102 THE NATURE OF ANIMAL LIGHT 



bacteria) and the mucous secretion of Scolopendra elec- 

 trica or the luminous matter of the glowworm to be this 

 material, noctilucin, which, ^4n moist condition, takes up 

 oxygen and gives oif CO2 and when dry appears like 

 mucin. ' ' Phipson says that it forms an oily layer over the 

 seas in summer (he probably refers to masses of dino- 

 flagellates), is liquid at ordinary temperatures and less 

 dense than water, smells a little like capryiic acid, is 

 insoluble in water but misceable with it, insoluble in alco- 

 hol and ether, dissolves with decomposition in mineral 

 acids and alkalies and contains no phosphorus. We can 

 see from this description that the word ^* noctilucin'^ does 

 not indicate a chemical individual, but it is the earliest 

 attempt to definitely designate the luminous substance. 



The idea of a definite substance oxidizing and causing 

 the light has been upheld by a number of investigators, 

 and many years later Molisch called this substance the 

 photo gen. He contrasts the '^photogen theory'' with 

 certain other views of light production, which may be 

 spoken of as ** vital theories," notably those of Pfliiger 

 (1875), who looked upon luminescence as a sign of intense 

 respiration, and of Beijerinck (1915), who regarded the 

 light as an accompaniment of the formation of living 

 matter from peptone. 



Fortunately biological science has advanced beyond 

 the stage where a living process can be explained by 

 calling it a vital process, and we must fall back upon the 

 idea of a photogen oxidizing with light production. In- 

 deed, it is now possible to go much further than this 

 and describe the properties of the photogen, but we must 

 not lose sight of the fact that it was recognized very 

 early in the history of Bioluminescence, that water. 



