100 Prof. li. Bun sen on Flame Reactions. 



16. Platinum Compounds. 



(a) In the oxidizing flame with soda they also yield a grey 

 spongy mass, giving white, ductile, lustrous particles on rubbing 

 up in a mortar. These are insoluble in either nitric or hydro- 

 chloric acid alone, but dissolve in a mixture of these acids, form- 

 ing, if the platinum is pure, a light-yellow solution, which is of 

 a darker colour if rhodium, iridium, or palladium is present. 

 The solution does not give with mercuric cyanide and ammonia 

 a white precipitate, but a light-yellow crystalline precipitate of 

 platinum-ammonium-chloride. 



(b) Stannous chloride colours platinum compounds yellowish 

 brown. 



17. Iridium Compounds are likewise reduced to metal when 

 heated with soda in the upper reducing flame, yielding a grey 

 non-lustrous powder which is not the least ductile. This is not 

 only insoluble in the separate acids, but also in aqua regia. 



18. Rhodium Compounds can be distinguished from those of 

 iridium only by the fact that the metallic powder, insoluble in 

 aqua regia, when fused with hydropotassic sulphate, is partially 

 oxidized, giving a rose-coloured solution. 



19. Osmium Compounds give, in the oxidizing flame, fumes of 

 volatile osmic acid, possessing an acrid chlorine-like smell and 

 attacking the eyes powerfully. 



20. Gold Compounds. — If gold is contained only in traces mixed 

 with large quantities of the matrix, it can only be concentrated 

 and detected by the old gold-test*". In other cases, however, a 

 few tenths of a milligramme can be detected by reduction with 

 soda on a charcoal splinter. The yellow lustrous ductile glo- 

 bules thus obtained can be flattened out to larger golden parti- 

 cles j these do not dissolve in nitric or hydrochloric acid, but are 

 p-retty easily soluble in aqua regia, yielding a light-yellow solu- 

 tion. If this is absorbed by a small piece of hlter-paper, the 

 purple of Cassius is obtained on moistening with stannous chlo- 

 ride. The solution remaining on the curved glass becomes 

 coloured brown from separation of reduced gold on addition of 

 ferrous sulphate, whilst the solution appears blue by transmitted 

 light. 



21. Silver Compounds. — If silver occurs only in traces in slags 

 or complex ores, it can only be detected by the well-known me- 

 thod of cupellationf. If, however, the silver compound is not 

 mixed with a very large amount of foreign matter, it can be de- 

 tected in very minute quantities by reduction with soda on the 

 charcoal splinter. The white ductile beads dissolve easily on 



* C. P. Plattner's Probierkunst, bearbeitetv. Th. Eichter, 1865, p. 541. 

 t Ibid. p. 512. 



