FLUORESCENCE. ,66 are short enough to affect the sense of sight. Thus if the extremely short ether-waves which do not affect our eyes, and which are called the ultra violet rays, or if the shorter visible I e.g. the violet and blue ones, are absorbed bj body, and if in return for the energy thus ab- i. longer visible rays, such as those in the n. yellow, or red. are emitted, the bodies are said to 'fluoresce,' and the entire phenomenon is called 'fluorescence.' This was first observed by Sir David Brewster for an alcoholic solution of chlorophyll. He found that when a beam of sun- light was passed through such a chlorophyll solution the path of the beam was marked by a brilliant red light, although the colors which were absorbed by the chlorophyll were the blue, yellow, and orange. It should be noted here that the fluorescent light, namely the red color, corresponds to a wave-length which is longer than that of the absorbed colors. This same phenomenon was observed by Herscbel in the case of a dilute solution of sulphate of quinine. If a beam of sunlight falls upon this liquid the por- tions of the surface where the light is incident exhibit a bright blue color, which is confined to the surface layer. Herschel also observed that if a beam of sun- light, after passing through a cell containing sulphate of quinine, is transmitted through a second cell of the same solution, there will be no fluorescence in the latter. This means that that constituent of sunlight which developed the blue fluorescenl color in the first solution was en- tirely absorbed by it, and that therefore the transmitted light contained no waves which were able to excite fluorescence in the second cell. The whole explanation of the phenomena of fluores- cence i- due to Sir G. G. Stokes. He has shown that it is exhibited in a greater or less degree by a great many substances, including ivory. bone, some kinds of paper, etc. The fact that it i- seen in many violet and green varieties of 'iluor-spar' suggested to Stokes the name 'fluores- cence' itself. Fluorescence is shown most vividly by so-called canary glass, which is glass colored with oxide of uranium; in most kinds of paraffin oil, and by solutions made from the bark of the horse-chestnut tree. In all cases, however, the law announced by Stokes and called by his name, viz., the fluorescenl light is of a longer wave-length than that of the absorbed waves which causes the fluorescence, is found to be true. Thus the waves absorbed by the quinine solution are in the ultra violet, as they are also in the case of the paraffin oils. Similarly, canary glass absorbs the violet and blue rays, and fluoresces with a greenish yellow color. It is thus evident that fluorescence offers a method for the study of ultra-violet spectra, be- cause if. for example, the solar spectrum be allowed to fall upon a screen which is moistened with some fluorescent substance, the positions of the 'Fraunhofer lines' will be conspicuous by the fact thai there is no light emitted al those places, whereas other portions of the screen where light is being absorbed « ill fluoresce. Umost all fluorescenl bodies case to emit light the instant the incident li"hi is stopped; hut, others, notably the sulphides of barium, trontium, ami calcium, continue to emit their fluorescent waves for some time after the in- cident light has been cut ol)'. Such bodies are called 'phosphore cent,' and the phenomenon is FLUORINE. called 'phosphorescence.' It should be carefully noted that this has nothing whatever to do with the ordinary luminosity of phosphorus itself, which is due to its slow oxidation. Consult Pres- ton, Theory of Light (New York, 1895). See Light. FLUORIDES (from fluor). The salts of hydrofluoric acid. See Fluorine. FLUORINE (from fluor). A non-metallic chemical element, one of the so-called halogens. Its elementary nature was first recognized by Davy, although it was as yet unknown in the free state, and remained unknown until 1887, when Moissan succeeded in isolating it from potassium hydrogen fluoride, by a process of electrolysis. Fluorine is not found native, hut occurs combined as fluorite, a calcium fluoride; as cryolite, an aluminum-sodium fluoride; ami, in smaller quan tities. in many other minerals, as apatite, time cerite, topaz, wagnerite, wavellite, yttrocerite; also in sea-water, mineral springs, and rivers, as well as in the sienis of grasses, and in hones and other animal substances. Fluorine (symbol, F or Fl ; atomic weight, 19.1) is a light, greenish-yellow gas of a pene- trating and disagreeable odor that has an ir- ritating effect on the eyes and on mucous mem- branes. It decomposes water, forming hydrofluoric acid with its hydrogen and setting free ozonized oxygen. Antimony, arsenic, boron, iodine, silicon, and sulphur are capable of burning in an atmos- phere of fluorine, and many organic substances, such as alcohol, when brought in contact with il, take fire. Hydrogen combines with it, even in the dark, forming the well-known hydrofluoric acid. Hydrofluoric acid is prepared by gently heat- ing one part of pure fluorite with about twice its weight of sulphuric acid in a leaden retort, the anhydrous acid thus produced being condensed in a receiver surrounded by a freezing-mixture. It is a colorless, mobile liquid that boils at 19.4° C. (67° F.), and solidifies to a white crystalline transparent mass at —102.5° C. ( — 152.5° F.), and has a powerful corrosive action on organic tissues, producing severe burns on the skin. For commercial use, the acid is made by passing the anhydrous vapor directly into a leaden receiver containing water, and is then obtained in dilute form. Owing to its corrosive nature, it must be preserved in lead, gutta-percha, or ceresine hoi ties. The aqueous acid has the property of dis- solving glass, and is therefore extensively used for etching on glass ; e.g. in marking the divisions on a thermometer tube. Other compounds of fluorine include hydro- fluosilicic acid, wheh is prepared by heating a mixture of sulphuric acid, fluorite, ami silica (sand or powdered ulass i . and passing the gas- eous silicon fluoride into water. It does not at- tack glass directly, hut on heating it decomposes with formation of free hydrofluoric acid, and this attacks the silica of the glass; hence its application for etching. Hydrofluosilicic acid combines with bases 1" form salts called flun- silicates or silioofiuorides, of which the most im- portant are those of potassium and sodium. (Vr tain compounds of fluorine have valuable anti septic properties, and a solution containing 0.61 of ammonium fruosilicate has been used as a wash for wounds. Fluorine compounds may also be
