160 ECONOMIC GEOLOGY 



capable of wide industrial application. Copper has been used 

 as a medium of exchange for many generations. In coinage, 

 gold and silver are too soft to resist abrasion. They are each 

 alloyed with copper in such proportions that the color and mal- 

 leability are not seriously impaired, while the hardness is ma- 

 terially increased. Copper is the hardening metal in the gold 

 and silver of jewelry. With platinum copper alloys in all pro- 

 portions. Cooper's gold, which so closely resembles 18 carat 

 gold and for which it is so largely substituted, sometimes con- 

 tains over 80 per cent, of copper. Mirror metal has 57.85 per 

 cent, copper and pen metal 13 per cent, of copper. 



The brasses are important alloys of copper and zinc. They 

 range from 95 per cent, of copper and 5 per cent, of zinc to 40 

 per cent, of copper and 60 per cent, of zinc. The most import- 

 ant brasses are those of about 40 per cent, of copper and 60 per 

 cent, of zinc. Some of these have a tensile strength of 40,000 

 Ib. to the square inch. English brass consists of 2 parts of 

 copper and one of zinc. Muntz metal consists of 3 parts of copper 

 and one of zinc. Dutch brass consists of 5 parts of copper and 

 one of zinc. Brazing metal of 9 parts of copper and one of zinc. 

 Naval brass which is so extensively used in condenser tubes 

 consists of 70 per cent, copper, 29 per cent, zinc, and one part 

 of tin. 



The bronzes are important alloys of copper and tin. Various 

 forms of bronze were used by the ancient Greeks and Romans. 

 Relics of bronze have been found in the Lake dwellings of Switzer- 

 land. The bronzes of the greatest technical value carry over 

 80 per cent, of copper. Those containing about 50 per cent, of 

 copper are called speculum metal, which is used for widely dif- 

 ferent purposes than the regular bronzes, as, for instance, the 

 silvering of glass reflectors and the specula for reflecting telescopes. 

 Gun metal consists of 9 parts of copper and 1 part of tin. 

 Bell metal consists of 80 per cent, copper and 20 per cent, of tin. 

 Silver is sometimes added in small quantities to bell metal to 

 increase its sonorous quality. Machinery brasses and bronzes 

 consist of alloys of copper, tin and zinc. 



Copper is used largely as bearing metals. One of these con- 

 sists of 70 per cent, copper, 15 per cent, of tin, and 15 per cent, of 

 lead. It is well known and widely utilized. Another consists 

 of 7 parts of copper and one of tin. Still another of 65 parts of 

 copper and 35 parts of tin. Hot boxes upon railroad trains often 



