PHOSPHORESCENCE.
Tilts is a name that has been applied to a property which many substances possess of emitting light under certain circumstances. Though the term would lead us to ima gine that it depended on the presence, or rather on the slow combustion of phosphorus, it has no connection with that, in any of the instances where it occurs, as far at least as chemical examinations have yet proceeded. Indeed, in every case, the whole of these processes are so mysteri ous, that we cannot pretend to throw any light on their real nature, and must, therefore content ourselves with treating of phosphorescence merely as a fact; as a branch of natural history, noting those circumstances with regard to it that are the most interesting.
We must however caution our readers against imagin ing that the various phosphorescences which we shall de scribe arise from a common cause, or have any connection; or that we have here placed them together from conceiv ing that they were all partsof one subject. On the contrary, it is most evident that the phosphorescence of minerals must depend on circumstances utterly different from those which cause vegetable matters and animals to yield light. In these two departments also there appears no analogy in the source of the luminous appearance ; nor are we even sure that among the animal creation possessed of this remarkable property, We have any reason to refer the light to a common cause. In truth, all these phenomena seem to be united by little more than a common term ; and, therefore, following the general usage, we shall proceed to treat of them in one article.
But if we cannot distinguish the causes of this appear ance, we can at any rate distinguish those by which it is excited, or the circumstances under which it appears. These seem to be five. Some substances emit light when their temperatures are raised; and this is more particu larly the case with mineral substances ; whether these are natural compounds, or minerals, properly so called, or chemical ones, such as the earthy metallic and alkaline salts, or the metallic oxides ; or, lastly, the simple earths. Others emit light after having been previously exposed to its action without any elevation of temperature ; a pheno mena which is also confined to the same classes of sub stances. Electricity excites it also in certain cases ; but the action of this cause is comparatively rare. As it is excited, in the fourth place, by friction, pressure, or per cussion, it is far better known, not only in many minerals, but, familiarly, in the case of sugar; and, finally, it ap pears to occur spontaneously, or, at least, without any ex citing cause which can be ascertained ; as is the case for the most part in animal and in vegetable substances.
In making these experiments by means of heat, it is ge nerally sufficient to place the substance to be tried on a heated body of iron or baked clay. The exceptions we shall note in their proper places hereafter. The heat must not be red, lest that light should interfere with the phos phorescence, and the substances must be examined in a dark room. When bodies are to be examined for their powers of absorbing light, it is sufficient to expose them to the direct rays of the sun for a few minutes, or even seconds. The mode of producing the results by electri city or friction, require no notice, except that the force or quantity of the latter that is required in different cases is very various.
Phosphorescence of Insects.
In the animal creation, the property of emiting light is confined to the insect tribe and to fishes ; appearing to be very limited in the former, but, as we hope to show hereafter, universal in the latter. We shall first describe the insects which possess this quality, premising at the same time that we do not imagine all the luminous species to be yet known.
This property has been observed in the genera Elater, Lampyris, Fulgora, Scolopcndra, Pausus, Limulus, Gala thia, and Lynceus ; but the observations of naturalists on most of these have been slight. We may notice the most remarkable. The Lampyris noctiluca is our common glow-worm, being an insect tolerably well known to most persons. In this animal the light proceeds from a pale spot on the under side of the abdomen towards the tail. Though without wings, it is the female of a winged beetle, so unlike, that the two would never be imagined to be long to one species. But the male also gives light, as has lately been ascertained ; yet not nearly so brilliant as that of the female. It is produced from four small points on each of the two last segments of the abdomen. Both the male and female have the power of extinguishing the light at pleasure; and they can also render it for a short time more brilliant than common. The Lampyris splendidula and hemiptera are also luminous ; and, in the former, the light is visible when the insect is on the wing.