PRECIOUS METALS 



107 



drogen flame. It is brittle and very hard. It is a powerful 

 catalytic agent when finely divided. It is obtained by igniting 

 Mimnonium clilnriridatc, which is obtained from osmiridium or 

 the platinum residues by a complicated process. It is found in 

 the Ural Mountains, Brazil, California, Borneo and Australia. 

 Its atomic weight is 193.1. 



Rhodium. Rhodium, symbol Rh, is a white, malleable, ductile 

 metal. Its specific gravity is 12.1 and its fusion-point is 2000 C. 

 It is insoluble in acids and unchanged in air. It is prepared from 

 platinum residues by first converting it into rhodium chloride 

 and then by reducing this by heating it with sulphur in a carbon 

 crucible. It forms alloys with platinum, gold, bismuth, and lead. 

 It is the most costly of all the metals, being worth five times as 

 much as gold. It occurs in small quantities in platinum ores and 

 in some native gold. It is found in the Ural Mountains and in 

 Brazil. Its atomic weight is 102.9. 



METALS OF THE PLATINUM GROUP 



The members of the platinum group occur intimately associated 

 with each other. Where one is present the other members in 

 larger or smaller quantity are usually present. Native platinum 

 contains from 16 to 40 : per cent, of the other metals of the group. 

 These often alloy with each other as in iridosmine and platinirid- 

 ium. They are all white or grayish- white, lustrous, ductile and 

 malleable metals. They are all permanent in the ordinary atmos- 

 phere. Osmium alone burns when strongly heated. All the 

 others are scarcely affected by oxygen at any temperature. Save 

 palladium, they are insoluble in any single acid, and often 

 aqua regia is without solvent action upon iridium and ruthenium. 

 Palladium fuses at the temperature of wrought iron, and is, 

 therefore, the easiest fused of any members of the group. The 

 order of fusibility of the other metals is, palladium, platinum, 

 ruthenium, iridium, osmium, the last of which has never suffered 



