1897.] on Diamonds. 491 



was formed, and with tolerable certainty we can estimate this 

 temperature hy the resistance of the specimen of graphite to this 

 reagent. 



The superficial dark coating on a diamond after exposure to mole- 

 cular bombardment I have proved to be graphite,* and M. Moissan f 

 has shown that this graphite, on account of its great resistance to 

 oxidising reagents, cannot have been formed at a lower temperature 

 than 3600° C. 



It is therefore manifest that the bombarding molecules, carrying 

 with them an electric charge, and striking the diamond with enormous 

 velocity, raise the superficial layer to the temperature of the electric 

 arc, and turn it into graj)hite, whilst the mass of diamond and its 

 conductivity to heat are sufficient to keep down the general tempera- 

 ture to such a point that the tube appears scarcely more than warm 

 to the touch. 



A similar action occurs with silver, the superficial layers of 

 which can be raised to a red heat without the whole mass becoming 

 more than warm. J 



This conversion of diamond into graphite is, I believe, a pure 

 effect of heat. In 1880 § Professor Dewar in this theatre placed a 

 cry.^tal of diamond in a carbon tube through which a current of 

 hydrogen was maintained. The tube was heated from the outside by 

 an electric arc, and in a few minutes the diamond was converted into 

 graphite. I will now show you that a clear crystal of diamond, heated 

 in the electric arc (temjjerature 3600° C), is converted into graphite, 

 and this graphite is most refractory. 



The diamond is remarkable in another respect. It is extremely 

 transparent to the Eontgen rays, whereas highly refracting glass, 

 used in imitation diamonds, is almost perfectly opaque to the rays 

 (Fig. 25). I exposed over a photographic plate to the X rays for a 

 few seconds the large Delhi diamond, of a fine pink colour, weighing 

 31J carats, a black diamond weighing 23 carats, together with an 

 imitation in glass of the pink diamond lent me by Mr. Streeter ; also 

 a flat triangular crystal of diamond of pure water, and a piece of glass 

 of the same shape and size. On development, the impression where 

 the diamond obscured the rays was found to be strong, showing that 

 most rays passed through, while the glass was practically opaque. 

 By this means imitation diamonds and some other false gems can 

 readily be detected and distinguished from the true gems. It would 

 take a good observer to distinguish my pure triangular diamond from 

 the adjacent glass imitation. 



Speculations as to the probable origin of the diamond have been 

 greatly forwarded by patient research, and particularly by improved 



* ' Chemical News,' vol. Ixxiv. p. 39, July 1896. 



t ' Comptes Eendus,' cxxiv. p. 653. 



% Proc. Roy. Soc. vol. 1. p. 99, June 1891. 



§ ' Proceedings of the Royal Institution,' Jan. 16, 1880. 



