There are several varieties of flies, and three species of beetles of the genus _Elater_, which have the power of emitting luminous rays. The great lantern-fly of South America is one of the most brilliant, a single insect giving sufficient light to enable a person to read. In Surinam a very numerous class of these insects are found, which often illuminate the air in a remarkable manner. In some of the bogs of Ireland a worm exists which gives out a bright green light; and there are many other kinds of creatures which, under certain circumstances, become luminous in the dark. This is always dependent upon vitality; for all these animals, when deprived of life, cease to shine.
At the same time we have many very curious instances of phosphorescence in dead animal and vegetable matter; the lobster among the Crustacea, and the whiting among fishes, are striking examples; decayed wood also emits much light under certain conditions of the atmosphere. This development of light does not appear to be at all dependent upon putrefaction; indeed, as this process progresses, the luminosity diminishes. We cannot but imagine that this light is owing, in the first place, to direct absorption by, and fixation within, the corpuscular structure of those bodies, and that it is developed by the decomposition of the particles under the influence of our oxygenous atmosphere.
The pale light emitted by phosphorus in the dark is well known; and this is evidently only a species of slow combustion, a combination of the phosphorus with the oxygen of the air. Where there is no oxygen, phosphorus will not shine; its combustion in chlorine or iodine vapour is a phenomenon of a totally different character from that which we are now considering. This phosphorescence of animal and vegetable matter has been regarded as something different from the slow combustion of phosphorus; but, upon examination, all the chemical conditions are found to be the same, and it is certainly due to a similar chemical change.
The luminous matter of the dead whiting or the mackerel may be separated by a solution of common salt or of sulphate of magnesia; by concentrating these solutions the light disappears; but it is again emitted when the fluid is diluted. The entire subject is, however, involved in the mystery of ignorance, although it is a matter quite within the scope of any industrious observer. The self-emitted light of the carbuncle of the romancer is realized in these remarkable phenomena.