88 



STUDIES IN LUMINESCENCE. 



appear to be a straight line, while under the same conditions the curve OP 

 would still be seen to be concave downward. 



The curves in Fig. 79 represent closely the facts deduced from Figs. 77 

 and 78. They also agree with the observations recorded in Chapter IV, 

 which show that the decay curve consists of two straight lines gradually 

 merging into one another. By changing the slopes and intercepts of the 

 lines in Fig. 79 representing the decay of the single bands, a decay curve of 

 the total intensity can be derived which will exactly lit the deductions 

 mentioned above. Dr. Pierce's later experiments on the decay in Sidot 

 blende (Chapter VIII) nevertheless make it seem very doubtful whether 

 this explanation of the form of the decay curve, and of its dependence upon 

 temperature, can be made to accord with all the experimental facts. 



The effect of heating the Sidot blende after it has been excited at room 

 temperature and allowed to decay at room temperature to a small fraction 

 of its initial intensity of phosphorescence is illustrated in Figs. 80, 81 , and 82. 

 The result of this process is the well-known phenomenon of thermo-lumines- 

 cence. The curves are plotted with intensities as ordinates and times, meas- 

 ured from the moment of heating, as abscissas. Higher temperatures increase 

 the intensity of the outburst of light, and cause the maximum to occur 

 sooner after the beginning of heating and to die away more rapidly. 



In the case of Fig. 80 the initial intensity immediately after excitation 

 was somewhat greater than I =12. The intensity was allowed to decay to 

 / = 0.8, when heating was begun. In the case of Fig. 82, the initial intensity 

 was somewhat greater than 1 = 37. 



Fig. 80. 



Effect of heating when the phosphorescence has decayed to an intensity equal to 0.8. Excited 10 

 seconds at room temperature. Curve A, heated to 307 C; curve B. heated to 270 C; 

 curve C, heated to 208 C; curve I), heated to 155 C; curve E, heated to i)<) C. 



The effect of heating after excitation may be considered in one of two 

 ways. Either it suddenly releases the energy represented by the phos- 

 phorescence, or else it sets up some new reactions in the powder. Though 

 the decay had reached a low intensity before heating was begun in the runs 

 shown in Figs. 80, 81, and 82, yet at this low intensity the decay was slow; 

 hence there may have been considerable energy still left, which heating 



