IOO 



STUDIES IN LUMINESCENCE. 



ture at which excitation and decay take place. These curves and the 

 curves given earlier in this article agree generally. Fig. 106 shows the 

 effect of excitation and decay at a temperature lower than any used in this 

 article. 



Effect of history previous to excitation. Excitation and decay at room temperature. Excited 30 seconds. 

 Curve .1, excited after action of infra-red; curve B, excited immediately after curve A; curve (', 

 excited immediately after curve B; curve D, before exciting for curve D, the powder was excited for 10 

 minutes and allowed to decay to an intensity corresponding to D =9 in the figure, at which time.the 

 excitation of 30 seconds was begun. 



In the case of calcium sulphide, it is not necessary to use a spectrophoto- 

 meter to prove that there is more than one band included in the phenom- 

 enon of phosphorescence, for under the influence of heat the color of the 

 powder can be seen to change from blue to green. 



Fig. 103. 



Curves of Fig. 102 repeated at a temperature of 133 C 



It has already been shown that the form of the phosphorescence decay 

 curve may be closely approximated on the basis of two bands, each follow- 

 ing the law 



T 



/ = 



(a+bt? 



This explanation was carried to some length in the earlier part of this 

 chapter. But there seems to be no way of combining two decays, each of 

 which follows this law so as to produce an upward bending. If it be sup- 

 posed that one band increases in intensity as the other decreases, then an 



