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B. Development of Light unaccompanied by any alteration in the Pon

derable Matter of Bodies. a. Development of Light after exposure to Light. A great number of bodies have the property of shining in the dark when they have previously been exposed to light: such bodies are said to exhibit phosphorescence by Insolation or Irradiation. The cause of this phenomenon is probably that the bodies, by being exposed to light, absorb a portion of it unaltered into their substance by adhesion, and subsequently give it out in a dark place,—because there the effort of the light to diffuse itself uniformly through the space devoid of light overcomes its adhesion to the ponderable matter.

Phosphori by Irradiation, Light-absorbers, Light-magnets, are transparent or opaque, colourless or slightly coloured, but never black substances.

The best phosphori by Irradiation are the following: Diamond (some diamonds however have no phosphorescence.) (Heinrich.)

Bonnonian Phosphorus. (1.) A paste made of gum tragacanth and powdered heavy spar free from iron and dried, is placed in layers between small coals in a wind-furnace (or in a crucible, — Wach) ignited for an hour, and transferred while yet warm into well stopped glass vessels. (John.) 3 or 4 per cent of magnesia mixed with the powdered heavy spar improves the phosphorescence considerably. (Wach.) (2.) Osann passes hydrogen gas over sulphate of baryta heated to redness in a tube. (3.) Daguerre fills a marrow-bone, as thick as can be procured, freed from fat and dried, with heavy spar pounded in a non-metallic mortar-lutes it—incloses it in a tube of iron-plate or cast-iron-surrounds and covers it completely with fire-clay-exposes the whole to a red heat in the furnace for at least three hours,—then removes the clay from the bone (which should be white after cooling,—a grey colour would show that it had not been heated long enough), breaks it up on paper--and preserves the white or pale yellow phosphorus thus obtained. If it be heated once or twice more in a fresh bone, its phosphorescent properties will be greatly increased. The sulphate of baryta used must be perfectly free from iron and other heavy metals.

Strontian Phosphorus may be prepared in a similar manner (1) from cælestin (John): its luminosity may be greatly increased by the addition of 3 or 4 per cent. of magnesia to the powdered cælestin.

Canton's Posphorus. (1.) Canton exposes a mixture of 3 parts of sifted and calcined oyster-shells and 1 part of flowers of sulphur to a strong fire for an hour. (2.) Grotthuss places oyster-shells—which have been previously cleaned and ignited by themselves for half an hour-in alternate layers with pounded sulphur in a crucible, the inner surfaces of the shells being turned downwards,—and heats the crucible in a windfurnace for at least an hour. The oyster-shells must be previously well burnt so as to remove all dark spots, and their inner surfaces must be cleaned from adhering ashes with a soft brush which will not injure them. The phosphorus is more luminous when the burnt oyster-shells are heated with sulphur in their entire state than when they are pounded. Moderate ignition for half an hour in contact with sulphur is generally quite sufficient: more powerful and longer-sustained ignition produces a phosphorus wbich is but faintly luminous. Pure lime heated with sul


phur yields a much weaker phosphorus than that produced from oystershells, because the latter contain a little magnesia. (Wach.) (3.) Dessaignes ignites gypsum mixed with flour.

Osann's Phosphori.—a. Antimonial Phosphorus : Formed by placing cleaned and ignited oyster-shells in alternate layers with finely pounded sulphuret of antimony in a well covered crucible, and heating the mixture for an hour : after cooling, the white pieces are to be picked out, the yellow and black ones thrown away.-6. Realgar Phosphorus: The same mode of preparation, but using realgar instead of sulphuret of antimony.c. Arsenical Phosphorus: A paste formed of neutral arseniate of baryta and gum tragacanth is dried and iguited for half an hour between coals or on an earthenware support: it has a greyish yellow colour.-d. Burnt oyster-shells treated as in a with orpiment instead of sulphuret of antimony, or-e. with mosaic gold, or-f. with cinnabar, or-9. with a mixture of sulphur and zinc-blende, or-h. with arsenious acid. Of all these phosphorescent compounds, the most luminous are a, b, and c.

Wach's Phosphori: a. Burnt oyster-shells thinly sprinkled with solution of artificial tersulphuret of arsenic, covered after drying with pounded sulphur, and ignited in a covered crucible, produce an excellent phosphorus.—6. Three parts of burnt oyster-shells disposed in alternate layers with 1 part of a mixture of 10 parts of flowers of sulphur and 1 part of oxide of antimony, and moderately heated in a covered crucible. - c. Similarly with oxide of zinc.-d. With oxide of cadmium.-. With peroxide of tin.—f. A solution of arseniate of ammonia is dropped upon calcined oyster-shells, which are then sprinkled with sulphur and ignited.—Similarly with chloride of antimony--h. With sulphate of zinc.--i. With sulphate of cadmium.-k. With proto-chloride of tin.--. Good phosphori are likewise obtained by igniting hyposulphite or sulphite of baryta, strontia, or lime, particularly hyposulphite of lime mixed with 3 or 4 per cent. of magnesia.

