n6 



STUDIES IN LUMINESCENCE. 



bright red heat. It is clear from the curves that the heat treatment has 

 decreased the initial intensity and made the decay less rapid; both these 

 changes may be observed by the eye alone if the two screens are excited 

 in a dark room. Willemite No. 3 which had been heated 2 hours was found 

 to give a curve whose points fell so nearly on curve M that it was not plotted. 

 The effect of heat upon the decay in ZnCl2+.rMnS04 is similarly 

 shown in Fig. 120. Curve H represents the decay in a sample of the sub- 

 stance taken as a dry powder, which was prepared by heating the mixture 

 only 2 hours at a bright red. Curve K represents the decay of a sample 

 heated 5 hours, and curve G after 7 hours heating. The curves show that 

 the first heating was not sufficient to bring out the initial brightness and 

 longer decay of the substance, and that there is a time limit to the heating 



02 .04 .06 



Seconds 

 Fig. 118. 



Showing the decay in different substances. 



Curve D, CaS, time of excitation 0.033 sec. 



E, CdSOi, " " *' 0.032 



F, CdCh, 0.031 



.08 



46 



42 



40 



.01 02 .03 .04 



Seconds 



Fig. 119- 



Showing effect of the heat treatment. 



Curve L, willemite No. 1, time of excita- 

 tion 0.022 sec. 



Curve M, willemite No. 2, time of excita- 

 tion 0.022 sec. 



necessary to produce maximum intensity; for on heating beyond this the 

 initial intensity becomes less. From the behavior of this artificial com- 

 pound it would seem that the natural willemite shown in Fig. 119 had 

 reached a maximum heat treatment already, for further heating decreased 

 its initial intensity. The slope of the curves in Fig. 120 indicate little change 

 in the rate of decay after the initial drop; unfortunately the intensity be- 

 comes so small that it was found impossible to carry the curves farther. 

 The readings in this region represent the average of a number of settings. 



The curves given in Fig. 121 show the effect of changing the time of 

 excitation in ZnClo No. 3. This was accomplished by making the opening 

 of the disk in Fig. 1 16 smaller. It was found that when the 60-cycle current 

 was used for the spark the curves taken with decreased time of excitation 



