DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING. 



87 



temperature the observed part of the decay curve is concave downward 

 and becomes less concave as the temperature is raised is substantiated by 

 several series of curves which are not reproduced. 



A possible explanation of the changes in the decay curve that occur 

 when Sidot blende is excited at different temperatures may be deduced as 

 follows : Let it be assumed that the law of decay for a single band in the 

 spectrum is 1 



I= (a + bt 



Let it be assumed that the phosphorescent spectrum of Sidot blende con- 

 sists of two bands, and let curves AB and CD, Fig. 79, represents the decay 

 of these bands. The decay of the total intensity can be computed by the 

 equation 2 



L^ia+bty] 



D 



and is represented in 



Fig. 



OP. This curve is concave 



79 by the curve 

 downward throughout but approaches a straight line with increase in time. 

 From Figs. 77 and 78 it can be seen that the effect of a higher temperature 

 is to hurry the decay. Hence the slope of the lines representing the decay 



Fig. 79. 



AB, decay of one band at a certain temperature; 

 CD, decay of the other band at the same tem- 

 perature; OP, decay of the total intensity due to 

 the two bands; EF, decay of the first band at a 

 higher temperature; GH, decay of the other band 

 at the higher temperature; MN, decay of the 

 total intensity at the higher temperature. 



of the separate bands 3 will be greater at a higher temperature. MN, Fig. 

 79, represents the decay of the total intensity at the higher temperature. 

 M N is concave downward throughout, but if the portion occurring within 

 the first few seconds were not drawn, the remainder of the curve would 



The law proposed by H. Becquerel. See Chapter XV. 



2 The law proposed by H. Becquerel for the decay of the total intensity when the phosphorescent spectrum 

 consists of more than one band. 



a The existence of two bands in the fluorescence spectrum of Sidot blende has been demonstrated in 

 Chapter IV. The presence of twosuchbandsinthe case of the powder studied by Dr. Pierce was established 

 by him by means of spectrophotometric measurements. Their maxima were found to be approximately 

 at 0.53 j and 0.41 m- The same bands might therefore be expected in the phosphorescence spectrum. 



