i2o THE POPULAR SCIENCE MONTHLY 



urable amounts of heat certainly shows that if it is indeed a combus- 

 tion, it is a most remarkable one and one which differs from any 

 analogous process known to us. The view that the light might be the 

 result of oxidation has, however, long been held. Eobert Boyle made 

 experiments on this point in 1667, and concluded that the light pro- 

 duced by shining wood and fish was not affected by the absence of air, 

 and was therefore not what we now call a combustion or oxidation. 

 Spallanzani, as the result of his studies on luminous sea forms, came to 

 the opposite conclusion, in which he was opposed by Macartney and 

 Carradori. More recently this phase of the subject has been studied by 

 Dubois, Watase and Townsend, all of whom have published very inter- 

 esting observations. As a result of these several observations the con- 

 clusion must be drawn that oxygen is essential to the process of the 

 production of physiologic light, and that we have in this phenomenon 

 a true but remarkable form of combustion. Of the mechanism of this 

 process we are still very ignorant. Dubois's theory is that the light is 

 produced as the result of the action of an " oxidase " (oxidizing fer- 

 ment), to which he has given the name " Lucif erase," upon a substance 

 of unknown composition, which he calls " Lucif erin," the latter being 

 oxidized by the atmospheric oxygen through the agency of the ferment. 

 It is a little early to accept this hypothesis finally, although it cer- 

 tainly presents some analogy to known processes — for example, the pro- 

 duction of the black pigment melanin through the action of the oxidase 

 tyrosinase upon the organic compound tyrosin. Phipson had already 

 described a substance he called " Noctilucin " as the active principle 

 of physiologic light; it seems possible that Phipson isolated and ana- 

 lyzed a culture of photogenic bacteria. 



In this connection the structure of the light organs of various ani- 

 mal forms has been given special attention. In general, the results of 

 studies on those forms having special photogenic organs have been es- 

 sentially similar. Briefly, the luminous organs appear to be masses of 

 cells of some special kind, possibly a fat-derivative, or according to 

 Macaire and Kolliker, an albuminous substance penetrated by a net- 

 work of trachea (tracheoles), and as the result of some chemical ac- 

 tion, apparently oxidation, taking place in these tissues, the light is 

 produced. Whether these tracheoles are in life filled with air or with 

 a liquid seems doubtful; the evidence is contradictory so far as given, 

 but it seems quite probable that they convey air. 



What is the purpose of this production of light? Of what value is 

 it to the forms which possess it ? This is another side of the " secret of 

 the firefly/' which has yet to be solved. Quite probably the function 

 bears some relation to the reproductive life of the insect. The females 

 of the local species (Photinus pyralis K.) give a very much less bright 

 light than, and are quite rare as compared with, the males; one female 



