A. Phosphorescence in Minerals.— In addition to the phosphorescence after insolation already alluded to (see Light, vol. xiv. p. 603) many minerals exhibit this property under other circumstances: (a) on heating to a temperature much below what is known as “red heat” (fluorspar, lepidolite, quinine)—this being often attended with a change in molecular structure or in specific heat; (6) on friction, as in the case of fused calcium chloride (Homberg’s phosphorus); (c) on cleavage, a property manifested by mica, the two split portions becoming electrified— the one positive, the other negative; (√) on crystallization, as boracic acid after fusion, or water on rapid freezing.

A few meteorological phenomena may here be mentioned. Rain has been seen to sparkle on striking the ground, and waterspouts and meteoric dust have presented a luminous appearance. The ignis fatuus, or will-o’-the-wisp, seen in marshy districts, has given rise to much difference of opinion: Kirby and Spence suggest that it may be due to luminous insects; but this explanation will certainly not apply in all cases, and it is perhaps on the whole more reasonable to believe that the phenomenon is caused by the slow combustion of marsh gas (methyl hydride).

B. Phosphorescence in Organisms. —The vegetable kingdom has furnished few instances of the property under consideration; the earliest on record took place in the year 1762, when a daughter of Linnaeus saw luminous emanations from a species of Tropaeolum, since which time a like appearance has been noticed in Helianthus annuus, Lilium bulbiferum, Calendula officinalis, Tagetes patula, and T. erecta, all of which are red or orange-coloured flowers. A few cryptogams have been seen to shine in the dark, e.g., Schistostega οsmundacea among the liverworts; Rhizomorpha subterranea, Fungus igneus in Amboyna, and other fungi in Brazil and Italy; and the mycelium (thread-like fibres) of other species growing in decayed wood is also occasionally luminous. There are also a number of small marine phosphorescent organisms {Pyrocystis, Peridiniuin), concerning which it is impossible to say with certainty whether they should be referred to the animal or the vegetable kingdom. But the most brilliant as well as the most varied and interesting cases of phosphorescence belong to the animal world, and there is not one of the larger groups which does not furnish some instances of it.

Nature of the Light.— The light emitted by different animals varies very much in colour: green has been noticed in the glow-worm, fire-flies, some brittle-stars, centipedes, and annelids; blue is seen in the Italian firefly {Luciola italica) ; and this and light green are the predominant colours exhibited by marine animals, although the beautiful Girdle of Venus and some species of Sal·pa and Cleodora appear red, and Pavonaria and other gorgonoids lilac. The curious lantern-fly { Fulgor a pyrorhynchus) has a purple light. One very remarkable instance is mentioned of an Appendicularia in which the same individual appeared first red, then blue, and finally green. In comparatively few cases has the light been examined by the spectroscope. Panceri states that in every instance observed by him it was monochromatic, the spectrum consisting of a continuous band without any separate bright lines; in Pholas this band extended from the line E of the solar spectrum to a little beyond F; in UmbeUula, examined on the voyage of the “Challenger,” it was sharply included between the lines b and D. Luminous Organs.— In the lowest forms of life and in many jelly-fish there seem to be no organs specially set apart for the production of light, this being emitted from the whole surface of the body; but even in the latter group a degree of specialization is found, for in some it is only the marginal sense-organs, in others the radial canals and ovaries, that are luminous. In other groups of animals the localization of the photogenic property in certain organs or tissues is universal, and these present the utmost variety in structure and situation. In the sea-pens {Pennatula) every polyp has eight luminous bands on the outer surface of the stomach; when the colony is touched the light commences at the point irritated and then spreads to other portions. Pyrosoma, a colonial free-swimming ascidian, has two small patches of cells at the base of each inhalent tube; the cells have no nucleus, but contain a material which appears from its chemical relations to be fatty; as in Pennatula, the light spreads from the irritated point. In the transparent pelagic mollusc {Phyllirrhoe) there are rounded cells connected with the nerve-twigs from which, as also from the ordinary cells of the nerve-ganglia, the light emanates. Several annelids {Chaetopterus, Tomopteris) have luminous organs at the bases of lateral processes of the body. The rock-boring mollusc {Pholas), whose phosphorescent properties were known as long ago as the time of Pliny, has three distinct luminous organs—(1) a curved band along the anterior border of the mantle, (2) two small triangular patches at the entrance of the anterior siphon, and (3) two long parallel cords situated within this latter; these are all covered with ciliated epithelium, like that of other parts of the mantle, but having granular contents.

