PHYSIOLOGIC LIGHT 119g 
stimulants to light production; examples of this class are mononitro- 
benzene, carbon disulfide and carbon tetrachloride; (2) those sub- 
stances which are neutral in their action, neither provoking lumines- 
cence nor inhibiting it; examples of this class are hydrogen and nitro- 
gen; (3) substances which poison the tissue and permanently prevent 
the production of light; examples of this class are bromine, sulfur diox- 
ide and iodine cyanide. Strychnine and other alkaloids cause the pro- 
duction of light, as do also certain poisons; oxygen appears to activate 
the production of light somewhat. 
Probably the most interesting fact so far developed by the chem- 
ical study of this phenomenon is that when photogenic tissues have 
been dried out, the dry tissue glows again when moistened with water 
in the presence of air. Carradori mentioned this fact in 1808, and 
quoted Spallanzani and Reaumur as having made the same observa- 
tion at earlier dates. Carus reported the same observation in 1864, and 
Dubois confirmed it some twenty years later. Professor Kastle and the 
writer have been able to perform the same experiment with the Ameri- 
can firefly; it is indeed a fact that the photogenic tissue of this insect 
may be dried, the dry material powdered, and the dry powder kept for 
some time away from access of moisture, and it will, when moistened in 
the presence of air or oxygen, glow again; indeed, by careful redrying, 
the same result may be obtained two or three times on the same speci- 
men of the dry material. Moreover, this dried tissue gives, when mois- 
tened, many of the same phenomena with chemical reagents as do the 
living insect and its freshly detached luminous organ. The property of 
thus glowing upon moistening after having been dried, does not appear 
to be confined to the luminous organ of the firefly, but appears to be a 
constant characteristic of luminous tissue as a class. The main deduc- 
tion from this fact is that at least three factors are necessary for the pro- 
duction of light by photogenic forms—water, oxygen and some ma- 
terial, as yet unknown, whose oxidation in the presence of water pro- 
duces light. 
Several theories have been advanced from time to time to account 
for the production of physiologic light. Probably the earliest view was 
that it was due to the presence of the element phosphorus. That this 
is not the case is best evidenced by the fact that there are only traces of 
this element present in the luminous tissues, and that which is present 
is in the form of phosphates. Yet this is the commonly accepted view 
of the cause of the phenomenon, and even as recently as 1880, Jousset 
de Bellesme suggested that the light might be produced by the spon- 
taneous combustion of phosphine. Carradori assumed that the luciole 
was capable of absorbing from the air or from its food, the “ material of 
light,” and of then emanating it again at pleasure. 
The fact that the light is unaccompanied by the evolution of meas- 
