STUDIES OK PHOSPHORESCENCE OF SHORT DURATION. 



123 



a definite match was obtained. The rest of the readings were made by 

 moving the standard lamp one division and turning the mirror away from 

 the zero point. The dimness of the phosphorescent light and the limit of 

 the machine allowed but seven or eight points at most. The data plotted 

 are the averages of three or four readings. The phosphorescence of this 

 group is a pinkish red. In Fig. 129 the curves are plotted with time of 

 decay as abscissas and i/y/I as ordinates, i/\/I being proportional to the 

 distance of the standard lamp from the photometer screen. The lower and 

 upper parts of the curves are straight lines, becoming concave downward 

 as they approach each other. Several of the curves happen to have points 

 taken at the bend and would indicate two straight lines meeting. All of 

 the curves plotted with /"* as abscissas show the upper and lower parts as 

 straight lines, gradually merging into one another. 



The curves are in fact of exactly the same type as the decay curves 

 obtained with Sidot blende. 1 A glance at the curves in Fig. 129 shows a 

 marked tendency toward parallelism in both parts. According to the 

 assumption of Wiedemann and Schmidt, that the light is emitted during the 



I23A56789 

 Percentage of concentration 



10 



Fig. 130. 



Showing relation between intensity and percentage concentration. 



.02 .04 .06 



Seconds 

 Fig. 131. 



Decay curves for three cadmium 

 compounds. The color of phos- 

 phorescence is green in each case. 

 Time of excitation = 0.025 sec. 



A , CdS0 4 + MnSOj - phosphores- 

 cence; B, CdSOi + MnCl; C, 

 CdSOi +MnCl different percent- 

 age. 



recombination of the products of the dissociation produced during excita- 

 tion, this would indicate the same coefficient of recombination of the ions for 

 the lower parts of these curves, and also the same, but slower rate in the up- 

 per parts. In this group the compounds that have the greatest initial inten- 

 sity have the longest period of decay, although there seems to be no relation 

 between the initial intensity and the time of decay. The cadmium com- 

 pounds (Fig. 131) show just the opposite relation. The different slant in 

 the two parts of the curves indicates a different coefficient of recombination 

 and a different rate of decay for the two parts. The coefficient and the de- 

 cay are smaller in the second part, showing a much slower process in the 

 giving back of the stored energy. 



The greatest intensity was shown by the compound of 0.8 per cent con- 

 centration. The relation between percentage concentration and initial 



'See Chapter V. 



