120 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. Robert 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, Watasé 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 “ Luciferase,” upon a substance 
of unknown composition, which he calls “ Luciferin,’ 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 Kélliker, 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 
