PHOSPHORESCENCE BY HEAT. 19 
when heated; nor do most of the so-called chlor- 
apatites, which contain chloride of calcium substi- 
tuted in part or wholly for the fluoride. 
Of all these substances, the most remarkable 
is fluor-spar (fluoride of calctum—fig. 3). When 
Fig. 3. 
thrown in the dark upon heated mercury, into 
boiling water, or on to a hot shovel, this mineral 
immediately emits a brilliant phosphoric light. 
Some specimens possess this property to a greater 
extent than others. A certain green variety of 
fluor-spar called Chlorophane becomes phospho- 
rescent at the low temperature of 20° or 25° (cen- 
tigrade), which is almost that of our summers. 
Rare descriptions of chlorophane become phos- 
phorescent in a dark room from the mere warmth 
of the hand. According to Landrm (Dict. de 
Minéralogie) some varieties are almost constantly 
luminous in the dark.* 
_ * Fluoride of calcium loses its phosphoric property after it 
has been once heated. Miller asserts (‘ Elem. of Chemistry’) that 
when a phosphorescent fluoride of calcium is dissolved in hydro- 
chloric acid, and then precipitated by ammonia, the precipitate 1s 
