FLU'ORES'CENCE (cf. Fr. finoreseenre, Port. fluoreseeneia, from Lat. fluor, flux, from 'lucre, to flow). When ether-waves are absorbed by a body which they have entered, their energy, as a rule, is distributed throughout the minute particles of the body, and sonic heat elfect is pro (li•ed, generally rise in temperature. As a result of this the body will now radiate more energy than before, and in so doing, will give out in the ether which are called heat-waves. i.e. their wavelength is long. Thus, a piece of red glass absorbs certain ether-waves. among them all the visible ones except those which combine to pro duce the sensation red in the Inman eye. The temperature of the glass rises, and it emits ether waves which are too Imig to affect the sense of sight. There are many bodies. however, in which the energy of the absorbed Ware; is not spent in producing rise in temperature. and the conse quent emission of long heat-waves, but is spent in producing the emission of ether-waves which 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 rays, e.g. the violet and blue ones, are absorbed by the body, and if in return for the energy thus ab sorbed, longer visible rays, such as those in the gteen. 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 heath 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 Herschel 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 fluorescent 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 is due to Sir G. G. Stokes. Tie 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 is seen in many violet and green varieties of 'fluor-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 hark of the ho•se-chestnut tree. In all cases, however, the law announced by Stokes and called by his name, viz., the fluorescent light is of a longer wave-length than that of the absorbed waves which causes the fluorescence. is fond to he 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 absorns 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 he 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 that there is no light emitted at those places, whereas other portions of the screen where light is being absorbed will fluoresce.
Almost all fluorescent bodies cease to emit light the instant the incident light is slopped ; but others, notably the sulphides of barium, strontium, and calcium, continue to omit their fluorescent waves for some time after tile in cident light has been cut off. Such bodies are called 'phosphorescent.' and the phenomenon is 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.