106 ECONOMIC GEOLOGY 



ture of cutlery and instruments of precision. The alloys of plat- 

 inum with antimony, arsenic, bismuth, cadmium and tin are 

 generally brittle. With iridium the alloys are hard and elastic, 

 permanent in moist and dry air, and susceptible of a high polish. 

 This is especially true when less than 25 per cent, of iridium is 

 present. Above that point it becomes difficult to draw the alloy 

 into wire or to hammer it into sheets. An alloy of platinum with 

 10 per cent, of iridium resists the corrosive action of metals far 

 better than pure platinum. It has been stated that up to the 

 beginning of the present century 25 per cent, of all platinum used 

 in the United States was iridium. We use alloys of platinum and 

 iridium under the name of platinum. Platinum does not amal- 

 gamate readily with mercury. Here it is unlike gold and silver. 



Palladium. Palladium, symbol Pd, has the color, luster and 

 appearance of platinum, but takes a finer polish. It is malleable 

 and ductile, and is the most easily fused of any metals of the 

 platinum group. It is usually found in the metallic state, some- 

 times with gold and silver, and also associated with platinum in 

 the ores of the latter metal. Palladium can scarcely be distin- 

 guished from platinum by its color. It is chiefly noted for its 

 great tendency to absorb hydrogen. It is with difficulty soluble 

 in nitric acid. It dissolves in boiling sulphuric acid, being more 

 easily attacked in the finely divided state. It melts at 1586 C., 

 and at a higher temperature yields a green vapor. It forms alloys 

 with gold, silver, copper, mercury, nickel, antimony, arsenic 

 and the platinum metals. Palladium is used chiefly for the grad- 

 uated surfaces of physical instruments and for coating silver 

 articles, especially mirrors, because it retains a polish and does 

 not tarnish. Its atomic weight is 106.7. 



Osmium. Osmium, symbol Os, has a specific gravity of 22.47. 

 It is a bluish metal, harder than glass and infusible in the oxy hydro- 

 gen flame. It crystallizes in cubes or rhombohedrons and is the 

 heaviest of all known solids. It burns in the air to the tetroxide, 

 which has a peculiar penetrating odor and is injurious to the eye. 

 It alloys with metals, notably iridium. It is found in the Ural 

 Mountains, Brazil, California, Borneo and Australia. It is 

 used in pointing gold pens and as bearings for compass needles. 

 Its melting-point is 2500 C. Its atomic weight is 190.9. 



Iridium. Iridium has a specific gravity of 22.42 and a fusion- 

 point of 1950 C. It is a hard, white, lustrous metal resembling 

 steel. It is malleable at red heat. It melts only in the oxhy- 



