292 THE BODY AT WORK 



photographic plate may be exposed for several minutes, almost 

 without changing, to the light of a firefly bright enough to 

 enable one to read with ease in a dark room ; whereas light of 

 equal brilliance from any other source would change it in the 

 fraction of a second. Nor are heat-rays mixed with the light. 

 Measurements show that the activity of the photogenic organs 

 does not give rise to any greater rise of temperature than would 

 occur in the case of any other gland. 



The contrast between the emission of light by an animal and 

 its production in any other manner is very striking when the 

 physical evidence, or want of evidence, of what happens in the 

 protoplasm which produces it is considered. The fact that 

 no heat accompanies the light precludes us from attributing 

 it to oxidation. If a firefly is enclosed in a vessel of oxygen, 

 its lamp burns no brighter clear evidence that its luminosity 

 has nothing in common with the burning of a match or the 

 glowing of a stick of phosphorus. Nor is the lamp put out 

 when the insect is suddenly exposed to great cold ( 100 C.). 

 It continues to shine until the cold kills it. There is no relation 

 between the luminosity of a firefly and the phenomenon 

 termed " phosphorescence " by physicists. Sulphide of cal- 

 ciumthe substance used for rendering matchboxes visible 

 in the dark returns light which it has absorbed. A firefly's 

 power of emitting light is in no wise affected by keeping it 

 for a long while in the dark. 



Like all other events in vital chemistry, the generation of 

 light by protoplasm is due to a process of fermentation. The 

 luminous organs may be crushed, and the mixture of ferment- 

 able substance and ferment extracted with water. The 

 extract is luminous. If an extract is prepared rapidly, and 

 evaporated to dryness in vacuo, the residue glows when 

 moistened with water. That two substances are present in the 

 extract, one (luciferin) fermentable, the other (luciferase) 

 a ferment, is proved by the following experiment : A certain 

 quantity of extract is divided into two portions. One part (A) 

 is allowed to glow until its capacity for emitting light is ex- 

 hausted. The other portion (B), as soon as it is separated, 

 is heated to 55 to kill the ferment. B still contains luci- 

 ferin ; A contains luciferase, although all its luciferin has 

 been used up. Recombined, the extract is luminiferous. 



