PLATINUM. 681 



double chloride of iridium and potassium. The precipitate 

 is collected on a filter, and washed with a dilute solution of 

 chloride of potassium. By igniting this double salt with twice 

 its weight of carbonate of potash to the point of fusion, the 

 platinum is reduced to the metallic state, while a portion of the 

 iridium remains as peroxide. The platinum is dissolved by 

 aqua regia in which the peroxide of iridium remains untouched. 

 To complete the separation of the iridium, the precipitation, by 

 chloride of potassium and ignition with carbonate of potash, 

 may require to be repeated several times. But when it is not 

 required to have platinum absolutely pure, the solution of the 

 ore is precipitated by sal ammoniac, and it is in this way that 

 much of the commercial platinum is procured, the insoluble 

 double chloride of platinum and potassium is washed with a 

 solution of sal ammoniac dried and heated to low redness ; hy- 

 drochloric acid and nitrogen escape and sal ammoniac sublimes, 

 while the platinum remains behind as a pulverulent mass, or 

 spongy and a little coherent. The small trace of iridium which 

 is left in commercial platinum increases greatly its hardness 

 and tenacity. 



Platinum is too refractory to be fused in our furnaces, but 

 at a high temperature, its particles cohere like those of iron, 

 and it may, like that metal, be welded. Hence, by heating a 

 mass of the spongy platinum, previously strongly compressed, 

 and submitting it to increasing pressure, the mass comes to be 

 so far compacted, that it may be forged with a hammer. Plati- 

 num as it comes from the hands of the workman is highly mal 

 leable and ductile. It is the densest body at present known ; 

 the specific gravity of platinum was fixed by Dr. Wollaston at 

 21.53. This metal may be fused by the oxihydrogen blow-pipe, 

 or even made to boil, and be dissipated with scintillations. It 

 is not acted upon by any single acid, not even by concentrated 

 and boiling sulphuric acid. Its resistance to the action of acids, 

 conjoined with its difficult fusibility renders platinum invaluable 

 for chemical experiments , and for some purposes in the chemical 

 arts, particularly for the concentration of oil of vitriol. 



The remarkable influence of a clean surface of platinum in de- 

 termining the combustion of oxygen and hydrogen, has already 

 been considered. This property platinum shares with osmium, 

 iridium, palladium and rhodium. It is exhibited in the greatest 

 degree by the highly divided metal, such as platinum sponge, 



