STUDIES OF PHOSPHORESCENCE OF SHORT DURATION. 



"5 



number that could pass while the screen is exposed. It would seem then that 

 the excitation might be uncertain and irregular. These excitations, how- 

 ever, follow each other very rapidly; for example, in curve B, Fig. 117, the 

 rotating disk makes a complete revolution in 0.108 second, and since 10 to 

 20 seconds were required to make a setting of the spectrophotometer, what- 

 ever changes take place in the spark during successive excitation, the reading 

 is an average value of the intensity at that point. In order to make sure 

 of this, readings were taken starting with the lowest intensity and ending 

 with the highest, then starting with the highest and following the decay to 

 the lowest value of intensity, the points plotted being an average of the two 

 settings for each point on the curve. 



If the substances here studied exhibit the same hysteresis phenomena 

 that have been found in Sidot blende and several other substances, 1 the 

 decay curves obtained by this 

 method are probably not the same 

 as would be observed if it were 

 possible to restore the substance 

 to a standard condition between 

 successive periods of excitation, 

 for example by the action of infra- 

 red rays. In using these curves 

 to test any theory of the decay of 

 phosphorescence, this fact must 

 of course be taken into considera- 

 tion. 



The curves in Fig. 1 1 8 indicate 

 a much slower decay than those 

 shown in Fig. 117. The initial 

 decay or first process, however, 

 is quite rapid. These curves were 

 more difficult to obtain, since the 

 maximum of intensity lay in the 

 yellow-red, while the substances 

 whose curves are given in Fig. 

 1 1 7 have a green phosphorescence. 

 On this account the points are less 

 accurately determined than in 



Fig. 117. It is evident, nevertheless, that the curves have the same 

 general character and consist of two straight lines merging into each other. 



All the substances mentioned in this chapter are excited to fluorescence 

 by kathode rays. The mixture containing CdS0 4 was especially brilliant 

 and exhibited kathodo-luminescence of an intense yellow color. They 

 were also excited to some extent by X-rays, the Cd v S0 4 again showing the 

 greatest intensity. In all cases the phosphorescence excited by X-rays 

 was too small to be measured with the present apparatus. 



The curves given in Fig. 119 show the change in the decay of willemite 

 after heating. Curve L shows the decay of the untreated willemite. 

 Curve M shows the decay after heating the willemite for 45 minutes at a 



.02 



04 



Seconds 



06 



Curve A, 

 second. 



Curve B, 

 second. 



Curve C, 



Fig. 117 Decay curves. 



Willemite No. 5, time of excitation 0.026 

 Andrews ZnCk, time of excitation 0.027 

 ZnCh No. 3. time of excitation 0.26 second. 



'See Chapters IV and V. 



