ALUMINA, in chemistry, one of the five proper earths. It W3.9 discovered by the alchemists that alum WSIS composed of sulphuric acid and an earth, the nature of which was long unknown; but Geof froy, and afterwards Margrafl; found that the earth of alum is an essential ingredi ent in clays, and gives them their proper ties, hence it was called argil ; but Mor veau gave it the name of ahttnino, because it it obtained in a state ofthe greatest pu rity from alum by- the following process. Dissolve alum in water, and add to the solution ammonia as long as any precipi tate is formecl. Decant offthe fluid part, and wash the precipitate in a large quan tity of water, and then allow it to dry. The substance thus obtained is alumina not however in a state of absolute purity, for it still retains a portion °fele sulphu ric acid with which it was combined irt the alum. But it may be rendered tole rably pure, hy dissolving the newly preci pitated earth in moriatic acid, evaporating the solution till a drop of it in cooling de posit,: small crystals, setting it by to crys tallize, separating the crystals, concen trating the liquid a second time, and se parating the crystals ultich are again deposited. By this process, most of the alum which the earth retained will be se. parated in crystals. If the liquid be now mixed with ammonia as long as any pre. cipitate appears, this precipitate, washed and dried, will be altimina nearly- pure.
Alumina has little taste: when pure, it has no smell ; hut if it contains oxyde of iron, which it often does, it emits a peculiar smell when breathed upon, known by the name of earthy smell. This smell is very perceptible in common clays. The speci fic gravity of alumina is 2.00. When heat is applied to alumina, it gradually loses weight, in consequence of the evapora tion of a quantity of water, with which, in its usual state, it is combined ;' at the same time its bulk is considerably dimi nished. The spongy alumina parts with its moisture very readily ; but the gelati nous retains it very strongly. Spongy alu mina,when exposed to aredheat,loses 0.58 parts of its weight ; gelatinous, only 0,43: spongy alumina loses no more than 0.58 when exposed to a heat of 130° Wedge wood ; gelatinous in the same temperature loses but 0.4825. Yet Saussure has shown that both species, after beingdried in the temperature of 60°, contain equal propor tions of water. Alumina undergoes a di minution of hulk proportional to the heat to which it is exposed. This contraction seems owing, in low temperatures, to the /oss of moisture ; but i n high temperatures it must be owing to a more intimate com bination of the earthy particles with each other ; for it loses on perceptible weight in any temperature, however high, after being exposed to a beat of 130° Wedge. wood..
Mr. Wedgewood took advantage of this property of alumina, and by means of it constructed an instrument for measuring high degrees oflieat. It consists ofpieces of clay of a determinate size, and an ap paratus for measuring their bulk with ac curacy ; one of these pieces is put into the fire, and the temperature is estimated by the contraction ofthe piece. The con traction of the clay pieces is measured by means of two brass rules, fixed upon a plate, the distance between which at one extremity is 0.5 inch, and the other ex
tremity 0.3 inch ; and the rules arc exact ly 24.0 inches in length, and divided into 240 equal parts, called degrees. These degrees commence at the widest end of the scale. The first of them indicates a red heat, or 947° Fahrenheit. The clay pieces arc small cylinders, baked in a red heat, and made so as to fit 1° of the scale. They are not composed of pure alumina, but of a fine white clay. Alumina is scarcely soluble in water ; but may be dif fused through that liquid with great fa cility. Its affinity for water, however, is very considerable. In its usual state it is combined with more than its own weight of water, and we have seen with what ob. stinacy it retains it. EVen this combina tion of alumina and water is capable, in its usual state of dryness, of absorbing 23 times its weight of water, without suffer ing any to drop out. It retains this water more obstinately than any of the earths hitherto described. In a freezing cold it contracts more, and parts with more of its water, than any other earth; a cii-cum stance which is of some importance in agriculture. Alumina has no effect upon vegetable blues. It cannot be christal lized artificially ; but it is found native in beautiful transparent crystals, exceeding% ly hard, and having a specific gravity of 4 . It is distinguised in this state by the name of sapp/iyr. It does not combine with me tals; but it has a strong affinity for me tallic oxydes, especially for those oxydes which contain a maximum of oxygen. Some of these compounds are found na tive. Thus, the combination of alumina and red oxyde of iron often occurs in the form of a yellow powder, which is em ployed as a paint, and distinguished by the name of ochre. There is a strong af finity between the fixed alkalies. and alu mina. When heated together, they com bine, and form a loose mass, without any transparency. Liquid fixed alkali dis solves alumina by the assistance of heat, and retains it in solution. The alumina is precipitated again, unaltered, by drop ping an acid into the solution. This is a method employed by chemists to procure alumina in a state of complete purity ; for alumina, unless it be dissolved in alkali, almost always retains a little oxyde of iron and some acid, which disguise its properties. Liquid ammonia is also ca pable of dissolving a very minute propor tion of newly precipitated alumina. Ba, rytes and strontian also combine with alu mina, both when heated with it in a cru cible, and when boiled with it in water. The result, in the first case, is a greenish or bluish-coloured mass, cohering but im perfectly: in the second, two compounds are formed ; the first, containing an ex cess of alumina, remains in the state of an insoluble powder ; the other, containing an excess of barytes or strontian, is held in solution by the water. Alumina has a strong affinity for lime, and readily en ters with it into fusion. None of the earths is of more importance to mankind than alumina ; it forms the basis of china and stone-ware of all kinds, and of the cruci bles and pots employed in all those manu factures which require a strong heat. It is absolutely necessary to the dyer sod calico printer, and is employed too, with the greatest advantage, by the fuller and cleaner of cloth.