sheet of solid gelatine. When the irradiation continues from 10 to 20 seconds, the difference is no longer perceptible. (Biot.)

Of all the coloured rays of the prism the violet (or the blue according to Grotthuss) and the invisible rays beyond the violet act most powerfully in producing phosphorescence. This power diminishes with the refrangibility: according to Heinrich, the red ray does not induce phosphorescence in the diamond; according to Seebeck and Grotthuss, a feeble luminosity is induced by the red ray, and according to Seebeck, by the invisible rays adjoining it. Phosphori which have been rendered luminous by colourless light, cease to shine much sooner in red light than in the dark; and instantaneously when exposed to red light concentrated by a lens. (Seebeck.) Similar opposition of effects is produced by light which has passed through coloured glasses. Light transmitted through blue glass makes Canton's phosphorus almost as luminous as colourless light concentrated by a lens; behind red glass, on the contrary, the phosphorus not only fails to acquire luminosity, but ceases to shine when previously irradiated, much sooner than it would if placed in the dark. (Seebeck.) Osann's phosphori a, b, and c become strongly phosphorescent under colourless, violet, and blue glass, faintly under light green and light yellow, very faintly under orange-coloured, and scarcely or not at all under red glass. (Osann.) Calcined oyster-shells become strongly luminous when exposed to day-light under dark violet glass (which when analysed by the prism appears to consist of violet, blue, and red), very faintly under bluish green, and scarcely at all under homogeneous red glass coloured with suboxide of copper. (Biot and E. Becquerel.) Bolog nian phosphorus prepared according to (3), half covered with a plate of blue glass and exposed to sunlight, becomes less luminous in the direct light of the sun than under the blue glass. (Daguerre.) The same result therefore as that obtained by Seebeck. It appears from this that colourless light contains rays which oppose the production of phosphorescence, viz. the orange, as shown by Seebeck. It is therefore the chemical rays of light by which phosphorescence is produced.

Chlorophane, Canton's, and the Bononian phosphorus insolated at a temperature of 31°C (-24° Fah.) shine at +10° C. (50° Fah.), longer and with greater intensity than when they have been exposed to light at +31° C. (88° Fah.) (Grotthuss). Phosphori newly prepared by ignition and exposed to light while yet hot acquire little or no luminosity; it is only when cool that they are susceptible of the influence of irradiation. (Osann.) Canton's phosphorus insolated at temperatures from 100° to 200° C, and then taken immediately into a dark room exhibits but a feeble light. (E. Becquerel.)

A merely momentary irradiation produces phosphorescence, though but for a short time; longer continued irradiation does not produce stronger or more lasting phosphorescence than irradiation for 10 seconds only. (Dessaignes, Heinrich.) Pulverized phosphori exposed in masses to light, shine afterwards only on the surface, and when stirred with a rod, exhibit dark streaks. (Osann.)

The phosphorescence is sometimes so weak that it can only be perceived by an eye which has been in the dark for a long time; but good phosphori often shine in twilight. Wach's phosphori are visibly phosphorescent even by daylight, and illuminate a dark room.

The duration of the phosphorescence varies greatly according to the nature of the body. Canton's phosphorus, after being exposed to sunshine for 10 seconds, shines for 10 hours according to Dessaignes, and for

5 days according to Grotthuss; chlorophane for 10 days according to Grotthuss, and 30 to 60 minutes according to Heinrich; Bolognian phosphorus prepared by (3), for 48 hours according to Daguerre, 1 hour according to Heinrich, 4 minutes according to Osann, and 80 seconds according to Dessaignes,-realgar phosphorus for some hours, antimonial phosphorus for 149 minutes, arsenical phosphorus for 34 minutes (Osann); diamond from 5 seconds to an hour; common fluor-spar from 1 to 30 minutes, most salts and organic substances from 6 to 20 seconds, minerals for a still shorter time. (Dessaignes.)

