CHAPTER IV, 



THE DECAY OF PHOSPHORESCENCE IN SI DOT BLENDE AND 

 CERTAIN OTHER SUBSTANCES.' 



The decay of phosphorescence was first studied by E. Becquerel. 2 In 

 the case of short-duration phosphorescence the phosphoroscope was used 

 for this purpose and the intensity of phosphorescence was measured for 

 different speeds of the rotating disk. Becquerel regarded it as probable 

 that the law of decay was of the form 



I = hc- ai (i) 



and found that the observations were in fact fairly well represented by 

 an exponential expression. In discussing these observations, however, 

 Becquerel tacitly assumed that no appreciable time was required for the 

 exciting rays to produce their full effect. Later investigations have shown 

 that this assumption is not justified. Since a change in the speed of the 

 phosphoroscope altered not only the time that elapsed between excitation 

 and observation, but also the duration of exposure, it is probable, therefore, 

 that the initial excitation was less at high speeds. Attention was directed 

 to this point by E- Wiedemann and later by H. Becquerel, who states that 

 a recomputation of the data shows a less satisfactory agreement with the 

 exponential law than at first appeared. 



For the long-time phosphorescence of the phosphorescent sulphides E. 

 Becquerel proposed an empirical expression of the form 



r n (c-ht) = cr m ( 2 ) 



For each of the seven substances tested this expression was found to show 

 a fairly good agreement with the experimental results throughout a con- 

 siderable range. In one case, namely, that of a calcium sulphide prepara- 

 tion giving an orange-red phosphorescence, the expression represented 

 the observations with considerable accuracy throughout the whole range, 

 the value of ;;; being 0.5. But in most cases it was not possible to find 

 values of m and c which would make the formula fit the experimental data 

 for the whole time of decay. The values of m that suited the observations 

 best lay between 0.5, for the calcium sulphide just mentioned, and 0.806 

 for another calcium sulphide having an orange-yellow phosphorescence. 



The same empirical formula has since been very generally used, among 

 others by Darwin 3 in 1881, and Ch. Henry 4 in 1892. The former worked 

 with Balmain's paint and found the best value for m to be 0.86. It can 

 not be said, however, that the experimental results were represented very 

 accurately by the formula. The substance used by Henry was Sidot blende, 



'An account of the work described in this chapter was presented to the American Physical Society at the 

 meeting held on Oct. 28, 1905. Physical Review, xxn, p. 279. 



2 BecquereI, "La Lumiere." See also Comptes Rendus, 51, p. 921, i860, and Annales de Chimie et de 

 Physique, series 4, 62, p. 5. 



Philosophical Magazine, 11. p. 209, 1881. 



4 Comptes Rendus, 115, p. 505, 1892. 



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