FLUORESCENCE (ante), the action of certain substances which absorb light waves of short wave-length and re-emit the same light energy in waves of greater length. Some experiments are thus: A beam of sunshine, thrown by a mirror into a dark room through a hole in a shutter, is made to traverse a sheet of violet-colored glass, or a pink containing a strong solution of copper ammonia-sulphate. All but violet and actinic rays are excluded, and the room is nearly dark. If we place in the violet beam a mass of uranic nitrate it blazes with green, illuminating the room. Potassic chromate, or potassic ferrocynanide, remains dark in the same beam. If the beam traverse a jar of water upon which float some chips of horse-chestnut bark, beautiful streams of blue run down as the water dissolves the esculine of the bark. A transparent solution of quinine appears opaque, with a luminous milky precipitate. Designs in paper, drawn with quinine sulphate, though invisible in common light, become luminous in the violet beam: if drawn with varnish thickened with thallene, the effect is yet more brilliant. The design may be cut from paper coated with thallene and pasted on other paper; in electric light which has passed through yellow, green, or red glass, nothing will appear, but in light through cobalt blue glass, the thallene sketch will glow like fire against a background of black velvet. If the light be analyzed by a prism, the that lene or quinine designs will become luminous not only in the blue or violet parts of the spectrum, but also in that part usually black, where the actinic rays fall. It will be remembered that a pencil of light is a compound which may be analyzed by a prism, the result being several pencils of light of different vibrating, or wave-lengths; that the longer waves are those of the red, the wave-lengths constantly diminishing to the violet, and to the extra-luminous or,cliemical rays, whose plane iu the spectrum is beyond the violet. Ordinary substances leave no power to vary the wavc-len ztlis, but either absorb the light or return it unchanged. A red object seems so because it absorbs all other light and returns the red rays; if the object is placed in light from which the red is removed, as in the green of the spectrum, the object having no red to return, returns nothing, and is black. Hence, an object may show very different color when the light
comes from it by transmission or reflection; for example, it may reflect only mit only green, and absorb the remainder. In 1832, prof. G. G. Stokes described a' series of observations and experiments and explained the nature of the action called fluorescent. The subject has also been investigated by Becquerel, Hagenbach, and Morton.
The power of exciting fluorescence exists in all rays, but is most notable in the very short rays of the violet. Bodies which have any capacity for fluorescence will show it in the violet, and may show it in other parts of the spectrum. For most bodies there are special wave-lengths which show fluorescence better than other wave-lengths which lie between. Fluorescent excitement and absorption are in some degree correlated; thus, in general, those rays which are most powerful exciters are most absorbed; so that if a beam of light has been sifted by a prism, those rays may be missing which most excite fluorescence. This applies to light, but not to bodies. Many absorbing bodies, as permanganate of potash, have no fluorescence whatever, while certain fluor escent bodies have very complex selective absorption.
The most powerfully fluorescent bodies known are the following: Solids : thallene, emerald green; chrysogen. light green; chrysene, yellow green; platino-cyanide of tarium, uranic salts generally, and especially certain phosphates, double oxychlorides, and sulphates, also canary-glass, emerald green; platino-cyanide of magnesium, red; platino-cyanide of potassium, blue; solarized thallene (petrolucene), blue; anthracene, purplish blue. Solutions: acid quinine sulphate in water, blue; alkaline or neutral esculine in water, blue; bichlor-anthracene in alcohol, purple; bisulpho-bichlor-anthra cenic acid in water, purple; extract of stramoniuni-seeds in water or alcohol, green; solution of morin, obtained from fustic or Cuba-wood in water, with alum, green; alco holic solution of chlorophyl, best obtained from tea-leaves exhausted with water pre viously, red. The list includes only some • of the more brilliantly fluorescing bodies, and might be greatly extended.