Phosphorescence, the property which some bodies possess of being luminous in the dark without the emission of sensible heat. Physicists generally recognize five kinds, designated as follows: 1, spontaneous phosphorescence; 2, phosphorescence from the effects of heat; 3, from mechanical action; 4, from the action of electricity; 5, by insolation, or exposure to the light of the sun. 1. Spontaneous phosphorescence is seen in certain vegetables and animals. The flowers of certain living plants, especially those of a bright yellow or red color, as the common marigold, sunflower, and oriental poppy, it is said, have been observed to emit flashes of faint light on fine summer evenings a little after sunset. Some plants also give out in the dark a faint continuous light, caused probably by the oxidation of some hydrocarbon which they secrete. The Phytolacca decandra (pokeweed) gives out a greenish light in the dark. The milky juice of the cipo de cananum, a Brazilian plant, emits light for several hpurs after being drawn. The rhizomor-pha subterranea, which grows in mines, emits light from its whole surface, and the same phenomenon has been observed in other subterranean plants.

More familiar examples of phosphorescence in living organisms are seen in animals, as the glowworm and the firefly, and in the myriads of marine radiates, polyps, and infusoria, which cause the magnificent displays of phosphorescence that are often seen at sea by night, especially within the tropics and in the temperate zones during the summer. Various causes have been assigned for the phenomena of animal phosphorescence, and the causes no doubt vary with different animals. In many it is produced at a particular period of life, and in the firefly and glowworm it is regarded as being produced by an act of the will. M. Jousset has found that the liquid which exudes from the crushed eggs of the glowworm is phosphorescent, and remains so till it dries. In marine animals, according to the observations of several naturalists, a subtile luminous matter is thrown off as a secretion produced by glands having this special function, and some assert that it contains epithelial cells in a state of fatty degeneration, the decomposing fat being the cause of the phosphorescence. The light is increased by exposure to pure oxygen gas. MM. Quoy and Gaimard, during a voyage in the tropics, having placed two animalcules in a glass of water, the whole mass of the liquid immediately became luminous.

The phosphorescence of decaying fish and other animal matter, and of wood (fox fire), is due to a peculiar species of slow combustion by which vibrations are excited capable of transmitting luminous rays. 2. Many solids become phosphorescent when thrown upon a heated surface, and when heated in any manner between 550° and 750° F. Such are the diamond, especially the yellow variety, certain specimens of fluor spar, oyster shells, paper, Indian meal, and numerous well dried organic substances. The light is entirely different from that of incandescence, and is generally of a blue or violet hue, instead of the dull red of incipient incandescence. When phosphorescence is produced by insolation or exposure to the rays of the sun or any intense source of light, the effect is generally greatly increased by raising the temperature of the substance at the same time. 3. Phosphorescence from mechanical action is observed when certain bodies are struck with a hammer or subjected to friction, or are broken or violently torn asunder. In many instances the effect is only coexistent with the cause; in other cases it remains for a considerable time.

Adularia, a variety of or-thoclase feldspar, if split by being struck with a hammer, emits at each stroke a light which often lasts for several minutes, and if ground in a mortar it will have the appearance in the dark of being all on fire. Quartz, fluor spar, rock salt, and sugar, when broken or pounded in the dark, exhibit phosphorescence. Light is sometimes emitted by bodies undergoing a state of change, especially when passing from an amorphous to a crystalline state, or during the act of crystallization from solution, and is probably closely allied to the phosphorescence of mechanical action, which latter is also often accompanied by electrical effects. 4. If a powerful electric discharge is passed through a lump of sugar, it will shine for several seconds afterward with a beautiful violet light. Many other non-conducting substances may be affected in a similar manner, but the effect never occurs with a good conductor, such as any metal. Bodies may lose the power of becoming phosphorescent by heat or by insolation, but it may be restored by the repeated passage of electric charges through them.

M. Alvergniat produces phosphorescence by the action of electricity on chloride or bromide of silicon, in the following manner: A vacuum is made with a mercurial air pump in a glass tube, when the liquid chloride or bromide of silicon is introduced, which fills the space with vapor; the exhaustion is then continued until the pressure is reduced to 12 or 15 millimetres, when the tube .is closed by a blowpipe flame. If it be now rubbed with a piece of silk, a bright glimmer will follow the movement of the rubber. The chloride produces a rose-colored light, and the bromide a greenish yellow. A similar phenomenon has often been observed in barometer tubes. 5. Phosphorescence by insolation, or exposure to the light of the sun, has been carefully investigated by A. E. Becquerel. Insolation produces phosphorescence most readily in those substances which are bad conductors of heat. It was first discovered in 1604 in sulphide of barium, but M. Becquerel has found that it may be excited in many other substances, the sulphides of calcium and strontium being those which exhibit it in the highest degree. When well prepared they will remain luminous in the dark for several hours after exposure to the sun's rays.

