Manchester Memoirs, Vol. Ixii. (1918), No. 10. 3 



heating, and if the salt is dissolved in water a strong alkaline 

 reaction is obtained. Giesel produced similar salts by exposing the 

 neutral salt to the vapours of sodium or potassium. It is evident 

 therefore that, in this case, chlorine is actually liberated and the 

 free sodium left dissolves in a deeper layer of sodium chloride 

 unreached by the cathode rays. 



Debierne found that certain dark violet fluor-spars smell of ozone. 

 When heated they lose their colour and thermo-luminescence and 

 also yield -helium in variable but small quantity. On exposure to 

 radium rays the violet colour was restored. Doelter has worked 

 extensively on the question of the colour of minerals, and has in- 

 vestigated the effect of temperature changes, radium, and ultra- 

 violet light. He states that increasing the quantity of radium used 

 in colouring a crystal does not diminish the time in strict proportion, 

 but he gives no reliable data in support of this statement. He also 

 states that the quantity of radium used has no influence on the final 

 colour produced. From slight differences in colour observed on 

 treating certain minerals with radium in nitrogen and oxygen, he 

 draws the doubtful conclusion that the radium rays exert a re- 

 ducing action. 



Ultra-violet rays were found in general to exert the opposite 

 effect to radium rays, substances coloured by the latter being de- 

 colourised or the original colour restored by exposure to the light 

 from an arc lamp. Glass and yellow diamond were found to be 

 exceptions to this rule. 



Doelter finally concludes that the colour of fluor-spar, topaz, 

 rose quartz, etc., is due to the formation of colloidal metals or other 

 colloidal substances, and that these metals oxidise when heated. 

 He also states that the velocity of coloration is dependent upon 

 the diffusion of the radium into the material. Since certain colours 

 may be induced by the action of radium through two glass tubes 

 and 0*5 cm. of lead, it is impossible to accept this last statement. 

 Again, since strong colours may be induced 2 cms. deep in a clear 

 solid fluor-spar crystal in a few hours by radium, and destroyed 

 again in a few seconds by gentle heat, it is inconceivable that 

 atmospheric oxygen can have any appreciable influence on the 

 colour. 



Experimental. — The minerals chosen for experiment were, as 

 far as possible, clear crystals of bodies which either were colour- 

 less or were known to occur in a colourless state in nature. 



The following properties were studied :— 



i. Colour changes (a) on heating 



(b) on treatment with radium or cathode rays 



(c) on heating after treatment (b) 

 ii. Luminescence (a) under influence of rays 



(b) on heating before treatment 



(c) on heating after treatment. 



Other minor effects, such as disruptive action, formation or 

 enlargement of cleavage planes, etc., were also observed from time 

 to time. 



