154 FIRST YEAR COURSE IN GENERAL SCIENCE 



ever, when used to cover large surfaces like the domes 

 of buildings. 



Copper is of great use because it is a good conductor of 

 electricity; many tons of new copper wire are brought into 

 use daily. Copper is abundant enough to be reasonably 

 cheap. It is made into coins of low denomination the world 

 over. In pure form, it is rolled into sheets for covering sur- 

 faces exposed to air or water. x A great amount of copper 

 is used on large buildings, to cover the valleys and gutters 

 in the roof surface. Copper is alloyed with tin to make 

 bronze, and with zinc to make brass. 



169. Silver. Silver has properties similar to those of 

 copper and gold, but in different degree. Silver is not af- 

 fected by pure air or water. In the air of houses, however, 

 it tarnishes somewhat, because such air often contains 

 impurities from furnace or illuminating gas. 



Both as solid and as plated ware, silver is useful for table 

 articles. It is not affected by many kinds of food, though 

 eggs and mustard discolor it because they contain compounds 

 of sulphur, which make a dark coating on silver. "Solid 

 silver" is alloyed with 10 or more per cent of copper to give 

 it greater durability. United States silver coins contain 10 

 per cent of copper. 



170. Aluminum. The lightest metal in common use is 

 aluminum, or aluminium. Its lightness and the fact that it 

 does not tarnish in air or water make it useful for kitchen 

 utensils, aeroplane frames, and parts of automobiles. It is 

 a good conductor of electricity. 



171. Mercury. Mercury is the only metal which is 

 liquid at ordinary temperatures. It is used in thermometers 

 and barometers and in many other scientific instruments. 

 As it will dissolve gold, it is used in stamp mills to separate 

 fine particles of gold from sand or from rock dust. The gold 

 is finally separated from the mercury by distillation. 



Mercury is used with other metals as gold, silver, or tin 



