I9I0.] ON PHOSPHORESCENCE AND FLUORESCENCE. 



275 



So marked are the fluctuations in the duration of phosphor- 

 escence in these substances on cooHng that at many temperatures the 

 effect dies down more rapidly than it can be followed with the 

 apparatus just described. It is, however, possible to secure an 

 almost complete record of the fluctuations of phosphorescence with 

 temperature by allowing the substance to cool slowly throughout the 

 entire range of temperatures. The photometer carriage is set at a 

 convenient distance and records made of the times required for the 

 phosphorescence to attain this intensity at the various temperatures 



tf-Jti.. 



S. i*<L. 



-/io" -/io* -io' -^0° 0° 

 Fig. 9. 



through which the substance passes in cooling. Measurements of 

 this sort were made by Mr. Putnam and myself in the case of a 

 number of phosphorescent sulphides. Three characteristic cases 

 are shown in the curves in Figs. 7, 8 and 9. Two of these sub- 

 stances are those for which the decay curves have been given in the 

 previous figures, namely, the phosphorescent zinc sulphide and the 

 Ba Bi K compound. In all three of these cases it will be noted 

 that the time required for the phosphorescence to fall to a given 

 intensity varies greatly with the temperature and that the curves 

 showing these changes have marked maxima and minima. In the 



