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Rh by Vincenzo Cascariolo, a cobbler of Bologna, in about 1602. This was followed by the discovery of a number of other substances which become luminous either after exposure to light or on heating, or by attrition, and to which the general name of "phosphoric" (from φῶς and φόρος, bringing light) was given. Among these may be mentioned Homberg's phosphorus (calcium chloride), John Canton's phosphorus (calcium sulphide) and Balduin's phosphorus (calcium nitrate). Of late years it has been found convenient to limit the strict meaning of the word "phosphorescence" to the case of bodies which, after exposure to light, become self-luminous (even if only for a fraction of a second). The general term "luminescence" has been proposed by E. Wiedemann to include all cases in which bodies give off light not due to ignition. This general term embraces several subdivisions. Thus, (q.v.) and phosphorescence are included under the same heading, "photoluminescence," being distinguished from each other only by the fact that fluorescent bodies emit their characteristic light only while under the influence of the exciting illumination, while phosphorescent bodies are luminous for an appreciable time after the exciting light is cut off.

Phosphorescence, in its restricted meaning as above explained, is most strikingly exhibited by the artificial sulphides of calcium, strontium and barium. If any of these substances is exposed for some time to day light, or, better, to direct sunlight, or to the light of the electric arc, it will shine for hours in the dark with a soft coloured light. The colour depends not only on the nature of the substance, but also on its physical condition, and on its temperature during insolation, that is, exposure to the sun's rays. Thus the phosphorescent light emitted by calcium sulphide may be orange yellow, yellow, green or violet, according to the method of preparation and the materials used. Balmain's luminous paint, a preparation of calcium sulphide, shines with a white light. The colour also depends on the temperature during exposure to light. Thus A. E. Becquerel found that the light given by a specimen of strontium sulphide changed from violet to blue, green, yellow and orange as the temperature during the corresponding previous insolation was 20°, 40°, 70°, 100° or 200° C. The duration of phosphorescence varies greatly with different substances. It may last or days or for only a fraction of a second.

As in the case of fluorescent bodies, the light produced by phosphorescent substances consists commonly of rays less refrangible than those of the exciting light. Thus the ultra-violet portion of the spectrum is usually the most efficient in exciting rays belonging to the visible part of the spectrum. V. Klatt and Ph. Lenard (Wied. Ann., 1889, xxxviii. 90), have shown that the phosphorescence of calcium sulphide and other phosphori depends on the presence of minute quantities of other substances, such as copper, bismuth and manganese. The maximum intensity of phosphorescent light is obtained when a certain definite proportion of the impurity is present, and the intensity is diminished if this proportion is increased. It appears likely that when a phosphorescent body is exposed to light, the energy of the light is stored up in some kind of strain energy, and that the phosphorescent light is given out during a more or less slow recovery from this state of stra1n. Klatt and Lenard have shown that the sulphides of the alkaline earths lose the property of phosphorescing when subjected to heavy pressure. Many fluorescent solutions become briefly phosphorescent when rendered solid by gelatin.

When the duration of phosphorescence is brief, some mechanical device becomes necessary to detect it. The earliest and best-known instrument for this purpose is Becquerel's phosphoroscope. It consists essentially of a shallow drum, in whose ends two eccentric holes, exactly opposite one another, are cut. Inside it are fixed two equal metal disks, attached perpendicularly to an axis, and divided into the same number of sectors, the alternate sectors of each being cut out. One of these disks is close to one end of the drum, the other to the opposite end, and the sectors are so arranged that, when the disks are made to rotate, the hole in one end is open while that in the other is closed, and vice versa. If the eye be placed near one hole, and a ray of sunlight be admitted by the other, it is obvious that while the sun shines on an object inside the drum the aperture next the eye is closed, and vice versa. If the disks be made to revolve with great velocity by means of a train of toothed wheels the object will be presented to the eye almost instantly after it has been exposed to sunlight, and these presentations succeed one another so rapidly as to produce a sense of continuous vision. By means of this apparatus we can test with considerable accuracy the duration of the phenomenon after the light has been cut off. For this purpose we require to know merely the number of sectors in the disks and the rate at which they are turned.

