ALUMINIUM. 139 



as, for instance, aluminium-bronze, an alloy resembling gold, 

 and composed of 10 parts of aluminium with 90 of copper. 



Aluminium is trivalent, and shows, like a number of other 

 elements (iron, chromium, etc.), the peculiarity that the double 

 atom Al 2 vi acts as a single hexavalent atom. 



Alum is the general name for a group of isomorphous salts, 

 composed of one molecule of the sulphate of an univalent metal 

 in combination with one molecule of the sulphate of a trivalent 

 metal, combining in crystallizing with 24 molecules of water. 

 The general formula of an alum is consequently M' 2 S0 4 , 

 M 2 ui 3S0 4 ,24H 2 O. M' represents in this case a univalent, M m a 

 trivalent metal. 



Alums known are, for instance : 



Potassium-aluminium sulphate, K 2 SO 4 , A1 2 3SO 4 , 24H 2 O. 

 Sodium-aluminium sulphate, Na 2 SO 4 , A1 2 3SO 4 , 24H 2 O. 

 Potassium-chromium sulphate, K 2 SO 4 , Cr 2 3SO 4 , 24H 2 O. 

 Ammonium-ferric sulphate, NH 4 S0 4 , Fe 2 3SO 4 , 24 HO. 



The officinal alum is the potassium-aluminium alum, a white 

 substance crystallizing in large octahedrons, soluble in 10 parts 

 of cold and 0.3 part of boiling water; this solution has an acid 

 reaction and a sweetish, astringent taste. 



Alum is manufactured on a large scale by decomposing cer- 

 tain kinds of aluminium silicates by sulphuric acid, when alu- 

 minium sulphate is formed, to the solution of which potassium 

 or ammonium sulphate is added, when, on evaporation, potas- 

 sium or ammonium alum crystallizes. 



Dried alum, Alumen exsiccatum, K 2 S0 4 .A1 2 3S0 4 = 516. This is 

 common alum, from which the water of crystallization has been 

 expelled by heating it for several days to a temperature of about 

 80 C. 



Aluminium hydrate, Aluminii hydras, A1 2 6HO = 156. Obtained 

 by adding water of ammonia or solution of sodium carbonate 

 to solution of alum, when aluminium hydrate is precipitated in 

 the form of a highly gelatinous substance, which, after being 

 well washed, is dried at a temperature not exceeding 40. 



K 2 S0 4 .A1 2 3SO 4 + 6NH 4 HO = K 2 SO 4 + 3[(NH 4 ) 2 SO 4 ] + A1 2 6HO ; 

 K 2 SO 4 A1 2 3SO 4 + 3Na 2 CO 3 + 3H 2 O = K 2 SO 4 + 3Na 2 SO 4 + 3CO 2 4- A1,6HO. 



