Young Folks' Library, Volume XI (of 20) / Wonders of Earth, Sea and Sky

by Unknown

The whole phenomenon is one that gives rise to a good many questions which it is not easy to answer, and this is especially true in the case of animal phosphorescence. What is the nature of the light? What are the conditions under which it is manifested? What purpose does it serve in the animal economy?

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.

This fact of itself might be taken as a strong indication of the chemical nature of the process to which phosphorescence is due. But the problem has been made the subject of further investigations which have thrown more light upon it. It was long known that there were various inorganic bodies besides phosphorus which emitted a phosphorescent light in the dark, at least after being exposed to the rays of the sun; but it was not till quite recently that any organic compound was known to phosphoresce at ordinary temperatures.

This discovery was made by a Polish chemist, named Bronislaus Radziszewski, who followed it up with a long series of experiments on the phosphorescence of organic compounds, by which he was able to determine the conditions under which that phenomenon was exhibited. In all the substances investigated by him in which phosphorescence was introduced he found that three conditions were essential to its production: (1) that oxygen should be present; (2) that there should be an alkaline reaction in the phosphorescing mixture—that is, a reaction such as is produced on acids and vegetable coloring matters by potash, soda, and the other alkalies; and (3) that some kind of chemical action should take place.

He found, moreover, that among the organic compounds that could be made to phosphoresce under these conditions were nearly all the fixed and ethereal oils. With reference to the phosphorescence of animals, this observation is important, for it has been shown in a great many cases that a fatty substance forms the main constituent in their luminous organs. This has long been known to be the case in the luminous insects belonging to the Lampyridæ and Elateridæ, as well as in the luminous centipedes; and the researches of Panceri, already referred to, on the luminous organs of many marine forms have shown that it holds good with regard to these also.

We may, therefore, conclude that substances fitted to phosphoresce under the conditions determined by the experiments of Radziszewski are generally, and probably universally, present in the luminous organs of phosphorescent animals. Now, what is to be said as to the occurrence of these conditions? The access of oxygen is in all cases easy to account for, but it must also be shown how the alkaline reaction is to be produced. We need not expect to find in animal organisms potash, soda, ammonia, and the other common alkalies; but it was established by experiment that the alkaline organic compounds cholin and neurin, which are present in animal tissues, would also serve to bring about the phenomenon of phosphorescence in the substances on which the experiments were made.