PHOSPHORESCENCE OF METALLIC SULPHIDES. 



501 



with different fluxes and proportions. While the persistence of the 

 green band at low temperatures seems to be a general property of 

 these substances the recrudescence of the red as in the example here 

 considered is not so apparent in other cases. In the other substances 

 of this class it must be regarded either as masked by the brightness 

 of the green component or entirely absent. In the former case one 

 might expect a modification of the green at the temperature in ques- 

 tion and in several specimens thus far studied this is clearly the case. 



The effect of cooling the phosphorescent sulphides of calcium 

 and strontium is analogous to that just described. 



The band of long duration is greatly weakened or disappears; 



FAST 

 GREEN -BLUE— — - +20* 



GREEN - 



SLOW 



A 



185 



Fig. 6. 



—VIOLET 



- -BLACK 



8 



the other, which in all these cases, so far as has been observed, is a 

 green band, persists. 



Since the band destroyed by cold is, however, blue or violet, the 

 change of color is in the opposite direction from that noted in the 

 case of the barium compounds, i. e., from blue or blue-green to green 

 instead of from red-yellow to green. 



The most striking example in this class is perhaps the combina- 

 tion (Ca, Bi, Na.SO^; CaF,, Na^B^O^) already referred to. The 

 greenish blue of early phosphorescence, as observed through the 

 sectored disk at -|-20°, goes over to a full green of short duration 

 in the cold portions of the tube, the phosphorescence of long dura- 

 tion being in both cases of a remarkably rich pure violet. See 

 Figure 6. 



It is obvious that the methods described in this paper bring out 

 from quite a new standpoint the structure of the phosphorescence 



