ALUMINA. 



Alumina has little taslc : when pure, it 

 hus no smell ; but if it contains oxyde of 

 i run, which it often does, it emits a peculiar 



snu-ll when breathed upon, known by the 

 name of earthy smell. This smell is very 

 preceptible 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 

 ; ls 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- 

 mia,when exposed to a red heat,loses0.58 

 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 being dried in the 

 temperature of 60, contain equal propor- 

 tions of water. Alumina undergoes a di- 

 minution of bulk proportional to the heat 

 to which it is exposed. This contraction 

 seems owing, in low temperatures, to the 

 loss of moisture ; but in 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 heat 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 of heat. It consists of pieces 

 ef 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 of the 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 are 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 are 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 whatob- 

 stinacy it retains it. Even this combina- 

 tion of alumina and water is capable, in 

 its usual state ofdryness, of absorbing 2$ 

 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 circum- 

 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 disting-uised in this state by the name 

 of sapph yr. 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 and 



