20 PHOSP HORESCENCE 
Common salt (chloride of sodium), chloride of 
mercury, arsenious acid, etc., are phosphorescent 
only at a temperature of about 200° (centigrade). 
White flocconous oxide of zine may be heated 
to a very high temperature without melting or 
volatilizing ; but whilst heated it is observed to 
turn yellow, becoming white again on cooling. 
Now, whilst this transformation of colour from 
yellow to white is gomg on, the oxide of zinc is 
seen to glow with a faint blue phosphoric lght. 
This change of colour and this emission of light, 
observes Baudrimont (in his ‘ Traité de Chimie,’ 
vol. 1.), seem to indicate that the oxide of zinc 
undergoes what is termed an isomeric modification 
(change of chemical properties) at a high tem- 
perature, and returns again to its primitive state 
on cooling. 
Bendant affirms that a crystal of fluor-spar 
which is very perfect and transparent, will not be- 
come phosphorescent by heat until one of its sur- 
faces has been roughened a little on a piece of 
sandstone; he states also that diamonds which 
have not been cut are not phosphorescent by heat, 
but that they become so as soon as they are cut 
or polished. 
phosphorescent. This is not the case, however, if the fluoride 
has been previously heated enough to destroy its phosphorescence. 
Solution and precipitation have therefore no power to destroy or 
to restore this curious propertiy. 
