DECAY OF PHOSPHORESCENCE PRODUCED BY HEATING. 



97 



The dotted vertical lines show the wave-lengths of the lines of the mercury 

 arc. The figure indicates the presence of stray light in the speetro-photom- 

 eter, which lowers the zero line with respect to the curve and unduly 

 exaggerates the ratio of the maximum of the band at 0.41 /x to the maximum 

 of the band at 0.54^. The 

 band in the blue is, how- 

 ever, evidently much more 

 intense than the other. 

 This fact was shown by 

 the color of the initial phos- 

 phorescence, which was 

 blue. 



The effect upon the de- 

 cay curve of varying the 

 length of excitation is 

 shown in Fig. 99. These 

 curves are plotted with 

 distances of the standard 

 light from the photometer 

 as ordinates and time 

 measured from the end 



of excitation as abscissas. For short excitations the curves are concave 

 downward throughout, but for longer excitations the bending is concen- 

 trated in the first part of the curves. These curves are similar to those 

 obtained with Sidot blende, except that the phosphorescence lasts much 

 longer. Hence it is possible to get points relatively nearer the origin. This 



Fig. 98. 



Fluorescence spectrum of calcium sulphide excited by the visible 

 spectrum of a mercury arc. 



/60. ZOO 

 Fig. 99. 



362 Sec. 



Effect of varying the length of excitation. Excitation and decay at room temperature. 

 Curved, excited 14.4 sec. Curve E, excited 59.9 sec. 



B, " 30.1 F, 150.3 



C, " 30.1 G, " 300.0 



D, " 60.2 H, " 600.0 



Curves .4, B, D, and F were taken on the second day and the remainder on the first day of the run. 



fact allows one to make a tentative deduction regarding initial intensity 

 and length of excitation at room temperature. A study of Fig. 99 indicates 

 strongly that the initial intensity is greater the longer the excitation, which 

 is the impression obtained when getting the curves. Though it is impossible, 



