86 



STUDIES IN LUMINESCENCE. 



were from a 16 c.p. incandescent lamp with a screen of thin vulcanite, 

 heated the powder slightly, but the wait of 5 minutes allowed it to cool 

 to approximately room temperature. 



The effect of temperature upon the decay curve, when the Sidot blende 

 is excited and allowed to decay at the same temperature, is shown in Fig. 77. 



Fig. 77- 



Effect of temperature during excitation and decay. Excited 40 seconds. Curve A, temperature 21 C; 

 curve B, temperature 37 C; curve C, temperature 6o C; curve D, temperature 85 C 



The curves are plotted with distances of the standard lamp from the photo- 

 meter screen as ordinates and time reckoned from the end of excitation as 

 abscissas. The ordinates are therefore inversely proportional to the square 

 roots of the intensities. Curves ^1 and B are concave downward through- 

 out, while C and D are approximately 

 straight lines. In fact, the points on 

 curve D indicate an upward bending. 

 The initial intensity evidently increases 

 and the decay becomes more rapid as 

 the temperature is raised. 



To further test the form of the curve 

 of decay at high temperatures, a series 

 of runs was made with constant length 

 of excitation and constant temperature. 

 Four runs of the series are shown in 

 Fig. 78. If the curves had been plotted 

 with the actual times as abscissas, they 

 would coincide. In order that the eye 

 may be able to distinguish readily the 

 points on each curve, the four curves 

 are separated by plotting curve D with 

 the abscissas marked on the figure and 

 displacing the remaining curves each 

 5 seconds farther to the right. It is 

 evident that the curves are straight lines throughout the time of observa- 

 tion, but it is probable that they would show a downward bending nearer 

 the origin. Studies of curves of very rapid decay, to be described in 

 Chapter VII, greatly strengthen this view. The conclusion that at room 



Fig. 78. 



Effect of temperature during excitation and 

 decay. Excited 400 seconds. Tempera- 

 ture 93 C 



