

1905.] the Beta and Gamma Rays of Radium. 507 



escent value did not further diminish, though the colour became rather 

 deeper. 



5. When the reddened crystals are removed from the action of the 

 rays they at once cease to phosphoresce. At the outset, therefore, 

 there are two well-marked characteristics in the behaviour of the 

 platino-cyanide. (1) The phosphorescence starts at a maximum value 

 which rapidly falls to 1/12 or thereby; and (2) there is no after 

 phosphorescence. 



6. Other observers have stated that platino-cyanide screens which 

 have been reddened by over-exposure to radium rays may be restored 

 to the sensitive state by exposure to sunlight.* I have exposed 

 reddened crystals on a glass plate to a condensed beam of sunlight for 

 several hours without being able to observe any restoration of colour 

 or phosphorescent quality. The observations here recorded seem to 

 suggest that the restorative effect of sunlight, where it has been 

 observed, must have been due to some conditions supplied by the 

 other materials of which the screen was composed. The present 

 observations show that the red crystals retain their new character 

 over a period of many weeks either in the dark or in diffused daylight, 

 and none of these observations have given any indication that the 

 change is other than permanent. 



7. The only way completely to restore the red crystals to the yellow 

 state is by solution in water and recrystallisation. The properties of 

 the crystals obtained in this way appear to be in all respects the 

 same as they were before exposure to the rays. It would appear, 

 therefore, that the reduction of the phosphorescent value is not primarily 

 due to chemical change in the salt. 



The fully developed crystalline condition of the yellow salt is 

 evidently necessary, if the highest phosphorescence is to be obtained. 

 Anything, therefore, which interferes with this condition impairs the 

 phosphorescence. 



8. When the water of crystallisation is wholly or partially expelled, 

 the crystals, either yellow or red, are left in the form of an amorphous 

 powder of a brick red colour. In this amorphous state the value of 

 the phosphorescence has fallen to 2/100 and the colour of the glow 

 is no longer green but red. As crystals reddened by exposure to the 

 ft rays lose water and become opaque on heating to 130 just as 

 the yellow crystals do, it may be concluded that the change from 

 yellow to red under the influence of the /3 rays is not due to the loss 

 of water of crystallisation. 



9. As the reduction of the phosphorescent value in the change from 

 the yellow to the red form is neither due to chemical decomposition 

 nor to loss of water of crystallisation, it occurred to me that it might 

 be akin to the change from the crystalline to the amorphous state, 



* Eutherford, Radio-activity,' p. 168. 



