18 



CHAPTEE II. 



PHOSPHOKESCENCE BY HEAT. 



MANY substances become phosphorescent when 

 they are heated to a certain temperature. Such, 

 for instance,, are fluor-spar, lime, sulphuret of cal- 

 cium, diamonds, etc. They manifest their phos- 

 phoric qualities when, after being pulverized or 

 broken into small fragments, they are thrown upon 

 a heated surface. 



Fig. 2. 



Some varieties of apatite (fig. 2 phosphate of 

 lime with fluoride or chloride of calcium) are 

 phosphorescent when heated, especially the 

 coarser varieties. I have proved that their phos- 

 phorescence is owing to the fluoride of calcium 

 which forms part of the mineral, for pure phos- 

 phate of lime does not show any phosphoric light 



