32 THE NATURE OF ANIMAL LIGHT 



the light production. A trace of alkali usually increases 

 and acid inhibits the fluorescence of solutions. 



Triboluminescence and Piezoluminescencb. — ^Under 

 this head are grouped a number of light phenomena which 

 at first sight may appear to be electrical in nature but in 

 reality are not. The light is produced by shaking, rubbing, 

 or crushing crystals, and only crystalline bodies appear 

 to show triboluminescence or piezoluminescence. A strik- 

 ing ease is that of uranium nitrate. Gentle agitation of 

 the crystals is sufficient to give off sparks of light which 

 much resemble the scintillations of dinoflagellates when 

 sea-water containing these animals is agitated. If Rom- 

 berg's phosphorus, which is fused CaClj, is rubbed on the 

 sleeve, it glows with a greenish light. Lumps of cane 

 sugar rubbed together will glow. Saccharin crystals will 

 also light if shaken and Pope (1899) found that the bluish 

 light of saccharin was bright enough to be visible in a 

 room in daytime. It only appeared from impure crystals 

 and freshly crystallized specimens. Other crystals, also, 

 have been found to lose their power of lighting after 

 a time. 



Among biological substances, cane sugar, milk sugar, 

 mannite, hippuric acid, asparagin, r-tartaric acid, ?-malic 

 acid, vanillin, cocaine, atropin, benzoic acid, and many 

 others show triboluminescence. A long list is given by 

 Tschugaeff (1901), by Trautz (1905), and by Gemez 

 (1905) . The spectrum is a short continuous one, the waves 

 emitted depending on the kind of crystal. Thus the color 

 of the light varies among different santonin derivatives 

 from yellow to green. In saccharin it is blue. 



Although the light produced by some living organisms 

 resembles triboluminescence in that it may be evoked by 



