PHOSPHORESCENCE OF METALLIC SULPHIDES. 



497 



cautions, as the atmosphere above the liquid air is very rich in 

 oxygen and conflagration of the insulating materials and even the 

 metal parts is likely to occur. 



The principal phenomena to be studied were those due to : 



r>. 



M J L B 



LIQUID AIR 



Fig. 3. 



1. The change of color during the decadence of phosphorescence. 



2. The effect on the color of phosphorescence when excitation 

 occurs at temperatures between -|- 20° and — 185°. 



Changes During Decay. 



To confirm the well-known phenomenon of change of color dur- 

 ing decay it is only necessary to mount one of the sulphides in the 

 phosphoroscope and excite with the disk running at normal speed, 

 i. e., 1,800 rev. a minute. The appearance as viewed through the 

 revolving disk is then that due to its phosphorescence during the 

 interval .0001 to .004 second (approx.) after excitation. If the 

 excitation be stopped by breaking the spark circuit the changes of 

 color as phosphorescence dies away can be followed for several 

 seconds. 



In the case, for example, of a sulphide known as No, 33 (Ba, 

 Cu, NaoB^Oy) the initial tint of red-yellow changes to a deep red. 

 Another preparation, No. 3 (Ca, Bi, NajSO^; CaF^NaoB^O^), which 

 is typical of many, appears bluish green through the rapidly revolving 

 disk and changes very rapidly after the close of excitation to a deep 



