Manchester Memoirs, Vol. Ixii. (1918), No. 10. 7 



first a green light on slightly heating. This reached a maximum, 

 and then died down, but before it disappeared a new violet glow 

 started which also reached a maximum and died down on further 

 heating, after which no light was emitted as the crystal was heated 

 to dull redness. . 



The Cornish chlorophane, which gave a brilliant green phos- 

 phorescence, had evidently been in close proximity to radio-active 

 matter giving y rays. This is quite in keeping with the known 

 occurrence of pitchblende and other uranium minerals in Cornwall. 



Cryolite. — A white semi-transparent specimen from Canada 

 was quite unchanged after prolonged exposure to radium, but 

 acquired a slight thermo-luminescence. 



Rock Salt, Sylvine, ■ Potassium Bromide and Iodide. — All these 

 alkali halides glowed with a bluish violet light under the action of 

 radium. 



Rock salt is coloured brown throughout its mass, but more 

 strongly nearest the surface exposed to the radium and on cleavage 

 planes further in the crystal. In a cathode-ray tube a similar brown 

 colour is produced on the surface only. 



Sylvine is coloured blue by radium, but the colour is very 

 evanescent and disappears after a few hours even when kept in the 

 dark. In a cathode-ray tube it is coloured deep violet, and chemic- 

 ally pure potassium chloride acquires the same colour, which is 

 much more stable than that produced by radium. 



Potassium bromide is coloured sea-green by radium, or cathode 

 rays, the colour being nearly as evanescent as that of sylvine when 

 produced by radium, but more stable when produced by cathode 

 rays. The time necessary to colour this specimen was much longer 

 than that for potassium chloride. 



Potassium iodide is coloured brown like sodium chloride, but 

 the colour is much more stable than that produced in any of the 

 other alkali halides. It was thought that the colour in this case 

 might be due to the liberation of iodine, but when a small crystal 

 which had been coloured deep brown was dissolved in water no 

 trace of iodine could be detected by the starch reaction. 



None of the above group showed more than an extremely faint 

 thermo-luminescence after radium treatment. 



Quartz. — This mineral shows a still greater variety of colour 

 than fluor-spar, but only a few of these varieties occur clear and 

 transparent. The best-known are rock crystal, rose quartz, 

 amethyst, cairngorm and citrine, all of which become colourless on 

 heating. The temperature necessary to destroy the colour varies 

 greatly — rose quartz requires a red heat, amethyst requires strong 

 direct heating in a blow-pipe flame, while nearly black cairngorm 

 may sometimes be made quite colourless and limpid by heating in 

 a soft glass tube. 



Prof. Rutherford found that radium exerts a powerful dis- 

 integrating action on transparent fused quartz, a tube of this material 

 becoming so rotten that it fell to pieces on being touched after being 

 left in contact with radium for a few weeks. No sign of this 



