From an Easy Chair

A correspondent lately described in a letter to a London newspaper what he believed to have been “a luminous owl,” which was seen flying about at night in Norfolk. He mentioned the well-known fact that the dense greasy patch of feathers on the breast of the heron is said to be luminous by many trustworthy observers. It is very probable that it was some carnivorous or fish-eating bird, which was thus seen in a luminous condition at night. The occurrence is much more in accordance with known facts than most people would suppose to be the case. Light, even strong light, is produced by many natural objects without the accompaniment of heat. We usually expect not merely fire where there is smoke, but heat—in fact, great heat, where there is light or flame. Yet there are many instances to the contrary, and the word “phosphorescence” is used to indicate a production of light without heat in reference to the fact that phosphorus is luminous, even when covered with water, although no appreciable heat accompanies the light such as we are accustomed to observe in ordinary “combustion” or burning.

There is more than one kind of phosphorescence. We separate the phosphorescence which is due to the oxidation of peculiar fatty matters in the bodies of plants and of animals (such as glow-worms) from that which is caused by the breaking or heating of crystals (white arsenic and apatite), or by longer or shorter exposure to the sun’s rays (luminous paint), or by radio-activity, or by electrical discharges in vacuum tubes.

The “luminous owl” of the above-mentioned correspondent and the luminous breast of the heron probably owe their strange appearance to the birds having smeared themselves with phosphorescent carrion or dead fish, the luminosity of which is due to bacteria. The simplest case of phosphorescence in living things is that of the almost ubiquitous phosphorescent bacteria, minute microbes like those which cause putrefaction. They can be obtained and cultivated from almost any sample of sea water. A thin slice of meat placed in a shallow dish of salt water, so as to be barely covered by the liquid, will in cool, damp weather, almost certainly become covered with the growth of this phosphorescent germ and appear brilliantly luminous. The populations of seaside towns have often been terrified by all the meat in the butchers’ shops suddenly becoming thus phosphorescent. The growth may be cultivated in flasks of salt broth. I have prepared such flasks, which, when shaken so as to introduce oxygen, give out a heatless blaze of light of a greenish colour, brilliant enough to light up a room. I once found a bone in a dog’s kennel which was brilliantly phosphorescent owing to this bacterium. I kept it for several days and showed it to Huxley as well as to other friends. A certain kind of phosphorescent bacteria are parasitic in the blood of sandhoppers, causing a disease which kills them. The diseased sandhoppers shine like glow-worms. I have found them abundantly on the sea shore near Boulogne and near Trouville, but not yet on the English coast. The bacteria can be seen with the microscope and inoculated from diseased luminous sandhoppers into healthy ones by using a needle to prick first the diseased and then the healthy creature.

The animals of the sea are often provided with secreting organs, producing a fatty body which can be oxidised and made luminous at the pleasure of the animal. Thus many marine worms and minute sea-shrimps give out brilliant flashes of light. Jelly-fish of many kinds, and the minute noctiluca, no bigger than a pin’s head, and the three-horned animalcule Ceratium tripos are the usual cause of the phosphorescence of the sea on our own coast. Deep-sea fishes are provided with large phosphorescent discs or plates on the surface of the body, which are sometimes furnished with lenses like a bull’s-eye lantern. Glow-worms and fire-flies and some tropical beetles are examples of insects which have fatty phosphorescent organs which they can illuminate (oxidise) at pleasure, under the control of the nervous system. Some of the West Indian phosphorescent beetles are remarkable for having “lights” of two different colours. In the marshes around Mantua the fire-flies are so abundant at the end of June that the air for miles is full of them, and the sight so extraordinary and beautiful as to be worth a long journey to see. I have seen fire-flies as far north as Bonn on the Rhine. Once I was nearly upset by a horse shying at a glow-worm on a bank in Worcestershire. Some moulds and well-grown toadstools are phosphorescent, and a phosphorescent earthworm, a peculiar species, now well known, was first of all discovered in the South of Ireland by the late Professor Allman. In the autumn I have often picked up the phosphorescent centipede, which is remarkable for the fact that the phosphorescent material is a kind of slime which exudes from the body—the creature leaving thus a luminous trail behind it as it crawls. The piddock, or pholas—a boring sort of mussel—has brilliant phosphorescent glands, and the boys at Naples love to munch these shell-fish at night, and then to alarm the passer-by by opening their mouths, and showing a brilliant green light within. Cases are recorded, but not recently, of persons suffering from tuberculosis becoming phosphorescent; a possible, but certainly a rare, occurrence. Animal and vegetable phosphorescence is varied in colour. The light emitted is blue-green, green, yellow, orange, and even red in different cases. It is always due to the oxidation of a separate fatty chemical body, which can in many instances be extracted, then dried, and subsequently made luminous by moistening with ether, in consequence of which oxidation by the oxygen of the atmosphere is facilitated.