LUMINESCENCE AND INCANDESCENCE 29 



teria will continue to luminesce although they are grown 

 in the dark for many weeks. Indeed strong light has 

 a bactericidal action on these fonns similar to that with 

 ordinary bacteria. With some marine forms light has 

 an inhibiting effect. They lose their power of lumines- 

 cence during the day and only regain it at dusk or when 

 kept in the dark for some time. Indeed, ordinary light 

 never has the effect of causing luminescence in the same 

 sense as it causes phosphorescence of CaS. 



Fluorescence is most efficiently excited by the cathode 

 rays of a vacuum tube. They not only cause the residual 

 gas in the tube to glow (electroluminescence) by which 

 their path may be followed with the eye, but also a vivid 

 fluorescence of the glass walls of the tube, yellow green 

 with sodium glass, blue green with lead and lithium glass. 

 LiClj in the path of cathode rays gives off a blue light; 

 in the path of anode rays a red light ; NaCl a blue cathodo- 

 luminescence and a yellow anodoluminescence. The spec- 

 trum of the latter is a line spectrum of Li or Na, showing 

 the characteristic red or yellow lines similar to those 

 observed where Li or Na is held in the bunsen flame. The 

 spectrum of the salts under excitation of cathode rays 

 is a short continuous one in the blue region. Fluorescent 

 spectra in general are of this nature, made up of short 

 bands of light in one or more regions. 



Diamonds, rubies and many minerals fluoresce bril- 

 liantly in the path of cathode rays. Some specimens of 

 fluorite (CaFg) show the phenomenon especially well, 

 whence the name fluorescence. Fluorescent screens of 

 barium platino-cyanide, willemite (Zn2Si04) Sidot blend 

 (ZnS) or Scheelite (Ca tungstate) are frequently em- 

 ployed to render visible X-rays. The luminous paint most 



