DIAMONDS. 225 



I will project a diamond on the screen and bombard it with radiant 

 matter before your eyes. Some diamonds visibly darken in a few min- 

 utes, while others, more leisurely in their ways, require an hour. 



This blackeuing is only superficial, but no ordinary means of cleaning 

 will remove the discoloration. Ordinary oxidizing reagents have little 

 or no effect in restoring the color. The black stain on the diamond is 

 due to a form of graphite which is very resistant to oxidation. It is not 

 necessary to expose the diamond in a vacuum to electrical excitement 

 in order to produce a change. 



I have already signified that there are various degrees of refractori- 

 ness to chemical reagents among the different forms of graphite. Some 

 dissolve in strong nitric acid; other forms of graphite require a mixture 

 of highly concentrated nitric acid and potassium chlorate to attack 

 them, and even with this intensely powerful 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 tempera- 

 ture at which the graphite was formed, and with tolerable certainty we 

 can estimate this temperature by the resistance of the specimen of 

 graphite to this reagent. 



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

 lar bombardment I have proved to be graphite,^ and M. Moissan ^ has 

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

 ing reagents, can not have been formed at a lower temperature than 

 3,0(100 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 graphite, while the mass of diamond and its con- 

 ductivity to heat are sufficient to keep down the general temperature 

 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. ^ 



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

 of heat. In 1880 "^ Professor Dewar, in this theater, placed a crystal 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 (temperature 3,600° 0.),is converted into graphite, and this graphite 

 is most refractory. 



' Chemical News, Vol. LXXIV, page 39, July 1896, 

 ^Comptes rendus, CXXIV, page 653. 

 3 Proc. Roy. Soc, Vol. L, page 9^, June 1891. 



* Proceedings of the Royal Institution, Friday evening meeting, January 16, 1880. 

 SM 97 15 



