280 MINERALOGY 



Phosphorescence is caused by the lengthening of the wave of the 

 absorbed energy, as the ultra-violet wave and the wave of the 

 Rontgen ray are too short to be detected by the eye, but when 

 lengthened to .00039 mm. affect the eye as light. 



Fluorescence is much like phosphorescence, only the phenomenon 

 continues during the actual exposure only, as in the barium plati- 

 nocyanide screen, used to visualize the Rontgen rays. Some white 

 fluorites fluoresce in the sunlight with a bluish, milky, or hazy 

 light. This is a property possessed by uranium compounds and 

 some compounds of boron. Uranium nitrate is used in the manu- 

 facture of fluorescent glass. 



