206 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



indicate the presence of less than the 0.001 of a gramme of 

 gold. He proposes the following procedure : The gold of 

 the sample under analysis is first separated in the usual 

 manner from foreign metals, and next converted by means 

 of sodium chloride into sodio-gold chloride (NaAuCIJ ; the 

 solution is then concentrated by evaporation. In order to 

 detect the presence of gold, an aqueous solution of potassium 

 sulpho-cyanide (KCyS) is employed, which should contain, 

 for one part of the salt, about fifteen to twenty parts of wa- 

 ter. About six grammes of this solution are poured into a 

 test-tube, and some drops of the suspected gold solution, 

 treated in the manner above described, are added. If gold 

 is 2^ resent, a red-orange turbidity is at once obtained, which 

 soon falls in the form of a precipitate. On gently heating 

 the contents of the test-tube, the precipitate dissolves, and 

 the solution becomes colorless. This reaction is said to be 

 so delicate that one drop of a solution of the sodio-gold 

 chloride (one gramme of the salt in forty grammes of water) 

 affords a decided reaction. The reaction indicates, accord- 

 ing to the observer, the formation of a double sulpho-cyanide. 

 1^,XXXII., IVI. 



PUEE PLATINUM AND IRIDIUM. 



Deville and Debray, obtaining pure platinum and iridium 

 in quantity, have found for these metals rather higher densi- 

 ties than are commonly ascribed to them. The platinum has 

 a specific gravity of 21.5, while that of iridium is 22.421. An 

 alloy containing eighty-five per cent, of platinum to fifteen 

 of iridium, with density 21.618, was found to be very ductile 

 and malleable, and yet quite rigid. Probably it would be 

 useful in the arts. An alloy containing only 66.67 per cent, 

 of platinum, on the other hand, was not malleable. Supple- 

 ment to 7 A^ December^ 1875. 



OSMIUM. 



According to Deville and Debray, osmium is the heaviest 

 substance known, having a specific gravity of 22.477. They 

 have prepared the metal in a crystalline form by passing the 

 vapor of osmic acid over red-hot charcoal. The crystals ap- 

 parently belong to the first or regular system, and are hard 

 enough to scratch glass. The color is grayish-blue, becom- 



