the only known oxide of aluminum. It occurs colorless as corundum, and colored by traces of oxide of chromium and cobalt in the ruby and sapphire. It is found in a few places in larger quantities in the form of emery, which is nearly pure alumina. It is very widely disseminated in nature in combination with other bases in the form of double silicates, constituting feldspars, micas, and a large series of important minerals, from the decomposition and disintegration of which clays are composed. It forms the greater portion of the crust of the earth, and, in the form of clay, affects the fertility of every soil. It is not taken up by plants, except in rare cases, nor is it found in the animal kingdom. There has recently been discovered a mineral in the vicinity of Baux, France, to which is applied the name bauxite. It differs materially from clay in being simply a hydrated oxide of alumina and iron without any silica. It is entirely infusible, and crucibles and fire brick made of it remain unchanged when ordinary fire-clay material loses shape and partially fuses. It is extensively employed in the manufacture of sulphate and other salts of alumina, and of the metal aluminum.
When perfectly pure, bauxite is composed of sesqui-oxide of alumina 52.00, sesquioxide of iron 27.00, and water 20.40; but its composition varies considerably, and some varieties contain small quantities of silica and lime. It differs especially from kaolin in not being a silicate but an oxide of alumina. Well known minerals analogous to it are gibbsite and diaspore. - Alumina may be prepared by adding ammonia to any of its soluble salts (alum for instance), when a gelatinous precipitate of the hydrate of alumina is thrown down, having the formula, according to the present chemical theories, of Al2H6O6=Al2O3, 3H2O, after being dried in the air. To obtain it more dense and free of iron, it is now customary to pass carbonic acid gas through a dilute and cold solution of alumi-nate of soda, prepared in the United States from the mineral cryolite, which is brought from Greenland to be used in the manufacture of glass and soap. The pure anhydrous alumina is prepared by the calcination of the hydrated oxide obtained as above described, or by expos-inn; ammonia alum to a lively red heat. A peculiar modification of alumina is obtained, according to Walter Crum. by long-continued boiling of acetate of alumina; the acetic acid is liberated, and there remains a hydrated oxide soluble in water.
A second modification of soluble alumina was discovered by Professor Graham, which can be obtained by the dialytic decomposition of a solution of hydrate of alumina in chloride of aluminum; it has properties differing from either of the other forms. - Properties of alumina. When pure it is a white, light powder, devoid of taste and odor, infusible excepting before the oxyhydrogen blowpipe, when it constitutes a viscous fluid that can be drawn in strings like melted quartz, and on cooling yields a crystalline mass sufficiently hard to scratch and cut glass. Calcined alumina is absolutely insoluble in water, but if it has not been heated to redness it combines with a certain portion of water with disengagement of heat. Hydrated alumina is white when moist, but becomes translucent by desiccation, and sometimes yellow if it has been precipitated in the presence of organic matter. Its affinity for coloring matter is so great that it readily absorbs the organic dyes from solutions, and has extensive application as a mordant, as a clarifying agent, and in the manufacture of lakes. The hydrate of alumina after calcination is soluble only with difficulty in acids, but readily soluble when freshly precipitated.
The hydrates, prepared according to the methods of Crum and Graham, are soluble in water and possess characteristic properties. Several of the metallic oxides, as soda and potash, if fused in a silver crucible with alumina, combine with it and form alurninates. The minerals corundum, sapphire, and ruby have been made artificially by Deville and Caron, by heating the fluoride of aluminum in a carbon crucible, underneath which is suspended a platinum capsule containing boracic acid; at an elevated temperature the fluorine reacts on the boracic acid and yields a fluoride of boron and a crystalline metallic oxide. By adding variable quantities of sesquioxide of chromium, good imitations of the ruby, sapphire, and corundum can be obtained; and it is said that Bonsdorf has made the mineral gibbsite by exposing a solution of aluminate of potash to the action of an atmosphere of carbonic acid. When metallic aluminum is heated to redness in the air or in oxygen gas, it burns brightly and is converted into alumina, 53.3 parts of the metal taking up 46.69 parts of oxygen to form the pure earth.
The compound thus produced is inferred to be sesquioxide because it is isomor-phous with the sesquioxides of iron and chromium, and is capable of replacing these oxides in combination in any proportion.