490 Mr. William Crookes [June 11, 



will be for the D line • 404, and the refraction equivalent, — 



^ d ' 



will be 4-82. 



After exposure for some time to the sun many diamonds glow 

 in a dark room. Some diamonds are fluorescent, aj^pearing milky 

 in sunlight. In a vacuum, exposed to a high-tension current of 

 electricity, diamonds phosphoresce of different colours, most South 

 African diamonds shining with a bluish light. Diamonds from other 

 localities emit bright blue, apricot, pale blue, red, yellowish-green, 

 orange, and pale green light. The most phosphorescent diamonds 

 are those which are fluorescent in the sun. One beautiful green 

 diamond in my collection, when phosphorescing in a good vacuum, 

 gives almost as much light as a candle, and you can easily read by its 

 rays. The light is pale green, tending to white. 



I will now draw your attention to a strange property of the 

 diamond, which at first sight might seem to argue against the great 

 permanence and unalterability of this stone. It has been ascertained 

 that the cause of phosphorescence is in some way connected with the 

 hammering of the gaseous molecules, violently driven from the 

 negative pole, on to the surface of the body under examination, and so 

 great is the energy of the bombardment, that impinging on a piece of 

 platinum or even iridium, the metal will actually melt. When the 

 diamond is thus bombarded in a radiant matter tube the result is 

 startling. It not only phosphoresces but assumes a brown colour, 

 and when the action is long continued becomes almost black. 



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

 radiant matter before your eyes. I do not like to anticipate a 

 failure, but here I am entirely at the mercy of my diamond. I 

 cannot rehearse this experiment beforehand, and it may happen that 

 the diamond I have selected will not blacken in reasonable time. 

 Some visibly darken in a few minutes, while others, more leisurely in 

 their ways, require an hour. 



This blackening is only superficial, but no ordinary means of 

 cleaning will remove the discoloration. Ordinary oxidising re- 

 agents have little or no effect in restoring the colour. 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 this change. 



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

 toriness to chemical reagents among the difi'erent forms of graphite. 

 Some dissolve in strong nitric acid ; other forms of graphite re- 

 quire 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 temperature at which the graphite 



