ALUMINUM. 161 



Alums known are, for instance : 



Potassium-aluminum sulphate, K 2 SO 4 , A1 2 (SO 4 ) 3 .24H 2 O. 



Ammonium-aluminum sulphate, (NH 4 ) 2 SO 4 , A1 2 (SO 4 ) 3 24H 2 O. 



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



Ammonium-ferric sulphate, (NH 4 ) 2 SO 4 , Fe 2 (SO 4 ) 3 .24H 2 O. 



The official alum, alumen, is the potassium 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 certain 

 kinds of clay (aluminum silicates) by sulphuric acid, when aluminum 

 sulphate is formed, to the solution of which potassium or ammonium 

 sulphate is added, when, on evaporation, potassium or ammonium 

 alum crystallizes. 



Dried alum, Alumen exsiccatum, K 2 SO 4 .A1 2 (SO 4 ) 3 = 516. 

 (Burnt alum.) This is common alum, from which the water of crys- 

 tallization has been expelled by heat. It is a white powder, dis- 

 solving very slowly in cold, but quickly in boiling water. 



Aluminum hydroxide, A1 2 (OH) 6 = 156. Obtained by adding 

 water of ammonia or solution of sodium carbonate to solution of 

 alum, when aluminum hydroxide is precipitated in the form of a 

 highly gelatinous substance, which, after being well washed, is dried 

 at a temperature not exceeding 40 C. (104 F.). 



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

 K 2 SO 4 A1 2 (S0 4 ) 3 + 3Na 2 CO 3 + 3H 2 O = K 2 SO 4 + 3Na 2 SO 4 + 3CO 2 + A1 2 (OH) 6 . 



The usual decomposition between a soluble carbonate and any soluble salt 

 {provided decomposition takes place at all) is the formation of an insoluble 

 carbonate ; according to this rule, the addition of a soluble carbonate to alum 

 should produce aluminum carbonate. The basic properties of aluminum 

 oxide, however, are so weak that it is not capable of uniting with so weak an 

 acid as carbonic acid, and it is for this reason that the decomposition takes 

 place as shown by the above formula, with liberation of carbon dioxide, 

 whilst the hydroxide is formed. (Other metals, the oxides of which have weak 

 basic properties, show similar reactions, as, for instance, chromium, and iron in 

 the ferric salts.) 



The weak basic properties of aluminum are shown also by the fact that alu- 

 minum sulphate, chloride, and nitrate, and even alum itself, have an acid 

 reaction, while the corresponding salts of the alkalies or alkaline earths are 

 neutral. 



Aluminum hydroxide shows considerable surface-attraction toward many 

 substances, which property is made use of in the art of dyeing, where the 

 hydroxide is used for retaining coloring matter upon the cotton-fibre. Prac- 



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