20 PHOSPHORESCENCE 



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 zinc 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 going on, the oxide of zinc is 

 seen to glow with a faint blue phosphoric light. 

 This change of colour and this emission of light, 

 observes Baudrimont (in his ' Traite de Chimie/ 

 vol. ii.), 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 property. 



