A primitive earth existing in great abundance in clays, earths, ochres, rocks, etc. Soils containing much of this substance are called argillaceous. It is found in a state of great purity in many precious gems, as the sapphire, topaz, emerald, garnet, beryl, etc. When pure, it is of a white colour, pulverulent, and soft to the touch. It adheres to the tongue, but is tasteless. It is insoluble in water, but is readily dissolved by acid3, and also by caustic, potash, or soda. If moistened with water, a very ductile and tenacious paste is formed, which, by heating, becomes exceedingly hard, and is, therefore, extensively employed in the manufacture of porcelain, earthenware, and all kinds of pottery. Bricks, tiles, crucibles, and stone ware, contain large quantities of alumina. It is infusible, per se, in the strongest heat of a furnace, but small quantities may be fused by the oxy-hydrogen blow-pipe. If mixed, however, with certain proportions of lime and silica, it fuses readily. Pure alumina may be obtained easily from the triple salt, containing ammonia instead of potash, by heating it intensely.

The acid and the alkali are dissipated by the heat, and the pure earth remains.

It is usually procured by adding a small quantity of solution of bicarbonate of potash to a solution of common alum, which precipitates the iron contained generally in that salt. The filtered liquid is then to be added to the liquid ammonia, which combines with part of the sulphuric acid of the alum, and precipitates the earth in a spongy mass. This must be washed frequently in distilled water, and then heated to dryness. According to the experiments of Sir H. Davy, it appears, that, like the other earths, alumina is a metallic oxide. By passing potassium in vapour over alumina heated to whiteness, a great part of the vapourized metal was converted into the alkali potash: - a decisive proof that alumina contains oxygen. By treating the chloride of aluminum with potassium, a grey powder is obtained, which, by burnishing, acquires metallic lustre. This powder burns with much splendour if heated to redness, and is converted to alumina by absorption of oxygen. The metallic base is called aluminum; and it is found that 100 parts combine with 8 of oxygen. Alumina has the unusual property of contracting by heat, and in pretty exact proportion with the intensity applied. On this property the ingenious Mr. Wedgewood constructed his pyrometer for estimating very high temperatures.

See Pyrometer. This useful earth has a powerful affinity for colouring matter, and also for greasy substances. • Fuller's earth and pipe-clay owe their useful properties to the large quantities of alumina they contain.