DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING. 



91 



the photometer. For some other work a strip of platinum which supported 

 a thin layer of the powder was heated by passing an electric current through 

 the strip. The flash in this case occurred too rapidly to be followed, the 

 maximum intensity being the only measurement possible. 



Fig. 86. 



Curves .4 and C, variation in temperature of furnace when 

 the powder is put in. Curves B and D, increase in tem- 

 perature of the powder. Curves .4 and B do not become 

 identical because they were not taken simultaneously and 

 the temperature changed slightly between the two runs. 



It is not easy to estimate the effect of gradual heating as compared with 

 more nearly instantaneous heating. The outside layer of the powder is 

 subjected to the temperature of the furnace, which does not vary widely. 

 It is only the inside layers which are heated as slowly, as the curves in Fig. 

 86 indicate. It is not believed that any material change is introduced in 



Fig. 87. 



Effect of varying the length of excitation. Ordinates show the intensities of luminescence, and abscissas 

 the times after heating began. The length of excitation and the temperature of the furnace are as 

 follows: 



Curve 6, 320 sec, 153. o C; curve 5, 160.0 sec, 155.5 C.; curve 4, 80.1 sec, 155. 5 C; curve 3, 

 40.0 sec, 155. 5 C; curve 2, 19.9 sec, 155. 5 C; curve 1, 10.0 sec, 155. o C. 



the relation of the various curves, though the actual form of the curves 

 may be changed more or less. 



The effect of varying the length of excitation is brought out in Figs. 87 

 and 88. As in previous curves, the phosphorescence was excited at room 



