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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, conimon 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 i 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, vnder 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, augito, 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), wbich 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 2009 and 325°, with minerals of the siliceous class between 250° and 375°, with oils between 94o 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 dark-or if the iron be brought in contact with gum, starch, horn, feathers-or if olefiant gas or ether vapour be made to rise against it—a pale, bluish white, lambent flame of various degrees of intensity will be produced. Tallow, paper, and cacao-fat exhibit phosphorescence at 149°, wax not below 204° C. (Williams, Pogg. 39, 490; also J. 6, 92.)

Bodies which have ceased to shine at a certain temperature again become luminous when more strongly heated; but they then require to be once more exposed to light before they will exhibit phosphorescence on being beated. The realgar and antimony phosphori prepared in a covered crucible and cooled in the dark do not become phosphorescent by heat; but if exposed to light and then kept in the dark for a long time, they emit light on being heated. (Osann.)—The application of a very strong heat produces a momentary and very vivid phosphorescence, but deprives most bodies, the heavier metals, their oxides and salts, for example, of the power of again becoming luminous: e. g., metal filings do not emit light when too much pressure has been used in the act of filing. (Dessaignes.)—Many other bodies may be deprived by half an hour's ignition of the power of emitting light when heated, e.g., precious stones, glass, quartz, clay, magnesia, heavy spar, strontianite, carbonate of lime, fluorspar, and many other salts of the alkalis and earths which partly lose their water of crystallization when ignited. The luminous power is however restored by electric discharges even when sent through a paste formed of the powdered substance mixed with water (this is the case with carbonate of lime, fluor-spar and heavy spar); in the case of salts which have lost their water of crystallization by ignition, the phosphorescent power is restored by exposure to the air, by breathing on them, or moistening them (some salts indeed, after being moistened with water, shine again, though but faintly, when heated, without previous exposure to light, -(Dessaignes, Grotthus); and in the case of strontianite and carbonate of lime, by heating to whiteness (Heinrich, Dessaignes). --Chlorophane, which has lost its phosphorescent properties by ignition, yields, by solution in hydrochloric acid and evaporation, crystals of fluor-spar which acquire little or no luminosity when heated; whereas if the chlorophane be fresh, or if its phosphorescent power after being destroyed by ignition has been restored by the electric spark, it will, when treated by hydrochloric acid, yield crystals which emit light on being heated. If ignited and unignited chlorophane be dissolved in hydrochloric acid and precipitated by ammonia, the precipitate of the former will shine when heated with a faint bluish white light, that of the latter with a bright emerald green light. If the hydrochloric acid solution of unignited chlorophaue be treated with sulphuric acid, the precipitated sulphate of lime shines almost as brightly as the chlorophane itself, but with a somewhat different light; if a solution of ordinary chloride of calcium be treated with sulphuric acid, a precipitate of gypsum is obtained which is quite destitute of phosphorescence." (Grotthus.) If ignited chloride of sodium be dissolved in one portion of water, and the same salt ignited and afterwards electrified be dissolved in another portion, and both solutions evaporated, the latter will evaporate more quickly than the former, efflorescing at the same time, and yield a salt which shines more brightly when heated than that which evaporates from the former solution. (Grotthuss.) Baryta, strontia, lime, magnesia, alumina, and silica do not lose by ignition the power of emitting light when heated. (Dessaignes.)

Pearsall, in the following experiments, placed the substance to be examined on ivory, and passed through it one or two shocks from a Ley. den jar having a surface of two square feet. He found that the same effect is produced when the spark is passed over the substance inclosed in a glass tube, whereas the vivid light between the charcoal points attached to the terminal wires of a voltaic pile of 100 pairs has no effect whatever. Chlorophane deprived by ignition of the power of emitting light when heated does not recover it by two days' exposure to the sun's rays, but regains it almost entirely by one electric shock (producing a green light) and still more powerfully by repeated shocks. Ordinary fluor spar electrified after ignition shines brightly on being heated, if kept for several weeks in the dark; but if after being electrified, it is exposed to the sun for the whole of this interval, it gives little or no light when heated. Apatite, electrified after ignition, shines with a citron-yellow light. Unignited fluor-spar, which does not shine when heated, acquires this property in a higher degree, the greater the number of electric shocks passed through it. Unignited fluor-spar becomes luminous when heated, emits after the action of electric shocks a more intense light, which, after several discharges have been passed, approaches more nearly to that of chlorophane, and exhibits colours different from those which it had before electrization. Minerals containing silica and alumina and not originally possessing the faculty of emitting light when heated, do not acquire this power by the action of electric shocks. A solution of phosphorescent apatite or fluor-spar in hydrochloric acid gives, when treated with ammonia, a precipitate which does not become luminous when heated, either after simple drying or after ignition and electrization; but a urinary calculus consisting of phosphate of lime becomes luminous when similarly treated. The crystals of fluoride of calcium which the solution of fluorspar in hydrochloric acid deposits on cooling likewise emit light on being heated. (Pearsall.)

c. Development of Light by Mechanical Force. Almost all bodies which acquire phosphorescence by insolation or by the action of heat, likewise become luminous by friction or percussion. The combined light thus disengaged from them is probably the same as that which is set free by heat; from which cause also the intensity of the light developed by percussion increases with the temperature.—Becquerel (Ann. Chim. Phys. 22, 33) refers the light developed by pressure to electric action: if for example two masses of ice come in contact at sea, they are brought by strong pressure into opposite electrical states;—and when the pressure ceases, the opposite electricities combine again and produce light. The development of light by friction is in many cases undonbtedly of an electric nature, e.g., in that of sulphate of quinin. Moreover, pressure produces heat—and this may excite phosphorescence by rise of temperature. There is probably also, according to Heinrich, a Light of Separation (Trennungs-licht), i. e., a development of light produced by destruction of cohesion as in splitting, tearing, or breaking; in few cases, e.g., iron pyrites, is the production of light a consequence of combustion. R. Böttger (Pogg. 43, 655) endeavours to prove that the light developed by rubbing flints together is electrical—from the fact that, like the electric light, it shows distinctly and separately the colours painted on a revolving circular dise; Doppler (Pogg. 49, 505) regards this proof as insufficient.

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