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CHAPTER II. 
PHOSPHORESCENCE 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 ther phos- 
phoric qualities when, after beimg 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 hght 
