1905.] 



the JJeta and Gamma Rays of 



513 



placed on the radium cell it phosphoresced with a pale greenish light ; 

 but after a few minutes the spot at which the rays entered glowed with 

 an orange red phosphorescence. This glow rose to a certain pitch, and 

 then continued fairly uniform throughout the exposure to the rays. 

 On removal from the radium at the end of six days the crystal con- 

 tinued to phosphoresce as brightly as it had done in the path of the 

 rays. Twenty-four hours later the crystal still phosphoresced, though 

 less brightly. On warming it to 80 the glow became brighter than it 

 had originally been. On cooling again to the ordinary temperature 

 the glow was feebler than it had been before heating. The reserves of 

 phosphorescing power had evidently been more quickly expended at 

 the higher temperature. 



28. The gradual disappearance of the phosphorescence was watched 

 day by day for a week. Four days after removal from the radium 

 the crystal could still be seen in complete darkness. Heated to over 

 100, the phosphorescence was much augmented, but the glow was 

 distinctly whiter than it had originally been. Five days after removal 

 from the radium the crystal had become quite invisible in complete 

 darkness, but on heating it to over 100 it again phosphoresced. On 

 the sixth day it still phosphoresced feebly when heated to 100, but 

 on raising it to 150 it glowed so brightly that it could be seen 

 distinctly in a dimly lighted room. The phosphorescence was now 

 even whiter and more diffused than it had been before. On the fifth 

 day the patch of yellow coloration had completely disappeared. 

 This disappearance coincided with the loss of phosphorescence at the 

 ordinary temperature and also with the development of the whiter 

 and more diffuse glow when the crystal was heated. On the seventh 

 day the crystal heated to 150 gave a feeble white phosphorescence. 

 On this occasion a crystal of calcspar, which had -not been exposed to 

 the radium, was heated side by side with the other which had. This 

 fresh crystal gave absolutely no phosphorescence at 150. 



29. A piece of potash glass exposed to the rays showed a feeble 

 fluorescence. After 60 hours it was removed from the radium, 

 Where the rays had entered the surface, a strong brown coloration 

 had taken place. This coloured patch phosphoresced feebly. Six 

 weeks later the glass was examined and showed no phosphorescence 

 at the ordinary temperature, but on heating to 100 it gave a faint 

 greenish glow. The depth of colour of the brown patch in the glass 

 showed no diminution of intensity at the ordinary temperature with 

 the lapse of time, but on heating to about 200 the coloration 

 completely disappeared. 



30. A lens of colourless flint glass, tested for lead by heating one 

 of its edges to the softening point in a gas flame, showed the usual 

 brown coloration of lead glass. The lens was placed on the radium 

 cell and at once phosphoresced feebly. After 12 hours' exposure to 



