90 



STUDIES IN LUMINESCENCE. 



the approach to temperatures at which the substance loses its power to 

 phosphoresce, the curves suggest by their approximation to horizontal 

 lines that the function of heating is merely to release the stored energy 

 more suddenly. 



If this explanation of the effect of heating is correct, then it would be 

 reasonable to expect that in the case of the curves of Fig. 77 the decay would 

 be more rapid the higher the temperature, which effect has already been 

 pointed out. 



Fig. 84 shows the change in the maximum ordinate of the curves of thermo- 

 luminescence as the temperature is raised. No weight is given to the exact 

 shape of the curves, the points being connected merely to aid the eye in 

 distinguishing them. The curves are in accordance with what would be 

 expected if the function of the temperature is to liberate suddenly the 

 phosphorescent energy. If this is the case, the greater the temperature 

 the quicker the energy will be liberated and the brighter the flash. The 

 relation of maximum intensity to temperature is shown more clearly in 

 Fig. 85. 



To ascertain the rapidity with which the powder within the furnace was 

 heated, measurements were made for each of the temperatures at which 



g. 83. 



Fig. 84 



Fig. 85. 



Fig. 83. Areas between the thermo-phosphorescence curves and the coordinate axes. Curve .4, excited 

 320 seconds (from Fig. &2) ; curve B, excited 160 seconds; curve C, excited 80 seconds (from Fig. 81) ; 

 curve D, excited 40 seconds; curve E, excited 20 seconds; curve F, excited 10 seconds (from Fig. 80). 



Fig. 84. Change in the maximum intensity as the temperature is raised. Maximum intensity vs. time of 

 maximum intensity measured from the beginning of heating. Curve A, excited 320 seconds (from 

 Fig. 82); curve B, excited 160 seconds; curve C, excited 80 seconds (from Fig. 81); curve D, excited 

 40 seconds; curve E, excited 20 seconds; curve F, excited 10 seconds (from Fig. 80). Each point on 

 a curve is for a given temperature. On each curve the lowest point is for 90 C. and for the other points 

 in consecutive order 155, 207, 267 , and 308 C, respectively. 



Fig. 85. Increase of maximum intensity of thermo-Iuminescence with increase of temperature. Curve A, 

 excited 320 seconds (from Fig. 82); curve B. excited 160 seconds; curve C, excited So seconds (from 

 Fig. 81); curve D, excited 40 seconds; curve E, excited 20 seconds; curve F, excited 10 seconds 

 (from Fig. 80). 



curves of thermo-Iuminescence had been obtained, and from these curves 

 were plotted showing the rise of temperature of powder and the variation 

 in the temperature of the furnace. Fig. 86 gives the curves for two of these 

 temperatures. The general conclusion from these measurements was that 

 the powder reached a constant temperature in a constant time independent 

 of the temperature. The periods of the two galvanometers used in the 

 temperature measurements were too short to affect the shape of the curves 

 appreciably. 



The method of heating employed possesses several advantages. The 

 temperatures are known and can be accurately reproduced as many times as 

 desirable. Further, the gradual heating allows the flash to be followed in 



