ii 4 



STUDIES IN LUMINESCENCE. 



of phosphorescence by rather wide variations in the percentages of the added 

 sodium. In most cases of the compounds recorded in Table 1 7 the amount 

 of sodium salt present was not far from 1 per cent. 



An inspection of Table 17 shows that the color of the phosphorescence 

 of cadmium salts with sodium ranges from blue to yellow. In all but two 

 cases the intensity was very small so small as to make it impossible to 

 obtain from these compounds reliable curves of decay with a spectro- 

 photometer. Doubtless all of these could be increased in intensity by find- 

 ing the suitable proportion of sodium. 



Table 17. 



DISCUSSION OF RESULTS. 



The results of the study of the substances just described are indicated 

 by means of decay curves plotted, like many of those which appear in 

 Chapters IV and VI, with I~- as ordinates, and times, counted from the 

 close of excitation, as abscissas. It will be seen that the curves have the 

 same general characteristics as those obtained by the measurement of 

 long-time phosphorescence. They may be regarded as consisting of two 

 straight lines merging into one another. 



The curves in Fig. 1 1 7 are decay curves typical of three of the substances 

 studied. In the case of curve A (willemite No. 5) the decay was so rapid 

 that it was difficult on account of the small intensity of the light to follow 

 the "second process" very far. 



In each curve one point has been plotted to the left of the zero on the 

 ordinates. This observation is made on the fluorescent light, i. e., it is the 

 intensity of the light coming from the screen when the mirror is in position 

 to reflect the light which comes from the screen during excitation. The 

 zero point, or the point where the exciting light fails to be reflected into 

 the spectrophotometer, was determined by placing a piece of white paper 

 in place of the screen. 



Since the induction coil furnished at most 120 discharges per second it 

 will be seen that, for the time of excitation employed in the measurements 

 illustrated in Fig. 1 17, three sparks per revolution of the disk is the maximum 