The colour of the light varies in an equal degree according to the nature of the substances. Most of them emit a white light, diamond and Bolognian phosphorus a yellowish red,-arsenical phosphorus a yellowish red, when longer ignited a reddish yellow, and after still longer ignition a colourless light,-Canton's phosphorus a light yellow rose-red or pale violet,-phosphori from oyster shells and cinnabar a yellow,-antimonial phosphorus a light green, or, when it has been ignited for a long time, a colourless light,-glucina and chlorophane a green,-strontian phosphorus a green or bluish light,—realgar phosphorus a blue light, in some places yellow and purple red, but after strong ignition a white light,-phosphori from oyster-shells and mosaic gold or blende a bluish,--calcined oystershells a red and in some parts a pale blue light. Wach's phosphorus a gives a blue, & a white, ca bright green, bluish in parts, d a deep yellow and e a white light; the part ƒ sprinkled with arseniate of ammonia appears of a fiery red with a green border; that moistened with chloride of antimony g, a yellowish red with lighter border; that with white vitriol h, a faint light with a bright bluish luminous zone, that moistened with sulphate of cadmium i, a bright yellow with bluish circumference; and that sprinkled with chloride of tin k, shines with a faint yellowish light surrounded by a luminous border. Since the border generally shines the most brightly, a minimum quantity of metallic salt seems best adapted to strengthen the illuminating power of the phosphorescent body. (Wach.)

This colour bears no relation to that of the light by which the phosphorescence has been developed. Thus diamond or Bolognian phosphorus insolated with the blue or violet ray, shines with yellowish red, and Canton's phosphorus under the same circumstances with light yellow or rose-coloured light. Hence Grotthuss concludes that the phosphorus has the property of converting colourless into coloured, and coloured light into light of another colour or white light; and he considers the differences between the several rays as resulting only from the different velocities of the lateral oscillatory movements of the rays, the direct velocity of propagation being the same for all.

Phosphorescence shows itself in all transparent media which do not alter the composition of the phosphorus; the diamond shines in water, hydrogen gas, nitrogen, and nitrous gas (Heinrich); chlorophane in water (Grotthuss); Canton's phosphorus in a vacuum and in all gases,-but . acid gases soon destroy its luminous power, and chlorine destroys it instantly (Dessaignes); similarly in water and alcohol (Grotthuss).

The phosphori formed from realgar and antimony retain their property of shining by insolation, when kept for a long time under water; the former does not shine more brightly in oxygen gas than in common air, (Osann.)

The phosphorescence of these bodies shows itself, according to Heinrich, as strongly at 12° as at +25°. A higher temperature

strengthens it but shortens its duration. When a body like Canton's phosphorus or chlorophane has ceased to shine at a certain temperature, it will shine again, even months after, when its temperature is raised, e. g. by the warmth of the hand, by boiling water, or the approximation of a hot iron: but it afterwards requires renewed insolation to make it shine again. (Canton, Grotthuss, Osann.) Canton's phosphorus insolated in a freezing mixture and removed to a dark place without being taken out of the mixture, shines as strongly as if it had been insolated and placed in the dark at the ordinary temperature; but after ceasing to shine in the freezing mixture, it again begins to emit light when raised to the ordinary temperature. (E. Becquerel.)--The Bolognian phosphorus prepared according to (3), and spread upon a plate which is carried in the open hand, causes the fingers to show through the plate, because where their warmth is conducted through the plate, the phosphorus shines more brightly. (Daguerre.)

Translucent substances, such as marble, likewise emit light from the surfaces of fractures formed during phosphorescence.

All phosphori retain their phosphorescent properties only so long as they are not chemically altered; hence some of them, such as the Bolognian and Canton's phosphorus, must be preserved in sealed tubes. Some of them, e. g. diamond dust, chlorophane, common fluor-spar and sulphate of potash, lose their power by ignition, but recover it when an electric shock is passed through them. (Dessaignes, Grotthuss)

