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

by Unknown

Accordingly, it seems fair to conclude that when all these conditions for the production of phosphorescence in a chemical laboratory are present in animal organisms, the phenomenon, when observed in these, is exactly of the same nature as that which is produced artificially. By that it is meant that animal phosphorescence is attended, like the artificial phenomenon, by a slow chemical action, or in other words, that the phosphorescent light is due to a gradual process of oxidation.

One curious circumstance has been discovered which lends still further probability to this explanation. It was mentioned above that among phosphorescent plants there are several species of Agaricus. Now, from one species of this genus, though not indeed one of the phosphorescent species (from A. muscarius) there has been extracted a principle called amanitia, which is found to be identical with cholin. In the light of the results derived from the investigations just referred to it is reasonable to draw the conclusion that, if sought for, this principle would likewise be found in the phosphorescent species in which the other conditions of phosphorescence are also present.

On this theory of the production of the phenomenon now under consideration, the effect of shaking or of vital action in giving rise to or intensifying the exhibition of the light is accounted for by the fact that by these means fresh supplies of oxygen are brought into contact with the phosphorescent substance. The effect of ammonia on the light emitted by the sea-slug Phyllirhoë bucephala, is also fully explained, ammonia being one of those alkaline substances which are so directly favorable to the exhibition of the phenomenon.

Nor is it difficult to account for the control which in some cases insects appear to have over the luminosity of the phosphorescent organs, exhibiting and withdrawing the light at will. It is not necessary to suppose that this is an immediate effect, a conversion of nerve force into light, and a withdrawal of that force. The action of the creature's will may be merely in maintaining or destroying the conditions under which the light is manifested. It may, for example, have the power of withdrawing the supply of oxygen, and this supposition receives some countenance from the observation cited from Kirby and Spence on the two captured glow-worms, one of which withdrew its light, while the other kept it shining, but while doing so had the posterior extremity of the abdomen in constant motion. But the animal may also have the power in another way of affecting the chemical conditions of the phenomenon. It may, for example, have the power of increasing or diminishing by some nervous influence the supply of the necessary alkaline ingredient.

But if animal phosphorescence is really due to a process of slow oxidation, there is one singular circumstance to be noted in connection with it. Oxidation is a process that is normally accompanied by the development of heat. Even where no light is produced an increase of temperature regularly takes place when substances are oxidized. We ought, then, to expect such a rise of temperature when light is emitted by the phosphorescent organs of animals. But the most careful observations have shown that nothing of the kind can be detected. It was with a view to test this that Panceri dissected out the luminous organs of so many specimens of Pholas. He selected this mollusk because it was so abundant in the neighborhood of Naples, where, his experiments were made; and in making his experiments he made use of a thermopile, an apparatus by which, with the aid of electricity, much smaller quantities of heat can be indicated than by means of the most delicate thermometer. The organs remained luminous long after they were extracted, but no rise in temperature whatever could be found to accompany the luminosity. Many experiments upon different animals were made with similar negative results by means of the thermometer.

The only explanation of this that can be given is probably to be found in the fact that the chemical process ascertained to go on in the phosphorescence of organic compounds on which experiments were made in the laboratory is an extremely slow one.

The so-called phosphorescence of most inorganic bodies is one of a totally different nature from that exhibited in organic forms. The diamond shines for a time in the dark after it has been exposed to the sun; so do pieces of quartz when rubbed together, and powdered fluor-spar when heated shines with considerable brilliancy. Various artificial compounds, such as sulphide of calcium (Canton's phosphorus, as it is called from the discoverer), sulphate of barium (Bologna stone, or Bologna phosphorus), sulphide of strontium, etc., after being illuminated by the rays of the sun, give out in the dark a beautiful phosphorescence, green, blue, violet, orange, red, according to circumstances. The luminous paint which has recently attracted so much attention is of the same nature. In these cases what we have is either a conversion of heat rays into light rays (as in the powdered fluor-spar), or the absorption and giving out again of sun-rays. In the latter case the phenomenon is essentially the same as fluorescence, in which the dark rays of the solar spectrum beyond the violet are made visible.

But we must now return to the other questions that have been started in relation to phosphorescence in animals. There has been much speculation as to the object of this light, and to the purposes it serves in Nature. Probably no general answer can be given to this question. It is no doubt impossible to show why so many animals have been endowed with this remarkable property; but we may consider some of the effects which the possession of it has in different cases.

In the first place, it will undoubtedly serve in many cases to afford light to enable the animal to see by, and in the Lampyridæ it would seem that the degree of luminosity is related to the development of the vision. In that family, according to the Rev. H.S. Gorham, the eyes are developed, as a rule, in inverse proportion to the luminosity. Where there is an ample supply of this kind of light the eyes are small, but where the light is insignificant the eyes are large by way of compensation. And moreover, where both eyes and light are small, then the antennae are large and feathery, so that the deficiency in the sense of sight is made up for by an unusual development in the organs of touch.