STUDIES OF PHOSPHORESCENCE OF SHORT DURATION. 



119 



determined for the different wave-lengths. The phosphoroscope was then 

 adjusted to give the decay at a later period, and the intensity for different 

 wave-lengths was measured in the same way. Both of these methods gave 

 the same results, i.e., that all portions of the band seemed to decay at the 

 same rate. The curves shown in Fig. 123 are typical for all the substances 

 studied. In none of the substances was there an indication of a shift of the 

 maximum of the curve as the decay went on. If any change occurs during 

 the decay it is too small to be detected by the method used. 



SPECTROPHOTOMETRY STUDY OF THE CADMIUM-SODIUM COMPOUNDS. 



The time of decay of these substances is so much longer than that of 

 the others considered in this chapter that the phosphoroscope already 

 described was not well suited to their study. A new instrument was 

 therefore devised for the purpose. This machine is a modification of the 

 phosphoroscope described by Kester, 1 but is so constructed that it is possible 

 to determine curves of decay 

 without changing the time 

 of excitation. 



Fig. 124 shows the top 

 view of the apparatus for 

 making the decay curves. 

 W is an iron pulley 45 cm. 

 in diameter mounted on a 

 vertical shaft. The mass of 

 the pulley being consider- 

 able, it acted as a balance 

 wheel on the driving motor 

 to keep the speed constant. 

 The speed was automati- 

 cally recorded on a chrono- 

 graph. The compound to 

 be studied was sifted lightly 

 over a strip of paper covered 

 with "zapon" varnish; the strip of paper was then placed on the rim of 

 the pulley. The spark gap S was mounted in such a way that it could 

 be moved to different points on the periphery of the wheel without in the 

 least disturbing the speed of the wheel or the continuity of the spark. This 

 made it possible to maintain a constant excitation and still obtain the inten- 

 sity of the phosphorescence at various times after excitation. The phos- 

 phorescent light to be studied enters the Lummer-Brodhun spectrophoto- 

 meter at Co. The comparison source is shown at G, and the telescope at T. 



Fig. 125 shows the phosphorescence spectrum of a cadmium-sodium- 

 manganate preparation taken with the apparatus just described and with 

 an incandescent lamp as the comparison source. 



Fig. 126 shows the phosphorescence spectrum of a cadmium-manganese- 

 chloride compound. The distribution of intensities in these two cases is 

 very similar and the spectra correspond closely with that of cadmium- 



.50 



Fig. 123. 



Curve showing the phosphorescence spectrum of ZnCl: No. 

 at different times after the exciting light was removed. 



'Kester: Physical Review, xxil, p. 280. 



