ALUMINA. 511 



lowish colour, arid is equal in hardness to the native crystallized 

 aluminous earth, corundum. 



Alumina; A1 2 O 3 ; 642.4, or 51.44. This earth is the only 

 degree of oxidation of which aluminum is susceptible, so far as 

 is known at present. In its constitution, alumina is presumed 

 to resemble peroxide of iron, because it occurs crystallized in 

 the same form as the native peroxide of iron, and the salts, into 

 which it enters, are strictly isomorphous with the corresponding 

 salts of that oxide. To three atoms of oxygen it must, there- 

 fore, contain two atoms of metal, such being the composition 

 of peroxide of iron. Aluminum is not known to enter into 

 any other combination in a less proportion than two equiva- 

 lents. 



Alumina occurs in a state of purity, with the exception of 

 a trace of colouring matter, in two precious stones, the sapphire 

 and ruby, the first of which is blue, and the other red. They 

 are not inferior in hardness to the diamond. Their density is 

 from 3.9 to 3.97- It may be obtained by calcining the sulphate 

 of alumina and ammonia, or ammoniacal alum, very strongly. 

 But alumina so prepared is insoluble in acids. It is obtained 

 in the state of a hydrate from common alum, by dissolving the 

 latter in boiling water, and adding a solution of carbonate of 

 potash till it no longer causes a precipitate ; a slight excess of 

 the carbonate may then be added, and the whole allowed to 

 digest at a gentle heat for some time, in order to decompose 

 a subsulphate of alumina which the alkali first throws dou n. 

 The precipitate, which is white, gelatinous, and very bulky, 

 must be carefully washed, by mixing it several times with a 

 large quantity of distilled water, allowing it to settle and 

 pouring off the clear liquid, or by affusion and decantation, 

 as it is said. The precipitate is then dissolved in hydrochloric 

 acid, the solution filtered, if not clear, and precipitated again 

 by ammonia or its carbonate. This last operation is necessary, 

 in order to get rid of a portion of carbonate of potash which 

 remains attached to alumina, when precipitated by that salt. 

 Ammonia cannot be substituted for potash in the first preci- 

 pitation, as it throws down a subsulphate of alumina, and does 

 not deprive the earth entirely of its acid The alumina of the 

 second precipitation is also in the state of a subsalt, unless 

 ammonia be added in excess, which is capable of decomposing 

 completely the subsalt from hydrochloric acid. The alumina 



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