As to the nature of the light, the principal question is whether it is a direct consequence of the vital activity of the organism in which it is seen, of such a nature that no further explanation can be given of it, any more than we can explain why a muscle is contracted under the influence of a nerve-stimulus; or whether it is due to some chemical process more or less analogous to the burning of a candle. The fact of luminosity appearing to be in certain cases directly under the control of the creature in which it is found, and the fact of its being manifested in many forms, as M. de Quatrefages found, only when muscular contraction was taking place, would seem to favor the former view. On the other hand, it is against this view that the phosphorescence is often found to persist after the animal is dead, and even in the phosphorescent organs for a considerable time after they have been extracted from the body of the animal. In the glow-worm the light goes on shining for some time after the death of the insect, and even when it has become completely extinguished it can be restored for a time by the application of a little moisture. Further, both Matteucci and Phipson found that when the luminous substance was extracted from the insect it would keep on glowing for thirty or forty minutes. In Pholas the light is still more persistent, and it is found that when the dead body of this mollusk is placed in honey, it will retain for more than a year the power of emitting light when plunged in warm water. The investigations of recent years have rendered it more and more probable that the light exhibited by phosphorescent organisms is due to a chemical process somewhat analogous to that which goes on in the burning of a candle. This latter process is one of rapid oxidation. The particles of carbon supplied by the oily matter that feeds the candle become so rapidly combined with oxygen derived from the air that a considerable amount of light, along with heat, is produced thereby. Now, the phenomenon of phosphorescence in organic forms, whether living or dead, appears also to be due to a process of oxidation, but one that goes on much more slowly than in the case of a lighted candle. It is thus more closely analogous to what is observed in the element phosphorus itself, which owes its name (meaning "light-bearer") to the fact that when exposed to the air at ordinary temperatures it glows in the dark, in consequence of its becoming slowly combined with oxygen. At one time it was believed that the presence of oxygen was not necessary to the exhibition of phosphorescence in organic forms, but it has now been placed beyond doubt that this is a mistake. Oxygen has been proved to be indispensable, and hence we see a reason for the luminous organs in the glow-worm being so intimately connected, as above mentioned, with the air-tubes that ramify through the insect.