This phosphorescence takes place in vacuo as well as in air or oxygen, and therefore the cause must be attributed to molecular action produced by the rays of light. Other phosphors which are excited by insolation are the diamond, particularly the yellow kind, most specimens of fluor spar, aragonite, calcareous concretions, chalk, apatite, heavy spar, fused nitrate of calcium (Baudoin's phosphorus), dried chloride of calcium, and a number of dried organic substances, as paper, silk, and cane, and also amber and milk sugar. Canton's phosphorus, prepared by heating sulphur with calcined oyster shells, will after exposure to the sun's rays emit a yellow light sufficient to show the time by a watch; even the light of an argand lamp will cause it to become phosphorescent. The Bolognese phosphorus, which is made by uniting heavy spar with gum traga-canth, gives out after insolation a bright light of more than a day's duration. It was found by M. Becquerel that the different rays of the solar spectrum had not the same power to render the substance phosphorescent.

The greatest effect is produced by the violet rays, or even a little beyond, the phosphorescent light emitted by the substance being developed by rays of greater refrangibility than they themselves, an action closely related to fluorescence. (See Fluokescence.) It may be said in general that the color emitted by phosphorescent substances varies as they are insolated by light of different parts of the spectrum. Thus Canton's phosphorus may shine with a greenish light if excited by rays from one part, while with undecomposed light it appears yellow. Phosphorescent tubes have been made in Germany and France for several years, and their preparation was kept a secret; but such tubes are now produced by several experimenters showing all the colors of the rainbow, and preparations may be made to imitate flowers and bright-colored insects, as well as landscapes. The substances, after being prepared, may be stirred in a powdered state in melted paraftine, and any design may then be painted with them on glass plates. The paratfine protects the powders from the action of moisture and prevents decomposition. The glass plates will glow for several hours after exposure to intense light with colors depending on the number and kinds of materials used.

A more permanent mode of preservation is to seal the mixture in glass tubes or flat bottles. Green light may be produced by heating hyposulphite of strontia 15 minutes over a Berzelius lamp, and then fusing in a blast lamp flame. Blue is obtained by heating precipitated sulphate of strontia in a current of hydrogen gas, then over a Bunsen burner for 10 minutes, and lastly over a blast lamp for 15 or 20 minutes. Should the light be yellowish, further heating with the blast lamp is required. Yellow phosphorescence is obtained by fusing six parts of sulphate of baryta (heavy spar) with one part of charcoal over a blast lamp. At first no phosphorescence follows the fusion, but after 24 hours the substance acquires the power of emitting an orange-yellow light after exposure to the rays of the sun. A calcium or magnesium light may be employed in place of sunlight. - Since it has been found that a great number of substances remain phosphorescent for a more or less appreciable space of time after exposure to light, it becomes a question whether all bodies whose particles are capable of being put into luminous vibrations by the action of the sun's rays do not give out light for some space .of time afterward.

M. Bec-querel invented an apparatus capable of measuring the duration of phosphorescence in different bodies, which is called a phosphoro-scope. The apparatus is so contrived that the interval between the time of insolation and observation can be made as small as desired, and measured with the greatest precision. A stationary cylindrical box, A, of blackened metal, has an opening in the form of a circular sector in each end, one being exactly opposite the other. One of these openings is shown at o. Within the box two circular screens, also of blackened metal, one at either end, are fixed to a common axis by which they are made to revolve. Each screen has four apertures, as shown at B, of the same shape as those in the cylinders, and at the same distance from the centre. These apertures are not opposite each other, but alternate, so that a ray of light cannot pass through the machine. The substance whose phosphorescence is to be examined is placed in a stirrup suspended from the upper side of the cylinder, and may be raised or lowered by means of a milled head screw.

The apparatus is placed in a window with the further side exposed to the sunlight, the machinery for turning it being inside, and the room darkened. "When the body is illuminated it cannot be seen by a person in the room, because when the further screen uncovers the outer aperture of the external cylinder the nearer screen closes the front aperture. On turning the screens one eighth of a revolution, the obstacle between the object and the eye of the observer is removed, and at the same time all source of illumination is cut off behind; and therefore if the object is now visible it must be by its own light, or in other words because of its phosphorescence. If it retains its phosphorescence for a longer time than it takes for the disks to make one eighth of a revolution, it will be visible; but if it parts with it in less than that time, it will be invisible. If the revolutions are 160 a second, the length of time between the illumination and observation would be 1/8 of 1/160, or 0.00078 of a second. Quartz, sulphur, metals, and liquids gave no appearance of phosphorescence.

The uranium compounds presented the most beautiful appearance, remaining brightly luminous 0.003 to 0.004 of a second after insolation.