I2 4 



STUDIES IN LUMINESCENCE. 



intensity is shown in Fig. 130 by plotting percentage concentrations as 

 abscissas and initial intensities as ordinates. The initial intensities were 

 obtained by extending the straight line of the first part of the curves in 

 Fig. 129 to the y axis and reading the y intercept from the scale. 



THE CADMIUM GROUP. 



The phosphorescence of the cadmium compounds is yellow-green. Fig. 

 131 shows the decay curves for three different compounds. These are 

 characteristic decay curves, and follow the general form. The compounds 

 CdS0 4 +NaC0 3 , CdS0 4 -f-NaC10 3 , CdS0 4 +NaCl, CdS0 4 -f-NaN0 3 , and 

 CdS0 4 +NaBr were too dim to measure, but when excited may be seen 

 in a dark room for several minutes. In this group the specimens that have 

 the most intense phosphorescence have the most rapid decay. 



SUBSTANCES OF SLOW DECAY. 



A number of phosphorescent sulphides, made by the method of Lenard 

 and Klatt, 1 were obtained from Leppin and Masche, Berlin. They consist 



of a sulphide, an active metal, and a flux. 

 Practically all long-time and short-time 

 decay curves have shown two distinct 

 processes, merging into one. By this 

 method it was possible, using long-time 

 specimens, to study the first part of the 

 first process, for the very first interval of 

 decay. The specimens were excited for 

 0.008 second and the first readings made 

 0.0014 second after excitation. The 

 curves shown in Fig. 132, plotted with 

 time as abscissas and I~- as ordinates, 

 are in all cases straight throughout the 

 range studied. These results, which are 

 in agreement with the results of Wag- 

 goner for Sidot blende, make it appear 

 quite improbable that the behavior of 

 the phosphorescent sulphides during the 

 early stages of decay can be represented 

 by an exponential law, as has recently 

 been predicted by Lenard. 2 It should 

 be pointed out, however, that the slant of 

 the straight lines in Fig. 132 is such as to indicate an initial rate of decay 

 far greater than that corresponding to what is usually called the "first 

 process," or " Momentan-prozess," in the decay of substances of this 

 class. It is possible that under the conditions of these experiments the 

 decay curves of the phosphorescent sulphides consist of three portions, 

 each of which is linear, or nearly so, when 7 _i is plotted against time. 



02 04- .06 



Seconds 

 Fig. 132. 



A, Sr-Bi-Na phosphorescence, green; /?, Sr- 

 Zn-F phosphorescence, green; C, Ca-Bi-Na 

 phosphorescence, violet; I), Ba-Cu-Li phos- 

 phorescence, orange-red. 



l See Kayser, Handbucn der Spectroscopic, vol. 4. p. 750. : Lenard Annalen der Physik, XXI, p. 6 4 t, 1910. 