The glow-worm {Lampyris spleηdiduld) has been investigated by Max Schultze; he finds that the male has a pair of organs in each of the two segments preceding the last in the abdomen; each organ consists of a pale transparent superficial layer, which gives off the light, and a deep opaque layer, whose function is less obvious, but which may serve as a reflector. Quite recently Emery has examined the Italian fire-fly, in which both male and female are luminous. As in the glow-worm, the organ consists of two layers: the dorsal contains large quantities of uric acid salts; while in the ventral layer there are clear cells arranged in cylindrical lobules, which surround vertically-disposed tracheal limbs—a structure comparable to the stellate tracheal cells of Schultze. The luminous organs are regarded as homologous to the “fat body” so common in insects. The ultimate branches of the tracheae ramify in these and terminate in peculiar star-like cells; nervefibres are also present. The Mexican fire-flies {Pyrophorus) are in most respects similar to the glow-worm, but have a pair of organs in the thorax and one in the abdomen, whilst the lantern-flies {Fulgora) carry their light at the extremity of a long curved proboscis. Many crustaceans are luminous, but in most cases it has not been observed from what part of the body the light emanates; in some instances, however (Tlιysanopoda ∖ Nyctiphanes ∖ norvegica, Euρhausia pellucida, <fcc.), there are small globular phosphorescent organs, which have often been described as eyes, beneath the thorax and between the abdominal swimmerets. Sars states that “these globules . . . constitute a highly complicated luminous -apparatus, the lenticular body of the organs, generally described as a true eye-lens, acting as a condenser, which . . . enables the animal to produce at will a very bright flash of light in a given direction.” Mr John Murray in the same place records the occurrence of a very brilliant display of this phosphorescence during the “Triton” expedition in the Faroe Channel.

Many deep-sea fish possess round shining bodies imbedded in the skin, either in the vicinity of the eye or along the sides of the body; some of these resemble modified eyes, whilst the structure of others recalls a glandular organ without the usual duct, and it is supposed that some or all of these are luminous organs, the lens in the former group acting as a bull’s eye (see Ichthyology, vol. xii. p. 684).

Dead and putrescent animals are not unfrequently phosphorescent; this fact has most commonly been observed in fish, though instances are not wanting in which the property has been manifested by molluscs and other animals, and even by the human body. Furthermore, a few startling but apparently well-authenticated instances are on record in which human beings have been luminous while yet alive owing to certain states of disease. Causes of Phosphorescence.— On this head it is at present impossible to write with certainty; it seems likely, however, from the variety of the effects produced by different chemical and physical agents, that the causes are manifold. In many instances light is only emitted after stimulation, either mechanically, chemically (by fresh water, milk, ammonia), or by electricity, though there are cases in which this last has no effect whatever. The fact that the nervous system is so often closely connected with the luminous organs indicates that the exhibition of the light is either dependent on the volition of the animal or is the reflex result of the stimulation of sensory nerves (Panceri). In the glow-worm the distribution of tracheae (air-tubes) throughout the photogenic apparatus, and the fact that carbonic acid extinguishes the light while oxygen intensifies it, suggest that it is due to some form of slow combustion, while the fatty contents of the luminous cells of this and many other animals point to the probability that a fat containing free phosphorus is the active agent in the process. Since a large number of luminous organs retain their power after the death of the animal, and even after desiccation and subsequent moistening, there seems no necessity to adopt the theory that we have to deal with an instance of the direct transformation of vital into radiant energy.

The well-known phosphorescence of the sea is due to the animals which inhabit it, except in a few cases in which it has been ascribed to putrescent matter. This was known as long ago as 1749, when Vianelli discovered in the waters of the Adriatic a luminous animalcule which was named by him Nereis noctiluca, and was probably the creature now known as Noctiluca miliaris. This minute animal swarms in countless myriads on the surface of the sea not very far from land, and is the commonest cause of its diffuse luminosity, although other low forms of life such as Peridinium (Ceratium) contribute in no small degree; and in mid-ocean another organism, Pyrocystis, which has often been mistaken for Noctiluca, appears to replace it, and is very abundant. The brilliant sparkling phosphorescence more rarely seen is caused by the presence of copepods and other small surface crustaceans.

Uses of Phosphorescence.— The service rendered by this property to its possessors is in many cases by no means obvious; indeed it would seem certain that to crustacean larvae and other surfaceorganisms surrounded by voracious enemies phosphorescence must be a “perilous gift.” It is possessed by so many anthozoa and jelly-fish, which have also stinging organs, that fish have perhaps learned to shun instinctively all phosphorescent animals; fishermen state that fishes avoid nets in which phosphorescent Medusae have become entangled; if such be the case, it would be possible for animals otherwise defenceless to obtain protection by acquiring this property. A similar hypothesis has been propounded with respect to the Italian fire-fly, although, as regards the glow-worm, it has been generally believed that the light serves to attract the opposite sex, and the same has been stated with respect to the earth-worm. The fact that so many deep-sea animals are phosphorescent, coupled with the discovery that many fish from those regions have large and normally-developed eyes whilst others have organs which appear to be adapted for the production of light, has led to the belief that this source of light becomes of great importance in the depths of the ocean where no sunlight penetrates,—an hypothesis which is known as the “abyssal theory of light.”