122 PLATINUM RESIDUES. 



ore are dissolved ; the kind B is obtained as a precipi- 

 tate by iron in the last mother liquor from the prepa- 

 ration of platinum. 



A. This residue is formed of grains and scales of irid- 

 osmine, together with pulverulent iridium with very 

 small quantities of palladium, rhodium, and platinum, 

 mixed with titanic iron, chromic iron, and sand. These 

 last three sometimes amount to 70 or 80 per cent., and 

 also contain traces of chlorides of silver and gold. To 

 extract the precious metals a variety of methods are 

 employed. 



The coarse granular kind is broken and ground as 

 finely as possible, to reduce the grains of the iron ores 

 to powder. It is then levigated with water, when most 

 of the irid-osmine is separated in larger grains and 

 scales. 



1. The levigated black powder is intimately mixed 

 with about its own bulk of decrepitated and finely- 

 powdered chloride of sodium, the mixture introduced 

 into a porcelain or glass tube, and gently ignited in a 

 slow current of undried chlorine-gas until the latter 

 commences to pass through the tube unabsorbed. 



The other end of the tube dips into a well-cooled, 

 tubulated receiver from the tubulure of which a gas- 

 tube conducts the excess of chlorine into milk of lime. 



By this process sodio-chlorides of iridium and of 

 osmium are formed. The greater portion of the latter 

 is decomposed by the moisture of the chlorine-gas, and 

 the osmic acid formed from it partly sublimes in the 

 receiver, and is partly conducted into the hydrate of 

 lime. 



The residue in the tube, when cold, is treated with 

 water, and is at last washed with hot water. 



The dark yellowish-red solution of iridium filtered 

 off from the iron ore is mixed with concentrated nitric 

 acid and distilled, when osmic acid passes over, dis- 

 solved in water. The liquid thus very much concen- 