Thermoluminescence.—Some bodies which do not emit light at ordinary temperatures in a dark room begin to do so if they are heated to a temperature below a visible red heat. In the case of chlorophane, a variety of fluor-spar, the heat of the hand is sufficient Many yellow diamond; exhibit this form of luminescence. It has been shown, however, that a previous exposure to light is always necessary. Sir James Dewar found that if ammonium platinocyanide, Balmain's paint and some other substances are cooled to the temperature of liquid air and exposed to light, they do not phosphoresce, but as soon as they are allowed to warm up to the ordinary temperature they emit a brilliant light. On the other hand, some bodies, such as gelatin, celluloid, paraffin and ivory, are phosphorescent at very low temperatures, but lose the property at ordinary temperatures.

Triboluminescence (from τρίβειν, to rub) is luminescence excited by friction, percussion, cleavage or such mechanical means. Calcium chloride, prepared at a red heat, exhibits this property. If sugar is broken in the dark, or two crystals of quartz rubbed together, or a piece of mica cleft, a flash of light is seen, but this is probably of electrical origin. Closely allied to this form of luminescence is crystalloluminescence, a phosphorescent light seen when some substances crystallize from solution or after fusion. This property is exhibited by arsenious acid when crystallizing from solution in hydrochloric acid.

Chemiluminescence is the name given to those cases in which chemical action produces light without any great rise of temperature. Phosphorus exposed to moist air in a dark room shines with a soft light due to slow oxidation. Decaying wood and other vegetable substances often exhibit the same property.

Electroluminescence is luminescence due to electrical causes. Many gases are phosphorescent for a short time after an electric discharge has been passed through them, and some solid substances, especially diamonds and rubies, are strongly phosphorescent when exposed to kathode rays in a vacuum tube.

See generally, Winkelmann, Handbuch der Physik, Bd. vi. (1906); E. Becquerel, La Lumière (1867).

The emission of light by living substance is a widespread occurrence, and is part of the general metabolism by which the potential energy introduced as food is transformed into kinetic energy and appears in the form of movement, heat, electricity and light. In many cases it is probably an accidental byproduct, and like the heat radiated by living tissues, is not necessarily of use to the organism. But in other cases the capacity to produce light is awakened on stimulation, as when the wind ripples the surface of the sea, or when the water is disturbed by the blade of an oar. It has been suggested that the response to the stimulus may be protective, and that enemies are frightened by the flash of light. In luminous insects and deep-sea fish the power of emitting light appears to have a special significance, and very elaborate mechanisms have been developed. The pale glow of phosphorescence has a certain resemblance to the light emitted by phosphorus, and it was an early suggestion that the phenomenon in living organisms was due to that substance. Phosphorus, however, and its luminous compounds are deadly poisons to all living tissues, and never occur in them in the course of natural metabolism, and the phosphorescence of life cannot therefore be assigned to the oxidation of phosphorus. On the other hand, it is certainly the result of a process of oxidation, as the emission of light continues only in the presence of oxygen. J. H. Fabre showed in 1855 that the luminous fungus, Agaricus, discharges more carbonic acid when it is emitting light, and Max Schultze in 1865 showed that in insects the luminous cells are closely associated with the tracheae, and that during phosphorescence they withdraw oxygen from them. In 1880 B. Radziszewski showed that many fats, ethereal oils and alcohols emit light when slowly combined with oxygen in alkaline fluids at appropriate temperatures. Probably the phosphorescence of organisms is due to a similar process acting on the many fats, oils and similar substances found in living cells. The colour varies much in different organisms; green has been observed in the glow-sworm, fire-flies, brittle-stars, centipedes and annelids; blue in the Italian fire-fly (Luciola italica); blue and light green are the predominant colours in the phosphorescence of marine organisms, but red and lilac have also been observed. The Lantern-Fly (Fulgora pyrorhynchus) is said to have a purple light, and E. H. Giglioli has recorded that an individual Appendicularia appeared first red, and then blue, and then green. P. Panceri, chiefly in the case of Salps, and S. P. Langley and F. W. Very in the case of Pyrophorus, have investigated the light spectroscopically, and