294 Mr. J. H. T. Roberts on the 



If oxidation be tbe mechanism of the disintegration of 

 the platinnm metals, the rate of disintegration should not 

 alter with time ; that this is so with pure metals is confirmed 

 by Sir "W. Crookes' curves* and by the observations of 

 Holborn and Austin on pure metals, and on alloys containing 

 small percentages of iridium ; the more rapid rate of dis- 

 integration, in some cases observed in the early heatings, is 

 due to the presence of traces of more easily oxidisable 

 impurities. 



Sir W. Crookes, in his paper on " The Volatility of Metals of 

 the Platinum Group" (loc. cit.), discusses the possibility of the 

 formation of an oxide in the case of platinum, but discards 

 the view on account of his theory that an oxide formed 

 at a high temperature would only dissociate at a still higher 

 temperature, not at a slightly lower temperature. Speaking 

 of the crystals which occur in a platinum resistance furnace 

 in the vicinity of the platinum strip, that is, upon the 

 porcelain tube, which is at a slightly lower temperature 

 than the strip itself, Sir W. Crookes says : " the mode of 

 occurrence of the beautiful crystals of platinum is against 

 the supposition that they are a product of the decompo- 

 sition of an oxide, for the crystals deposit upon a part of 

 the apparatus that is at a slightly lower temperature 

 than the bulk of the metal, and it is inconceivable that 

 platinum should combine with oxygen to form a volatile 

 oxide at one definite temperature, and part with this 

 oxygen, and come down in metallic crystals, at a little 

 lower temperature." Later on Sir W. Crookes says : " I 

 must therefore come to the conclusion that platinum is 

 absolutely non-volatile at 900° C, a temperature easily 

 obtainable in an analytical laboratory, and that the formation 

 of crystals of the metal in the electric furnace is a true case 

 of sublimation." 



It must be pointed out, however, that if the disintegration 

 of platinum were a simple case of volatility, it would not 

 become zero in a vacuum ; and that there are many examples 

 recognized in physical chemistry and in accordance with 

 the laws of thermodynamics, of the formation of compounds 

 at high temperatures, and their dissociation at lower tempe- 

 ratures provided the cooling is slow. In a platinum 

 resistance furnace, since the hot air round the strip is 

 confined, we have exactly the right conditions for the 

 dissociation of the oxide and the formation of crystals ; 

 the oxygen being used over and over again and acting 

 merely as the temporary carrier of the metal. In this sense y 



* Proc. Roy. Soc. 1912. 