Lastly, among good phosphori may be enumerated : Homberg's Phosphorus (chloride of calcium, which Homberg formed by melting 1 part of sal-ammoniac with 2 parts of slaked lime); Baldwin's Phosphorus (nitrate of lime fused till the nitric acid begins to decompose); many kinds of fluor-spar, as the chlorophane of Nertschinsk (Grotthuss), and a variety of fluor-spar from Dauuria (Schw. 49, 259); strontianite; arragonite; calcspar; marble; stalactites; chalk, and slightly-burnt oyster-shells.

Less powerfully luminous, according to Heinrich, are: Crystallized boracic acid, sal-ammoniac, sulphate of potash, nitre, crystallized carbonate, borate, and sulphate of soda : rock-salt, witherite, radiating heavy spar from Bologna, marienglas, fibrons gypsum, alabaster, artificial sulphate of lime, (common fluor-spar—Grotthuss), crystallized sulphate of magnesia, crystallized alum, arsenious acid, pharmacolite, freshly prepared flowers of zinc, sulphate of mercury, tartar, benzoic acid, loaf-sugar, sugar of milk, bleached wax, white paper (especially when it has been heated almost to burning: yellow and red paper are nearly as phosphorescent as white, dark blue paper is not at all so—(Grotthuss); egg-shells, corals, snails, pearls, bones, teeth, ivory, leather, and skins of men and animals.

The following are phosphorescent in a tolerably high degree: Tartario acid; also seeds, grain flour, starch, crums of bread, gam-arabic, feathers, cheese, yolk of eyg, muscular flesh, tendons, isinglass, glue, horn—all well dried; moreover, the alburnum of trees, bleached linen, bleached cottonyarn, and other bleached vegetable fibres.

Moderately phosphorescent are: Ice, oxide of antimony, sulphate of zinc, white lead, iron pyrites, alum-slate, basalt, potter's clay, fuller's earth, bark of trees, amber.

Feebly luminous are: Cælestin, smalt, magnetic iron ore, red ochre, undried seeds, flour, and starch; also, according to Grotthuss, blue carboDate of copper (Kupfer-lazar), and beryl.

Very feebly and often not at all luminous are: Glass, silica, rockcrystal, amethyst, cornelian, prase, heliotrope, sapphire, corundum, chrysolite, spinell, emerald, topaz, tourmalin, hyacinth, garnet, melanite, leucite, adularia, common felspar, zeolites and other minerals ; chloride of zinc, yellow blende, wood, most kinds of resin and gum, silk, and animal substances not well dried.

The following, according to Heinrich, exhibit no phosphorescence: Water and all other liquids, sulphur, graphite, all metals in the free state, baryta, strontia, lime, apatite, red lead, red oxide of mercury, fresh parts of plants, unbleached yarn of hemp and flax, mineral pitch, fossil tar, coal, jet, turf, charcoal. Moreover, according to Dessaignes, all metallic sulphurets except orpiment.

According to Dessaigues, phosphorescence is also exhibited by: Glucina, phosphorite from Estremadura, orpiment, flowers of sulphuret of antimony (spiessglanz-blumen), sulphate and phosphate of lead, protochloride of tin, a mixture of peroxide of tin and oxide of lead, and imperfectly slaked baryta, strontia, and lime.

These bodies will not shine in the dark unless they are first exposed to light: even the Bolognian and Canton's phosphorus, which are prepared by ignition, do not shine when left to cool in the dark and not first exposed to light (John): neither do the realgar and antimonial phosphori, even when heated to 100° C. (Osann.). Most of these bodies require to be exposed to the direct rays of the sun. The Cantonian and Bolognian phosphorus, diamond, paper, chlorophane, sulphate of potash and common salt, are rendered luminous by reflected sunlight; the five substances first nained, by strong lamp-light; the Bononian phosphorus, and Osann's phosphori a, b, c, by the light of phosphorus burning in oxygen gas. (Phosphorus burning in oxygen gas under a bell-jar makes Canton's phosphorus but very feebly luminous, because the light passes through the glass-E. Becquerel.) The last three phosphori (not the Bolognian) are also rendered phosphorescent by the light of sulphur burning in oxygen gas, and even by the light of a tallow candle, placed at the distance of a foot; the antimonial phosphorus, likewise by the light of white-hot iron at the distance of a foot; (in the last experiment the phosphorus was placed in a dish surrounded with ice); burnt Oyster-shells, by the light of burning alcohol impregnated with common salt; Canton's phosphorus and some diamonds are also rendered luminous by moonlight. The intensity of the emitted light is however always proportional to that of the light by which the phosphorescence has been excited. Bodies may be rendered luminous by irradiation, even when immersed in water. (Heinrich, Dessaignes, Osann.)

Canton's phosphorus, after being exposed to daylight for two seconds, exhibits the greatest luminosity when immersed in water: the light appears to be somewhat fainter when the substance is freely exposed to day-light; fainter again when the light falls upon it through a plate of rock-crystal 7 inches thick ; still fainter when it passes through blue glass, and faintest of all when the light reaches the phosphorus after passing through a plate of white glass 3 millimetres thick, or through a 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, b a white, c a bright green, bluish in parts, d a deep yellow and e a white light; the part f sprinkled with arseniate of ammobia 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 sanie 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 conmon air. (Osann.)

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

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