The effect of insolation in rendering these bodies phosphorescent may be replaced by that of the electric light produced by passing the charge of a jar through them. Canton's phosphorus becomes beautifully luminous when placed in a tube of blue or colourless glass over which an electric charge is passed (in a yellowish red tube no phosphorescence is produced). (Seebeck.) The intensity of the phosphorescence increases up to a certain degree with the strength of the shock; sometimes only a streak of light appears following the course of the spark, sometimes the whole body shines. (Heinrich.) The phosphorescence produced by electricity has the same colour and the same duration as that induced by insolation (Dessaignes); according to Grotthuss it is brighter. The electric spark produces phosphorescence, not by mechanical disturbance or electric action, but by its light. Canton's phosphorus or green fluor-spar becomes luminous when the spark of an electrical battery having a surface of 2 square mètres is passed over it, at any distance from 1 decimètre to 3 mètres; (at which last distance the electrical effects are no longer perceptible); but the greater the distance, the weaker is the light. Several sparks at a great distance produce as much phosphorescence as one close at hand. If the Cantonian phosphorus exposed to the electric spark be placed under glass coloured red with suboxide of copper or under yellowish green glass, it does not become luminous; under blue glass it becomes very faintly luminous, under violet or colourless glass rather more so (and the thinner the glass the greater is the effect) but not nearly so bright as when it is exposed without any covering to the action of the spark. (E. Becquerel.) Under a plate of rock-crystal, smoky topaz, or gypsum (Marienglas) Canton's phosphorus becomes much more powerfully luminous than under a plate of colourless glass, even of less thickness; the phosphorus also becomes luminous when covered with two plates of rock-crystal with water between them. If it be covered with opaque paper in which there is an aperture 1 millimetre in diameter, and the electric spark passed over this aperture, it will be found, on removing the paper in the dark, that

the phosphorescence is at first confined to a small circle, whence it gradually diffuses itself over the whole phosphorus, then gradually diminishes and disappears. (Biot & E. Becquerel.)

b. Development of Light, produced by the action of Heat.

Almost all bodies which are capable of becoming phosphorescent by insolation and have ceased to shine at ordinary temperatures-and others likewise become luminous when heated in the dark. It seems therefore that the bodies at the ordinary temperature contain a certain quantity of light so intimately combined with them, that it cannot diffuse itself through a dark space by virtue of its own elasticity; but that the capacity of bodies to fix light diminishes as their temperature rises.

The substances which exhibit phosphorescence when heated are not only almost all those which acquire the same property by insolation, but likewise those diamonds which do not become luminous by insolation (Heinrich); also baryta, strontia, lime, magnesia, alumina, apatite, the filings of several metals (zinc and antimony are the most luminous, gold and silver the least; mercury also exhibits a very faint luminosity according to Dessaignes, none at all according to Heinrich)-very many metallic oxides, both hydrated and anhydrous, almost all earthy minerals, e. g., red sapphire, rock crystal, red felspar, red mica, asbestus, steatite (Wedgewood); wernerite, dipyre, tremolite, harmotome (Hauy); heavy spar, anhydrite, bitterspar, datolite, green sapphire, brown adamantine spar, common quartz, amethyst, grey hornblende, blue, yellow, and white topaz, rubellite, cyanite, spodumene, petalite, sodalite, green, brown, and black mica, lapis-lazuli, obsidian, mesotype, tabular spar, augite, glassy actynolite, sphene, anatase, black titaniferous sand, tungstate of lime, sulphate of lead, arseniate of lead, red silver (Brewster); baryto-calcite. (Children, Ann. Phil. 24, 115.)-Sulphate of quinin and sulphate of cinchonin shine when moderately heated; resin of guiacum, mastic, sandarach, olibanum, myrrhs, galbanum, and ammoniacal resin at their boiling points; gum arabic, marsh-mallow root, and Florentine violet-root, at a heat at which they begin to char, perhaps, therefore, in consequence of a slow combustion. (Jonas, Br. Arch. 17, 250.) Comp. Bottger. p. 200. Likewise wax, fat, and volatile oils, and many other organic bodies, shine when heated, their phosphorescence being, however, due to slow combustion; the same remark applies to the luminosity of the filings of several metals, which may perhaps be due partly to the combustion of the metal, partly to that of oil adhering to it.-The sparkling observed by Döbereiner (Schw. 41, 221) on heating chlorate of potash with powdered peroxide of manganese (or with fine quartz-sand) (Schweigger), in a glass tube likewise results from chemical combination.

