22 FROM AN EASY CHAIR 



found more or less frequently coloured in various tints. 

 Nevertheless, it is the fact that in very few cases have 

 chemists been able to prove by analysis what precisely is 

 the cause of the colour in any given crystal or precious 

 stone, although they may strongly suspect this or that 

 as the colour-giving impurity. The actual quantity of 

 a metallic impurity sufficient to give a tint is so 

 excessively minute that the chemist finds it impossible 

 to determine what it is by examining one small precious 

 stone. He has not a sufficient bulk of material to 

 operate on. 



Having reached this point, we can see that such 

 potent disturbing agents as the rays of radium 

 penetrating, a colourless, or faintly-coloured, crystal 

 may determine oxidation or other chemical combination 

 within the crystal of traces of metal (iron, cobalt, man- 

 ganese, chromium) already present there, and so give 

 it an increased colour or an altogether new tint. In 

 1905 (therefore long before the recent French experi- 

 ments had shown that the radium rays will act in this 

 way on corundum, the " base variety " of sapphire and 

 ruby), Sir William Crookes published an account of his 

 experiments as to the action of the radium rays on the 

 diamond. " Some fine colourless crystals of diamond," 

 writes Sir William Crookes in 1905, " were embedded 

 in radium bromide, and kept undisturbed for more than 

 twelve months. At the end of that time they were 

 examined. The radium had caused them to assume a 

 beautiful bluish-green colour, and their value as ' fancy 

 stones' had been materially increased." On another 

 occasion Sir William found that a yellowish " off colour " 

 diamond had its tint changed to a pale blue-green when 

 embedded for six weeks in a tube with radium bromide. 

 (I have seen this stone.) He also has succeeded in 

 improving the clearness of diamonds by exposing them 

 to radium rays. Everyone who has experimented with 

 radium knows that it causes the sjlass which may be 



