DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING. 



93 



The effect of delay between the end of excitation and the beginning of 

 heating was investigated at some length. The general character of the 

 results obtained is given in Fig. 90 and the subsequent diagrams. 



Fig. 90 shows the way in which the outburst of thermo-luminescence 

 diminishes in intensity, in the case of a substance excited for a given time 

 and subsequently heated in a furnace of given temperature, as the interval 

 of time before the beginning of heating is increased. 1 Figs. 92 and 93. in 

 which time is measured from the beginning of heating, indicate clearly 

 a shift in the time of reaching the maximum of intensity of thermo- 

 luminescence in the same direction as that already noted in the case 

 of increased length of excitation. The energy of phosphorescence does 

 not manifest itself so rapidly after 

 long delay in heating or long excita- 

 tion as after short delay or short ex- 

 citation. For any given excitation 

 and temperature the curves tend to 

 coincide after a given time, which 

 may be taken to indicate that the 

 interval of time occupied by an out- \ 

 burst of thermo-luminescence is in- 

 dependent of the time which has 

 elapsed since excitation. 



Table 16 gives the approximate 

 duration of the flash of thermo- 

 luminescence from nine sets of 

 observations under varying condi- 

 tions of excitation and heating. Fig. 91. 1 



While the times stated are neces- 

 sarily somewhat inaccurate, they indicate that brief outbursts follow 

 short exposures and high temperatures of the furnace and vice versa. 



X 



-* 



f 



400 



800 



1200 Sec. 



Fig. 92. 



Effect of delay in heating. Time 

 measured from beginning of 

 heating. Excited 320 seconds 

 at room temperature. Tem- 

 perature of furnace 03 C 

 The time between the end of 

 excitation and the beginning 

 of heating is as follows: 



Curve 1, 2.0 sec; curve 2, 21.9 

 sec; curve 3, 41.5 sec; curve 

 4,8i.5sec; curves, i62.osec; 

 curve 6, 332.0 sec. 



JThe location of the crests C, D, E, F, G, H. I (Fig. 90) is along a curve which suggests in its form the 

 ordinary curve of phosphorescence, and when we apply the usual criterion, i.e., plotting I~ -^ and times, we 

 get the curve shown in Fig. 9 1 , which has the significant form already discussed in Chapter IV of this treatise . 



