24 THE NATURE OF ANIMAL LIGHT 



The luminescence whicli appears in a vacuum tube 

 when an electric current is passed, through it is sometimes 

 spoken of as electroluminescence. As electrolumine,scence 

 and also thermoluminescence are really special cases of 

 phosphorescence or fluorescence and tribo- and crystallo- 

 luminescence are closely allied, the classification has only 

 the merit of emphasizing the means of producing light. 

 Let us examine each kind in turn in order that we may 

 place the light of animals, organoluminescence or bio- 

 ^uminescence (or hiophoto genesis), in one of these classes. 

 All are examples of "cold light," light produced at tem- 

 perature far below those observed in incandescent solids. 

 In this category should be placed also the light from salts 

 in the bunsen flame, for flame spectra and line spectra in 

 general, while only obtained at relatively high tempera- 

 tures, are not to be confused with the purely temperature 

 radiation from the incandescent particles of carbon in a 

 gas or candle light. The sodium or lithium flame, etc., 

 is not a simple function of temperature and has been 

 spoken of as a luminescence, pyroluminescence. As the 

 luminescence of organisms could in no manner be re- 

 garded as a pyroluminescence, occurring at temperatures 

 far above those compatible with life, a consideration of 

 this form of luminescence will be omitted. Some otheV 

 low temperature flames are known, such as that of OS2 in 

 air, rich in ultra-violet rays, despite its relatively low 

 temperature. While these are of interest to the physicist 

 and chemist, they can have no direct bearing on the lumi- 

 nescence of animals and their consideration will also be 

 omitted. (See Bancroft and Weiser, 1914-1915.) 



Thermoluminescence. — Some substances begin to emit 

 light of shorter wave-length than red, well below 525°. 



