102 ECONOMIC GEOLOGY 



of silver are now refined in the United States. The bath con- 

 sists of a solution of nitric acid, silver nitrate and copper citrate. 

 The silver and copper are both dissolved at the anode. The 

 copper remains in solution, the silver is precipitated at the 

 cathode, the gold remains undissolved. 



Uses of Silver. Silver was used by the ancients practically as 

 early as gold. Silver is used very extensively in the arts and sci- 

 ences, as in jewelry, tableware, coinage, silver bullion as a me- 

 dium of exchange, photography, mirrors, for optical apparatus, 

 in plating and in very many alloys. Copper lowers the melting- 

 point of silver and makes the metal harder, but does not decrease 

 the malleability, or materially impair the color. This is by far 

 the most important of the silver alloys, and in coinage nine 

 parts of silver to one part of copper produces a coin that resists 

 wear through friction. Silver mixes with lead in all proportions 

 when molten but segregates upon cooling. Therefore, silver- 

 lead alloys lose their homogeneity. Silver alloys readily with 

 cadmium, producing a soft, white, malleable, and ductile alloy. 

 Silver alloys readily with mercury and produces silver amal- 

 gams. Silver unites with tin, zinc, and bismuth in the forma- 

 tion of important alloys, generally ductile and malleable. With 

 platinum, silver forms a hard alloy, that is used very extensively 

 in dentistry. Silver unites with palladium and with rhodium; 

 in fact, silver unites with all useful metals save iron and cobalt. 



PLATINUM: ITS PROPERTIES, OCCURRENCE AND USES 



Properties. Platinum, symbol Pt, is one of the rare metals. 

 It has a specific gravity of 2 1.46, silver white with a grayish tinge, 

 ductile, malleable, sectile, with a luster less brilliant than that 

 of silver. Its melting point is 1780 C. Its atomic weight is 

 195. In the finely divided state it is black. The presence of 

 minute impurities render platinum hard and brittle. In the 

 electric crucible Moissan volatilized it, but its boiling point is 

 unknown. It is unaffected by heat in both dry and moist air. 

 It is insoluble in all single acids, but is readily soluble in aqua 

 regia. 



Ores of Platinum. Native platinum; sperrylite, PtAs2, which 

 is the most important ore of the metal; platiniridium, an alloy 

 of platinum and iridium; osmiridium, an alloy of osmium and 

 iridium; native osmium and irridium contain small quantities 



