124 PLATINUM RESIDUES. 



heated to boiling (whereby red sesqui-cyanide of rho- 

 dium may be precipitated), and the platinum precipi- 

 tated by sulphate of copper. The violet precipitate, 

 a mixture of double cyanide of platinum and copper, 

 and of double cyanide of iridiurn and copper, is washed 

 with hot water and then boiled with caustic baryta. 

 The cyanides of platinum and of barium, very solu- 

 ble in hot water, but scarcely soluble in cold, may be 

 easily separated from the colorless and more soluble 

 salts of iridium by crystallization. 



The iridium reduced by hydrogen and freed from 

 iron, may contain ruthenium and rhodium. To extract 

 the first it is fused with caustic potassa and chlorate of 

 potassa, and afterwards to extract the latter with bisul- 

 phate of potassa. (See No. 70.) 



The sesquioxide of iridium from the metal obtained 

 by the calcination of the double chloride of ammonium, 

 may be brought to a coherent mass by a strong pres- 

 sure, and heating to a white heat; it is then placed in 

 a burnt lime crucible, and fused by an oxyhydrogen 

 jet. 



The metal may be extracted from the osmic acid by 

 the process given in No. 70. 



A single treatment of a platinum residue is not 

 generally sufficient, but it must be repeated several 

 times. 



2. The minerals mixed with the platinum metals 

 are separated by fusing the residues with a flux, and 

 lead, which dissolves the noble metals. The platinum 

 residue is mixed with (not more than 300 or 400 

 grms. at a time) an equal weight of granulated lead, 

 and one and a half times its weight of litharge, and 

 melted in a crucible with a thick bottom, until com- 

 pletely fused. It is stirred from time to time with an 

 earthenware rod to unite the grains of metal, the cru- 

 cible is taken from the fire (before the oxide of lead 

 has penetrated it), gently struck a few times and left 



