On the Action of Radium Emanations on Diamond. 47 



" On the Action of Eadium Emanations on Diamond." By Sir 

 WILLIAM CROOKES, F.E.S. Eeceived June 9, Eead June 16 

 1904 



When diamonds are exposed to the impact of radiant matter in 

 a high vacuum they phosphoresce of different hues, and assume a 

 dark colour, becoming almost black when the bombardment is long 

 continued.* 



Some diamonds blacken in the course of a few minutes, while others 

 require an hour or more to discolour. f This blackening is only super- 

 ficial, and although no ordinary means of cleaning will remove the 

 discoloration, it goes at once when the stone is polished with diamond 

 powder. The fact that the black stain is not affected by ordinary 

 oxidising reagents would seem to show that it is not due to a layer of 

 amorphous carbon; but it might be graphite, which is much more 

 resistant to oxidation. Becquerel has shown that graphite is con- 

 verted into graphitic oxide by long digestion in a warm mixture of 

 potassium chlorate and strong nitric acid, while diamond even in a 

 very finely powdered state is absolutely unaffected by the mixture.^ 



Some forms of graphite dissolve in strong nitric acid ; others require 

 a mixture of highly concentrated nitric and potassium chlorate to dis- 

 solve them, and even with this intense oxidising agent some graphites 

 resist longer than others. M. Moissan has shown that the power 

 of resistance to nitric acid and potassium chlorate is in proportion to 

 the temperature at which the graphite has been formed, and with 

 reasonable certainty we can estimate this temperature by the resistance 

 of the graphite to this reagent. 



Judging from the long time required to remove the superficial 

 darkening from diamond, the graphite is as resistant as that formed at 

 the temperature of the electric arc. 



On one occasion when I had blackened the surfaces of diamonds by 

 molecular bombardment in vacua M. Moissan was present, and took some 

 away with him for further examination. He subsequently reported the 

 results in the * Comptes Rendus.' He heated the diamond to 60 in an 

 oxidising mixture of potassium chlorate and fuming nitric acid prepared 

 from mono-hydrated sulphuric acid and potassium nitrate fused and 



* ' Phil. Trans., 3 1879, Part II, p. 658, par. 625. 



t At a lecture before the Koyal Institution on June 11, 1897, I exposed a flat 

 made crystal of diamond to radiant matter bombardment before the audience for 

 about 5 minutes, a strip of metal covering part of the stone. On removing the 

 diamond from the vacuum tube and projecting its image on the screen with the 

 electric lantern, the image of the darkening waa very apparent. 



t ' Ann. de Chim. et de Phys.,' [4], vol. 19, p. 392. 



Vol. 124, No. 13. 



