ALUMINIUM 467 



From time to time more of the oxide is added and the melted 

 aluminium (m. p. 650 to 700) is tapped off. The oxide must be 

 made from carefully purified bauxite, as the metal itself can not 

 be purified commercially. In 1866 it cost $250 to $750 per kilo- 

 gram but now sells at about 50 cents. 



Properties. The metal has a lower specific gravity (2.6) 

 than any other metal that could be put to the same uses (sp. gr. 

 iron 7.8, copper 8.8). It has malleability and the foil is taking 

 the place of tinfoil to some extent for wrapping foods. It has 

 considerable tensile strength, and is a good conductor of elec- 

 tricity. When heated, the metal burns brilliantly. 



In the air it acquires only a slight film of closely adhering oxide. & 

 This film prevents ^itjromjactmg upon water (hot,or cold). 

 the surface is cleaned and amalgamated wi?R fnCTcuiyTby dipping 

 in mercuric chloride solution, however, this metal acts as a contact 

 agent, and hydrogen is rapidly displaced: 



2A1 + 6H 2 -> 3H a | + 2A1(OH) 3 1 . 



Uses. The largest quantity of aluminium is consumed by 

 steel-makers. When added in small amount (less than 1 : 1000) 

 to molten steel, it combines with the gases, and gives sound ingots 

 free from blow holes. Next to this comes its use for long distance 

 transmission of electricity. A cable of the requisite capacity 

 is larger than one of copper for the same current, but is lighter 

 and puts less strain on the supports. Cooking^ Yessels of alumin- 

 ium are not corroded and are largely used. Cameras and opera 

 glasses are made of it. On account of its lightness, it is used 

 extensively in the metal parts of dirigibles and aeroplanes. Pul- 

 verized aluminium, mixed with oil, gives a paint which protects 

 iron admirably. 



Aluminium bronze (copper, with 5 to 12 per cent aluminium) 

 has a brilliant golden yellow color and is stable in air and easily 

 worked. Magnalium (containing 1 to 2 per cent of magnesium) 



