PHOSPHORESCENCE OF METALLIC SULPHIDES. 



499 



sulphides, and those of which the persistent band is of shorter wave- 

 length. 



Thirty-four sulphides examined by the method already described 

 showed color changes in accordance with the foregoing classification. 



Color Changes Due to Cooling. 



Since the intensity of the band of rapid decay vanishes in a small 

 fraction of a second, ordinary observations of the phosphorescence 

 of these sulphides pertain, as has already been pointed out, to the 

 persistent band alone. It is not possible moreover to isolate the rapid 

 band by the use of the phosphoroscope, since during the brief period 

 immediately following excitation we have both bands present and the 

 color due to their combination. 



The slow bands, however, are greatly diminished in intensity 

 by cooling the phosphorescent substance and are often reduced almost 

 or quite to the vanishing point. Thus it is possible by lowering the 

 substance during the experiment to temperatures approaching that o£ 

 liquid air, to observe the color and intensity of the rapid band, by 

 itself. 



When we cool the lower end of a tube of the BaCu sulphide No. 

 33 with liquid air, as described in a previous paragraph, and observe 



Fig. 4- 



its phosphorescence through the sectored disk, the red-yellow of the 

 upper (warm) end merges gradually into a brilliant green occupy- 

 ing the cooler regions below. A red-yellow patch occurs still lower 

 down, while at the very bottom where the temperature approaches 



