PRECIOUS METALS 105 



retorts in the manufacture of crude sulphuric acid, in which case 

 it is alloyed with 2 per cent, of indium. On account of its irit'in- 

 il>ility and the fact that its coclliricnt of expansion is nearly the 

 same as glass, platinum is used to connect outside copper wires 

 with the carbon filament in incandescent lamps. .The thickness 

 of t lie filament varies from 0.01 to 0.012 in. Platinum is used in 

 the manufacture of platinum spoons, dishes, crucibles, combs, 

 foil and wire. Liebig, in his chemistry letters, states that with- 

 out platinum it would be impossible in many cases to make an 

 analysis of many silicates, and thus the composition of most 

 minerals would remain unknown; without platinum the composi- 

 tion of our organic compounds would likewise remain unknown. 

 Platinum is used extensively by balance makers for weights; is 

 used in surgical and scientific instruments of precision; for the 

 points of stylographic pens; for the balance wheels and hair 

 springs of non-magnetic watches; for obtaining a silver color on 

 porcelain; for platinum plating; for oxidizing silver; for the fuses 

 of electrolytically exploded cartridges; for use with high-grade 

 explosives like dynamite. It is used with iridium as an electrode 

 for the electrolysis of alkaline chlorides, where an alloy of 15 per 

 cent, of iridium can be rolled to a thickness of 0.007 of a milli- 

 meter and yet have sufficient resistivity to be used on an indus- 

 trial scale. Platinum is used also in the manufacture of the 

 platinum salts of commerce. It is also used in the making of 

 platinized asbestos. The United States dental and electrical 

 uses of platinum equal 50 per cent, of the world's output. 



The Alloys of Platinum. Platinum alloys readily with gold 

 and silver, and these alloys have been discussed in the treatment 

 of silver. Platinum alloys readily with copper in all proportions 

 at high temperatures. The alloys are extremely hard and less 

 liable to tarnish than the ordinary brasses and bronzes. With 

 81.25 per cent, copper the alloy is a golden yellow, closely resem- 

 bling 18 carat gold. It is both malleable and ductile and sus- 

 ceptible of a high polish. Both platinum and copper alloy read- 

 ily with about 4 per cent, of zinc. The alloys are extensively used 

 in jewelry; mathematical instruments, and chronometer wheels. 

 Platinum bronze is an alloy of platinum, nickel, and tin. With 

 nickel, platinum forms a white, malleable, magnetic alloy. This 

 is capable of a high polish and is permanent in moist and dry air. 

 The presence of 3 per cent, platinum prevents steel from rusting, 

 and is therefore of great industrial importance in the manufac- 



