SALTS OF ALUMINA. 515 



of its transparency, hardness and brilliancy, is reckoned among 



the precious stones. They are, with their formulee after Ber- 



zelius : 



Topaz. . . 3(A1 2 3 , Si O 3 ) + (Al a O 3 + A1 2 F 3 ) 

 Pyknite . . 3(A1 2 Og, Si O 3 )+A1 2 F 3 . 

 The sulphocyanide of aluminum crystallizes in octahedrons, 



which are persistent in air. 



SALTS OF ALUMINA. 



Sulphate of alumina; A1 2 O 3 , 3SO 3 + 18HO ; 2145.8 + J024.6, 

 or 171.95 -f 162.23. Obtained by dissolving alumina in sulphu- 

 ric acid. I c crystallizes with difficulty in thin flexible plates of a 

 pearly lustre, has a sweet and astringent taste, and is soluble in 

 twice its weight of cold water, but does not dissolve in alcohol. 

 When heated it fuses in its water of crystallization, swells up, 

 and forms a light porous mass, which appears at first to be inso- 

 luble in water, but dissolves completely after a time. Heated to 

 redness it is entirely decomposed ; the residue is pure alumina. 

 This salt has been found in the crystalline form, in the volcanic 

 island of Milo in the Archipelago. Sulphuric acid and alumina 

 combine in several proportions, but this is considered the 

 neutral sulphate, as it possesses the same number of atoms of 

 acid, as it contains of atoms of oxygen in the base. 



Another sulphate of alumina (A1 2 O 3 , 3SO 3 + A1 2 O 3 ) is ob- 

 tained, according to Maus, by saturating sulphuric acid with 

 alumina, which contains twice as much alumina as the neutral 

 sulphate. After evaporation this subsalt presents itself in a 

 gummy mass, which dissolves in a small quantity of water, but 

 is decomposed when the solution is diluted with a large quan- 

 tity of water, or boiled ; in that case the neutral salt remains 

 in solution, and the following salt precipitates. Subtrisulphate 

 of alumina (A1 2 O 3 , 3SO 3 -f 2A1 2 O 3 ) precipitates on adding am- 

 monia to the sulphate of alumina, as a white insoluble powder, 

 which is not decomposed by an excess of ammonia. It contains 

 besides 46.9 per cent of water, or 9 atoms. This subsalt forms 

 the mineral aluminite, which is found near Newhaven in Eng- 

 land, and at Halle in Germany. 



Alum, sulphate of alumina and potash ; KO, SO 3 + A1 2 O 3 , 

 3SO 3 + 24HO; 3236.9 + 2700, or 259.38 + 216. Sulphate of 

 alumina has a strong affinity for sulphate of potash, in conse- 



