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CHAPTER XXXIX 

 ALUMINIUM 



THE family of tnvalent elements to which the metal aluminium 

 belongs includes the non-metal boron (p. 363), and several rare 

 metals. 



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Occurrence. Aluminium, although it does not occur free, is 

 the third element in order of quantity (see p. 19) . Its silicates, such 

 as clay (kaolin) HAlSi0 4 , mica . KAlSi0 4 , and felspar KAlSi 3 8 , are 

 amongst the most plentiful minerals. The oxide A1 2 3 occurs as 

 corunflum. samphire, rufyy, and emery (impure form). Bauxite is 

 a valuable hydrated oxide. Garnets Ca 3 Al 2 (Si0 4 ) 3 are mined and 

 pulverized to make " sand " paper. Cryolite 3NaF,AlF 3 (Greek, 

 ice-stone) is imported from Greenland. 



Manufacture. The making of aluminium on a large scale 

 originated in C. M. Hall's discovery (1886) that the oxide could be 



electrolyzed in solution in molten cryo- 



~ lite. Iron boxes (Fig. 110), about 5 

 by 3 feet and 6 inches deep, are lined 

 with a compressed mixture of coke and 

 tar which is afterwards baked. The 

 lining forms the cathode, while the 

 'oxygen is liberated at the anodes 

 a series of rods of carbon about 3 

 FlG - 110 inches in diameter which are attached 



copper rods. The cryolite is melted (1000) by the arcs 

 struck by the carbon rods. The latter are then raised some- 

 what, the aluminium oxide is added, and some coal (which floats) 

 is thrown in to cover the surface and obscure the blinding glow. 



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