140 METALS AND THEIR COMBINATIONS. 



The usual decomposition between a soluble carbonate and 

 any other 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 aluminium carbonate. The basic properties of alu- 

 minium 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 

 hydrate 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 aluminium are also shown by 

 the fact that aluminium sulphate, chloride, and nitrate, and 

 even alum itself, have an acid reaction, while the corresponding 

 salts of the alkalies or alkaline earths are neutral. 



Aluminium hydrate shows considerable surface-attraction 

 towards many substances, which property is made use of in the 

 art of dyeing, where aluminium hydrate is used for retaining 

 coloring matter upon the cotton-fibre. Practically this is accom- 

 plished by precipitating aluminium hydrate from solutions 

 containing coloring matter, which latter is carried down and 

 precipitated upon the fibre by the aluminium hydrate, or by 

 impregnating the articles to be dyed with this compound and 

 placing them in the colored solutions. 



Aluminium oxide, A1 2 3 = 102, is obtained as a white, tasteless 

 powder by either burning the metal or by expelling the water 

 from the hydrate by heat : 



A1 2 6HO = A1 2 O 3 -r 3H 2 O. 



Aluminium sulphate, Aluminii sulphas, A1 2 3S0 4 18H 2 = 666. 



A white, crystalline powder, soluble in about an equal weight 

 of water; obtained by dissolving the oxide or hydrate in sul- 

 phuric acid. 



Aluminium chloride, A1 2 C1 6 = 267. This compound is of in- 

 terest on account of being the salt from which the metal is ob- 

 tained. Most chlorides may be obtained by dissolving the 



