518 ALUMINUM. 



fibre, probably from a similar attraction of surface. When this 

 solution is concentrated by evaporation, alum crystallizes from 

 it, generally in the cubic form, and the excess of alumina is pre- 

 cipitated. 



Sulphate of ammonia may be substituted for sulphate of po- 

 tash in this double salt, giving rise to ammoniacal alum., which 

 agrees very closely in properties with potash alum. 



Sulphate of alumina also combines with sulphate of soda 

 forming soda alum, which crystallizes in the same form as 

 common alum, and also contains 24 HO. Crystals are obtained 

 by mixing the constituent salts, and leaving a concentrated solu- 

 tion to spontaneous evaporation ; or by pouring spirits of wine 

 upon the surface of such a solution contained in a bottle, which 

 deposits crystals as the alcohol gradually diffuses through it. 

 This salt effloresces in air, as rapidly as sulphate of soda. It is 

 very soluble in water, 10 parts of water at 60 dissolving 1 1 

 parts of this salt. 



Sulphate of alumina also combines with the sulphate of 

 protoxide of iron, when dissolved with that salt and a consider- 

 able admixture of sulphuric acid (Klauer). The double salt 

 was found to contain 1 eq. protosulphate of iron (FeO, SO 3 ), 

 1 eq. sulphate of alumina (A1 2 O 3 , 3SO 3 ), and 24 eq. of water 

 (24HO), which indicates a similarity in composition to alum. 

 But it is deposited in long acicular crystals, which do not belong 

 to the octohedral system, and has therefore no claim to be con- 

 sidered an alum. A similar salt with magnesia was obtained in 

 the same way. Another combination of the same class, con- 

 taining the sulphate of manganese, forms a white fibrous mineral 

 found in a cave upon Bushman's river in south Africa. This 

 salt has been carefully examined by Apjohn and by Kane, and 

 found to contain 25 HO. It is probable that if the proportion 

 of water in Klauer's salts were accurately determined, it would 

 be found to be the same. These salts may be represented as 

 compounds of a magnesian sulphate, retaining its single atom 

 of constitutional water, with sulphate of alumina ; the manganese 

 compound thus : 



MnO, SO 3 , HO + A1 2 O 3J 3SO + 24HO. 



Certain salts have been formed, isomorphous with alum, and 

 strictly analogous in composition, in which the alumina is re- 



