stimulants to light production; examples of this class are mononitrobenzene, carbon disulfide and carbon tetrachloride; (2) those substances which are neutral in their action, neither provoking luminescence nor inhibiting it; examples of this class are hydrogen and nitrogen; (3) substances which poison the tissue and permanently prevent the production of light; examples of this class are bromine, sulfur dioxide and iodine cyanide. Strychnine and other alkaloids cause the production 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 chemical 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 Eeaumur as having made the same observation 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 American 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 specimen of the dry material. Moreover, this dried tissue gives, when moistened, 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 deduction from this fact is that at least three factors are necessary for the production of light by photogenic forms—water, oxygen and some material, as yet unknown, whose oxidation in the presence of water produces 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 spontaneous 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-
