SILVER, GOLD, PLATINUM 525 



The metal is not affected by air, water, or adds, excepting 

 aqua regia. In the latter instance, chloroplatinic acid H 2 PtCl 6 is 

 formed. 



Platinum adsorbs hydrogen and oxygen. When finely divided, 

 so as to present a large surface, it catalyzes powerfully many 

 chemical actions (pp. 262, 314). The forms used are platinum 

 black, a powder precipitated from chloroplatinic acid by an active 

 metal (e.g. zinc) ; platinum sponge, a porous mass made by decom- 

 posing ammonium chloroplatinate by heat; and platinized as- 

 bestos, made by dipping the asbestos in chloroplatinic acid 

 solution and heating. 



It unites, slowly, with chlorine and bromine. It combines also 

 with carbon, phosphorus, and silicon, and alloys itself with many 

 metals, so that reactions which liberate any of these elements can 

 not be carried out in vessels of platinum. It also acts upon fused 

 caustic alkalies giving platinates. 



Uses. Since the metal does not melt in the Bunsen flame, and 

 is not affected by many substances, it is employed in laboratory 

 operations in the forms of wire, foil, and crucibles. Much of the 

 metal is used by dentists, and in photography. Having the same 

 expansibility as glass, it is fused through the bottoms of incan- 

 descent lamps, to connect the filament with the exterior. Since 

 the metal is not oxidized by the air, even when heated, it is used 

 for electrical contacts. The metal is employed most extensively 

 for jewelry. 



On account of the increasing demand, and the failure of the 

 Russian supply, platinum is now (1922) worth $93.00 per ounce 

 troy, or more than four times the value of gold (nominally $20.67 

 per ounce). 



Other Metals of the Platinum Group. Associated in 

 small quantities with platinum in nature are five other metals 

 ruthenium, rhodium, palladium, osmium and iridium. These 



