524 SMITH'S INTERMEDIATE CHEMISTRY 



$50,000,000 from the United States. In 1912 the world yield 

 was $475,000,000, but the increase in the cost of working has 

 caused many mines to be shut down. 



Properties. Gold is yellow in color. It is the most 

 malleable and ductile of metals. It melts at 1075. To enable it 

 to resist wear, it is alloyed with copper. Pure gold is " 24 karat " 

 fine. British gold coins are 22 karat, and American coins 21.6 

 karat (90 per cent gold). 



Gold is not affected by oxygen or by hydrogen sulphide. It 

 does not interact with any single acid. It combines directly, 

 however, with chlorine and bromine. It dissolves in aqua regia 

 (hydrochloric and nitric acids, mixed). This happens, not be- 

 cause aqua regia is more active as an oxidizing agent than the sub- 

 stances it contains, but because it oxidizes, and also furnishes the 

 chloride-ion Cl~ required to produce the exceedingly stable nega- 

 tive ion of chlorauric acid HAuCl 4 , namely AuCl 4 ~. 



Uses. Most of the metal is used in coins and bars as a medium 

 of exchange. It is beaten into gold leaf. It is employed in 

 making potassium chloraurate KAuCl 4 for toning photographs. 

 Gold plating on silver and other metals is carried out by using 

 a gold anode and a bath of sodium aurocyanide NaAu(CN) 2 

 solution. 



PLATINUM Pt 



Occurrence and Extraction. Platinum is found in the free 

 condition in alluvial sand, chiefly in the streams of the Ural 

 Mountains and the Caucasus and in Colombia. The separation 

 from osmium, iridium, and other metals, which accompany it, is 

 a complex operation. 



Properties. Platinum is a malleable and ductile metal with 

 a greyish-white luster. It melts in the oxy-hydrogen flame (m.-p. 

 about 1780), but not in the Bunsen flame. 