Phosphorescence is not induced by heating in bodies which fuse or volatilize at a high temperature, e. g. the hydrates of potash and soda, nitre, the nitrates of strontia and lime, and ammoniacal salts, which at most become slightly luminous when gently heated (Dessaignes); neither does it occur in incombustible liquids (Heinrich).

The lowest temperature capable of inducing phosphorescence is not only different in different substances, but likewise varies in different specimens of the same substance. With Canton's phosphorus, chlorophane (Pallas), many diamonds, and white topaz (J. Phys. 55, 60), which have ceased to shine in the dark at ordinary temperatures, the heat of the hand or the breath,-and with the first mentioned substance, immersion

in oil of vitriol or nitric acid, which produces heat, is sufficient to excite phosphorescence; with common fluor-spar the temperature must be raised to between 63° and 100° C., with phosphorite from Estremadura and adularia to 100°, with diamond between 100 and 250°, with the natural forms of carbonate of lime between 200° and 325°, with minerals of the siliceous class between 250° and 375°, with oils between 94° and 250°. In this respect it is indifferent whether the body is in the form of lumps or powder, and whether the heated support consists of glass, clay, porcelain, iron, copper, silver, mercury, or tin, or whether the substance is thrown into hot water.

Bodies which become strongly phosphorescent by insolation, generally also shine brightly after being heated, and conversely: a considerable degree of phosphorescence is however acquired by hard minerals when heated. The longer a body shines by insolation, the longer also, generally speaking, does it shine after heating; and with the same body, the phosphorescence produced by heat lasts longer than that excited by insolation, with the exception of diamond, fluor-spar, Canton's phosphorus, and other bodies, which shine for a long time after insolation, and, on the contrary, for a shorter time after being heated. The intensity of the light is also directly proportional, its duration inversely proportional, to the degree of temperature to which the body is raised.

With most bodies the light is soft and streaming, with metal filings and certain heavy metallic oxides it is sparkling. (Dessaignes.) The colour of the light bears no relation to that of the phosphorescent body, and is more variable than when produced by insolation, being sometimes white, sometimes violet, blue, green, yellow, or reddish: the same body often exhibits several of these colours at different stages of the process of heating, sometimes in the order just mentioned, sometimes in the reverse order, but always in such a manner that some of the colours are passed over. (Heinrich.)-The light emitted may be resolved by the prism into a coloured spectrum, just like ordinary light.

Inorganic bodies shine equally well in common air, oxygen, nitrogen, hydrogen, or carbonic acid gas, or in vacuo, water, or oil; organic substances, on the contrary, shine only in air, or still better in oxygen gas; their phosphorescence is therefore to be regarded as a phenomenon of combustion. Only in the case of linseed oil, Dessaignes was able to distinguish a fainter luminosity, which occurred at 125° C., even in carbonic acid gas, from the stronger phosphorescence which was produced at a higher temperature, and only when oxygen was present. The phosphorescence of boiling linseed oil ceases when the air is removed by the airpump, and recommences when it is again admitted. (Grotthuss.)-Quinin, sulphate of quinin, and sulphate of cinchonin, do not shine so strongly during the time that they are heated (on paper over a lamp) as they do 30 or 40 seconds after the removal of the lamp: the luminosity begins at the edge, extends towards the middle, and often lasts several minutes. Other salts of quina and other organic salifiable bases exhibit no phosphorescence. (R. Böttger, Ann. Pharm. 33, 342.)—Luminous characters may be traced on paper with a piece of iron heated somewhat below redness. The vapour which rises from paper heated by contact with hot iron is also luminous; so likewise are wood and sugar when touched with a hot iron. (Grotthuss.)-The wick of a tallow candle, which has been extinguished in the dark, so as not to leave a single spark alight, remains faintly luminous for some seconds. If sulphur, wax, tallow, fat oil, camphor, resin, or caoutchouc be rubbed upon hot iron not luminous in the