ALUMINA, the most abundant of the earths (q.v.), is the oxide of the metal alumin ium (q.v.), the formula being It occurs in nature abundantly in combination with silica, associated with other bases. The most familiar of its native compounds is felspar, a silicate of A., and potass This is one of the constit uents of granite, and of several other igneous rocks. Certain varieties of these, by ex posure to the atmosphere, become completely disintegrated, passing from the state of hard, solid rock, such as we are accustomed to see in building-granite, into soft, crum bling, earthy masses. It is the felspar which undergoes the change, and it appears to be owing to the action of rain-water charged with carbonic acid, which dissolves the potass and some of the silica of the felspar, leaving the excess of silica and the A. still united. It is not known, however, why certain specimens of granite are rapidly corroded and crumbled down, whilst others have resisted for ages the same causes of decay. By such a process of disintegration as we have described, the clays of our arable soils are pro duced. Clay consists of silica and A. in a state of chemical combination. It never is pure A., but the quantity of silica united to the latter is variable. When it is pure, clay is quite white, as we see in the porcelain clay of Devonshire and Cornwall, which is derived from colorless felspar. More frequently, clay is red, owing to the presence of oxide of iron; or black, from the diffusion through it of vegetable matter.
From alum, A. is prepared by adding to a solution of the former, water of ammonia, as long as it occasions a precipitate. The A. appears as a voluminous, white, gelatinous substance, consisting of the oxide of the metal combined with water. When A. is pre cipitated from a solution containing coloring matter, such as logwood, etc., it carries down the color chemically united to the flocculent precipitate; in this way are formed the colored earths, called lakes (q.v.). A. in the state of precipitate, after being gently dried, is readily soluble in acids and in alkalies; but if heated to whiteness, it loses the associated water, contracts greatly in bulk, and forms a white, soft powder, not at all gritty, and. with difficulty soluble in alkalies and acids. A., as generally prepared, whether hydrated or anhydrous, is insoluble in water, possesses no taste, and does not alter coloring matters; but some time Mr. Walter Crum obtained A. in an allotropic form, in which it is soluble in water. It is quite different, therefore, in properties from the alkaline earths, and is a much weaker base. In the anhydrous state it absorbs water with great readiness without combining with it, so that it adheres to the tongue, and is felt to parch it. Clay retains this property; and the ends of tobacco pipes are glazed to
prevent adhesion to the lips or tongue. A. is not fusible by a forge or furnace heat, but it melts before the oxyhydrogen blow-pipe into a clear globule, possessing great hard ness. It occurs in nature in a similar state. The more coarsely crystallized specimens form the emery which is used for polishing; the transparent crystals, when of a blue color, owing to a trace of metallic oxide, constitute the precious gem the sapphire, and, when red, the ruby. A., in common with other sesquioxides, is a feeble base. The salts it forms with the acids have almost all a sour taste, and an acid. action on coloring matter.
Al, eq. (old) 13.7, (new) 27.4—is one of the metals present in clay, granite, and other rocky and earthy substances. It was discovered by Withler in 1828, and was re-examined by him in 1846, when he obtained the metal in minute glob ules or beads, by heating a mixture of chloride of A. and sodium. In 1855, the French chemist Deville showed, as the result of a series of experiments, that A. could lie pre pared on a large scale and in a compact form without much difficulty. The mineral ery olite found in Greenland, which is a double fluoride of A. and sodium, was the ore first used for its manufacture; but bauxite, a mineral found in France, and consisting chiefly of alumina, or oxide of A. and oxide of iron, has more recently.been employed as a con venient source of the metal. An aluminate of soda is first obtained by heating the bauxite with soda ash in a furnace, and separating it (the aluminate) from the insoluble portions by lixiviation. When carbonic acid is added to the solution, pure alumina is thrown down. The alumina is then formed into balls with common salt and charcoal, which are heated in an earthenware retort through which chlorine gas is passed. In this part of the process, the charcoal combines with the oxygen, and the chlorine with the A,; the latter sublimes over with the common salt (chloride of sodium), and is collected as a double chloride of A. and sodium. When this double chloride is heated in a rever beratory furnace with fluxes and metallic sodium, the latter seizes the chlorine combined with the A., which is then set free, and falls to the bottom ready to he cast into ingots for use. A. was made for sonic years near Newcastle-on-Tyne, by Mr. J. Lothian Bell, but the demand for it not being sufficient to keep the work constantly in operation, it was stopped, and the metal is now chiefly manufactured in France.