9 8 



STUDIES IN LUMINESCENCE. 



with this apparatus, to get a measurement much nearer the origin than 

 0.8 of a second, yet the eye is sensitive to part of the change of intensity 

 before this, giving one a means of estimating roughly the amount of change 

 before the lirst measured point. The saturation effect is prominent, as 

 was the case with Sidot blende. Saturation is shown both by the change 

 of initial intensity and by the change in slope of the curves as the excitation 

 is increased. 



In Fig. 99 some of the curves were obtained one day and the remainder 

 on another day. One would expect curves B and C to coincide. The 

 difference between them is probably due to the fact that the infra-red 

 exposure does not reduce the powder to a standard condition. Curves D 

 and E agree more closely than B and C. A curve, not shown in the figure, 

 excited for 300 seconds, coincides with G. This coincidence may have 



100 



20 40 60 80 



Seconds 



Fig. 100. 



Decay curves at different temperatures. Excited 2 minutes. 



120 



Curve 1, temp, o 



2, 



3. 



4. 



5. 



excitation and decay, room temp. 

 66 C. 

 102 



145 



180 



been due to chance, but it is what would be expected, because the effect 

 of previous history becomes of less importance as the length of excitation 

 is increased. 



That the lack of agreement of curves B and C is due to the previous 

 history of the powder is substantiated by the fact that curve A was observed 

 immediately before curve B, and curve G immediately before curve C. No 

 deductions can be made for curves D and E, because the history of the 

 powder previous to the excitation for curve E is not known. Curve D 

 followed B immediately, but some preliminary work without infra-red was 

 done before the infra-red treatment preceding curve E. 



The changes produced in the decay curve by varying the temperature at 

 which excitation and decay take place are shown in Fig. ioo. The part 

 that previous history plays in these curves can not be estimated accurately, 



