8 On Radiant Matter. 



is under the influence of the radiant discharge ; but many 

 other substances possess this phosphorescent power in a 

 still higher degree than glass. For instance, here is some 

 of the luminous sulphide of calcium prepared according to 

 M. Ed. Becquerel's description. When the sulphide is 

 exposed to light even candlelight it phosphoresces for 

 hours with a bluish white colour. It is, however, much more 

 strongly phosphorescent to the molecular discharge in a 

 good vacuum, as you will see when I pass the discharge 

 through this tube. 



Other substances besides English, German, and uranium 

 glass, and Becquerel's luminous sulphides, are also phos- 

 phorescent. The rare mineral Phenakite (aluminate of 

 glucinum) phosphoresces blue ; the mineral Spodumene (a 

 silicate of aluminium and lithium) phosphoresces a rich 

 golden yellow ; the emerald' gives out a crimson light. 

 But without exception, the diamond is the most sensitive 

 substance I have yet met for ready and brilliant phos- 

 phorescence. Here is a very curious fluorescent diamond, 

 green by daylight, colourless by candlelight. It is 

 mounted in the centre of an exhausted bulb (Fig. 3), 



FIG. i. 



and the molecular discharge will be directed on it from 

 below upwards. On darkening the room you see the 



