270 



NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 



Three such curves for the Ca, Bi sulphide No. 3, plotted with 

 /"^ as ordinates, are shown in Fig. 6; four for the Sr, Pb sulphide 

 No. 13 in Fig. 7 and three for the Ba, Cu sulphide No. 33 in Fig. 8. 

 A notable feature of all these curves is the existence of two so-called 

 linear processes the first of steeper slope and therefore indicative of 

 a more rapid decay of phosphorescence than the second. This form 



Fig. 6. 



of curve, as is well known, is characteristic of phosphorescent sub- 

 stances in general, the only well established exceptions being those 

 occurring in the case of the uranyl salts. ^' As regards the relation 

 of the two processes recorded in these diagrams to what appear as 

 the first and second processes in the usual study of the long time 

 phosphorescence of such sulphides, it is clear that the second process 



1- Nichols, Proc. Nat. Academy of Sciences, II., p. 328, 1916. 



