PHOSPHORESCENCE, a name given to a variety of phenomena due to different causes, but all consisting in the emission of a pale more or less ill-defined light, not obviously due to combustion. The word was first used by physicists to describe the property possessed by many substances of themselves becoming luminous after ex-posure to light. Such bodies were termed " phosphori," and the earliest known appears to have been barimn sulphide, which was discovered by Vincenzo Cascariolo, a cobbler of Bologna, at the beginning of the 17th century. See Puosmenus. Subsequently, when certain animals were observed to be similarly endowed, the word "phos-phorescent " was applied to them also. It is clear, how-ever, that the light derived from previous exposure to light, which thus becomes, as it were, stored up, is hardly comparable with that which is produced by living proto-plasm and evidently under the control of the nervous system. It has been suggested that this latter should have a special name appropriated to it, and here it will certainly be convenient to divide the subject into two heads in accordance with this distinction.
A. PlioSPHORESCENCE tO the phosphorescence after insolation already alluded to (see LIGHT, v01. 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 mani-fested by mica, the two split portions becoming electrified - the one positive, the other negative ; (d) 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 lunch 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. - Tile 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 Linnams saw luminous emana-tions from a species of Tropxolum, since which time a like appearance has been noticed in IIelianthus annwus, Litho?' Indlaferum, Calendula offieinalis, 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, Sehistostega osmundacea among the liverworts ; Rhizo-nzorpha 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 2 (Pyroeystis, l'eridinium), concerning which it is impossible to say with certainty whether they should be referred to the animal or the vege-table kingdom. 13ut 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.
Krature 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 fire-fly- (Luciola italica) ; and this and light green are the pre-dominant colours exhibited by marine animals, although the beautiful Girdle of Venus and some species of Sa/pa and Cleodora appear red, and l'avonaria and other gorgonoids lilac. The curious lantern-fly- (Fulgora pyrorhynchns) has a purple light. One very remarkable instance is mentioned of an Appendieularia in which the same individual appeared first red, then blue, and fina,lly green.3 In comparatively few cases has the light been examined by the spectroscope. Pa,nceri 4 states that in every instance observed by him it was monochromatic, the spectrum consisting of a continu-ous band without any separate bright lines ; in Pho/as this band extended from the line E of the solar spectrum to a little beyond I' ; in Umbellula, examined on the voyage of the " Challenger," it was sharply included between the lines b and IV L'UlitinOUS Organs. - Th 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 surfac.e 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 ra.dial canals and ovaries, that are luminous. In other groups of animals the localization of the photogenic property in certain organs or tissues is univer.s.al, 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 com-mences at the point irritated and then spreads to other portions. Pyrosoina, 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 (Cketopterus, Tomopteris) have luminous organs at the bmes of lateral processes of the body. The rock-boring mollusc (Photos), whose phos-phorescent 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.1 The glow-worm (Lampyris splendicluto) has been investi-gated by Alax Schultze ; 2 110 filH1S 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 agi a reflector.3 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 verti-cally-di.sposed 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 trachece ramify in these and terminate in peculiar star-like cells ; nerve-fibres are also present. The Mexican fire-flies (Pyrophorus) are in tnost 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 lumin-ous, but in most eases it has not been observed from what part of the body the light emanates ; in some instances, however (Thysanopoda [.217-yct;phanes] norvegica, Euphausia pellueida, &c.), there a-re small globular phosphorescent organs, which have often been described as eyes, beneath the thorax and between the abdominal swimmerets. Sars 5 states that " these globules . . . constitute a highly com-plicated 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." Air John Alurray in the same place records the occurrence of a very brilliant display of this phosphorescence during the " l'riton " expedition in the Faroe Channel.
Aiany deep-sea fish possess round shining bodies im-bedded in the skin, either in the vicinity of the eye or along the sides of the body ; some of these resemble modi-fied eyes, whilst the structure of others recalls a glandular organ without the usual cluct,6 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, VOI. 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 pro-perty 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.7 Causes of Phospitorescence. - On this head it is at present impos-sible 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 exhibitien of the light is either deilendent on the volition of the animal or is the reflex result of the stimula-tion of sensory nerves (Panceri). In the glow-worm the distribu-tion of trachem (air-tubes) throughout the photogenic apparatus, and the fact that carbonic acid extinguishes the light while oxygen in-tensifies 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 num-ber of luminous organs retain their power after the death of the animal, and even after desiccation and subsegnent 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 a,go as 1749, when Vianelli discovered in the waters of the Adriatic a luminous animal-cule which was named by Min Xereis goctiluca, and was probably the creature now known as Xocalitea 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 Perieliniain ((Atoning) contribute in no small degree ; and in mid-occan another organism, Pyroeysns, which has often been mistaken for Nocti/acci, appears to replace it, and is very abundant. The brilliant sparkling phos-phorescence 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 1.■y no means obvious ; indeed it would seem certain that to crustacean larvte and other surface-organisms 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 ..4/cdusa, have become entangled ; if such be the case, it would be possible for animals otherwise defenceless to obtain protection by acquiring this property.9 A similar h3Tothesis has been proponuded with respect to the Italian fire-fly," although, as regards the glow-wortn, it has been generally believed that the light serves to aura( t the opposite sex, and the same has been stated with respect to the oartli-worm. The fact that so many deep-sea animals are plux-phor-escent, 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 prodnel ion 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 pcnetrates, - an hypothesis which is known as the " abyssal theory of light." (W. E. HO.)