ALUMINA. 



Alumina has little taste : when pure, it 

 has no smell ; but if it contains oxyde of 

 iron, which it often does, it emits a pecu- 

 liar smell when breathed upon, known by 

 the name of earthy smell. This smell is 

 very pi'eceptible in common clays. The 

 specific gravity of alumina is 2.00. When 

 heat is applied to alumina, it gradually 

 loses weight, in consequence of the eva- 

 poration of a quantity of water, with 

 which, in its usual state, it is combined ; 

 at the same time its bulk is considerably 

 diminished. The spongy alumina parts 

 with its moisture very readily ; but the 

 gelatinous retains it very strongly. Spon- 

 gy alumina, when exposed to a red heat, 

 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 

 Wedgewood ; gelatinous in the same tem- 

 perature loses but 0.4825. Yet Saussure 

 has shown that both species, after being 

 dried in the temperature of 60, contain 

 equal proportions of water. Alumina un- 

 dergoes a diminution of bulk proportional 

 to the heat to which it is exposed. This 

 contraction seems owing, in low tempera- 

 tures, to the loss of moisture ; but in high 

 temperatures it must be owing to a more 

 intimate combination of the earthy parti- 

 cles with each other ; for it loses no per- 

 ceptible weight in any temperature, how- 

 ever high, after being exposed to a heat 

 of 130 Wedgewood. 



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 

 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 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 

 scarcelv soluble in water ; but maybe 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 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 crystallized 

 artificially ; but it is found native in beauti- 

 ful transparent crystals, exceedingly 

 hard, and having a specific gravity of 4. 

 It is distinguished in this state by the name 

 of sapphyr. 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. Tliis is a 

 method employed by chemists to procure 

 alumina in n 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 



