1.1 



KLUOBORIC ACID. 



FLUOR INK. 



1:4 



of the earth, at which height the density must be 

 inoooosivablv mull ; and since it is not necessary to suppose that the 

 ethereal fluid beyond the atmosphere of the earth U more denne or m< >re 

 elastic th.ui the atmosphere at that height, it may readily be admitted, 

 that whatever may be the effect of luch a fluid in resisting the motion 

 of a comet, it can produce no sensible retardation of the movement* of 

 the planet*. 



The crepuscular light which ha> been obeerved at the cusps of 

 Venus, and the changes which take place on the apparent dine of 

 Man and Jupiter, .iff. ml indication* that thoee planets hare atmospheres, 

 though, with our instruments, such atmospheres may not be rendered 

 sensible by their action on transmitted light If, about a planet, an 

 atmosphere were formed in consequence of the attraction exercised by 

 the planet on the ethereal fluid supposed to fill all space, since that 

 fluid may hare no greater density and elastic power than the atmos- 

 phere of the earth at the height of five semi-diameters of the latter 

 above its surface, the attraction of the planet may be incapable of 

 T~y**""g it sufficiently dense to produce any sensible effect in re- 

 fracting light to the earth. Again, the height and the law of the 

 variations in the density of the strata in an atmosphere which may be 

 formed about a planet by the vapours arising from waters existing on 

 its surface, would depend on the temperature ; and this we have no 

 means of knowing : but assuming it to be equal to the mean tempe- 

 rature at the surface of the earth, the height of such atmosphere would 

 be very small compared with that of the existing atmosphere about 

 the earth. An atmosphere of either of the kinds here indicated would 

 serve to account for the rare occurrence of remarkable refractions in 

 the phenomena of the immersions and emersions of Jupiter's satellites, 

 or the occultation of star* by the moon. 



For the properties of elastic fluids, see also ELASTICITY ; AIR ; GAS ; 

 VAPOUR ; and EVAPORATION. 



FLUOBOHIC ACID. [KI.I-OUIXE.] 



KLL'oliESi 'KNCE (from fluor spar), is a term which has recently 

 been employed to designate a phenomenon the nature of which 

 remained till lately unknown. 



It has long been known that certain substances have the property of 

 yielding solutions which, though perfectly bright by transmitted light, 

 exhibit a sort of coloured opalescence by reflected light, quite different 

 from the transmitted colour. The bark of the horse-chestnut, for 

 instance, contains such a substance, to which, on account of the pro- 

 perty, the name SfhiUentof was formerly given by some Gertnan 

 chemists. Sir David Brewster discovered that on admitting a beam of 

 sun-light condensed by a lens into an alcoholic solution of the green 

 colouring matter of leaves, the path of the rays in the fluid w.-u- 

 visible as a beam of blood-red light. This phenomenon, which he 

 designated internal ditptriio, and which he seems to have attributed 

 to the reflection of light from suspended [articles, he has observed and 

 studied in a great many instances, among many others in the case of a 

 green variety of fluor spar, which appears deep blue by reflected light. 

 This blue colour Sir David showed U not superficial, but arise* from 

 internal dispersion. (' Edinburgh Transactions,' vol. xvi., part 2, 

 reprinted in the ' Phil. Mag. 1 for June, 1348.) 



In the ' Philosophical Transactions,' for 1845, Sir John Herschel 

 describes a remarkable phenomenon, which he discovered in relation to 

 the blue colour exhibited by dilute solutions of Halts of quinine. The 

 blue colour, he found, came mainly from a narrow stratum adjacent to 

 the surface by which the light enters the fluid, but the blue rays thus 

 produced traverse the fluid freely. The incident light, however, after 

 having once passed through a portion of the fluid of very moderate 

 thickness, although apparently unchanged, has undergone some mys- 

 terious analysis, whereby it U rendered incapable of again producing a 

 similar blue stratum in another portion of the solution on which it 

 falls, or of causing the deep blue reflexion at the surface of the green 

 fluor above-mentioned. 



In reflecting on this peculiar analysis of light discovered by Sir 

 Sir John Herschel, Professor Stokes was led to discover that the blue 

 light shown by solutions of quinine is produced, not by the blue rays 

 of the spectrum, but by the more refrangible and mostly invisible rays 

 which are known to exist. ( Phil. Trans. 1 for 1862, p. 463.) The 

 most direct mode of proving this by experiment consists in forming a 

 pure spectrum [ DlSPKRSIOS or LIGHT], and placing in it the solution 

 of quinine. Commencing about the middle of the violet, and extending 

 from thence onward* far into the region of the more refrangible invisible 

 rays, the path of the incident rays within the fluid is marked by a 

 sky-blue light, which emanates in all directions, as if the fluid were for 

 the time being self-luminous. The blue ray* produced exhibit on 

 analysis a continuous spectrum within certain limits. They have the 

 properties of other blue rays of like composition, and accordingly freely 

 traverse the fluid, which is transparent with respect to blue light. 

 The invisible rays, however, by which the larger part of the effect is 

 produced, are absorbed with great energy, and thus it i* that the light 

 which has traversed a moderate thickness of the fluid is unaffected 

 when judged of merely by the eye, on account of the invisibility of the 

 rays of which it is deprived, and ye* has lost ite property of prc.lu. -ing 

 the blue stratum in a solution of quinine. Inasmuch a* the incident 

 rays an absorbed, and in their stead there issue in all directions, 

 from the part of the fluid in which the absorption takes place, 

 rays of a different refnmgibility, the effective portion of the incident 



light may be said to have duuuitd iu nfrangibiltty, an expression 

 which briefly indicate* the most striking of the obeerved facts of the 



pi :. ..--.:. 



This change of refrangibility, when once suggested, proved to be 

 extremely common, and to embrace the more striking instance* of 

 internal dispersion mentioned by Sir David Brewster : among others, 

 that of the variety of fluor spar, of which mention ha* already been 

 made. When a .fooroenU body (or one which possesses the pro)>erty 

 in question) is examined in a pure spectrum, it is found that, Mgi 

 at a point of the spectrum varying with the substance examined, and 

 continuing from thence onwards in the direction of increasing refrangi- 

 liility, the incident rays in being absorbed cause the substance to emit 

 rays of a different refrangibility, which observation shows to be alaayi 

 lower than that of the active rays. The colour of the emitted ray* 

 depends only on their refrangibility, having no relation whatsoever 

 colour of the active rays, or to the circumstance of their belonging to the 

 visible or invisible p.irt of the spectrum. Thus, fluorescent substances, 

 by emitting light under the influence of the invisible rays, rein!- 

 presence or absence of such rays, and in case of their present- 

 course, a matter of direct ocular inspection, and in this sense they may 

 be said to reader visible the invisible rays. The appearanci 

 produced are in many cases very striking to witness, and several 

 remarkable effects may be produced by ordinary daylight, by the aid of 

 absorbing media (see ' Phil. Trans.' for 1853, p. 385). It is readily 

 shown also that glass is opaque with regard to the rays of very high 

 refrangibility, but quartz transparent ; and by using prisms and a lens' 

 in which quarts takes the place of gloss, the solar spectrum is shown to 

 extend to a distance, measured from the extreme red, more than 

 double the length of the visible spectrum ; while with tin- 

 light a spectrum is obtained no less than six or . i-_'l>t times .1 

 as the visible spectrum. (' Proceedings of the Royal Institnn. : 

 1853.) 



As to the cause of fluorescence, Professor Stoke* suppose* ( Phil. 

 Trans.' for 1852, p. 548) that the incident ethereal vibrations 

 the ultimate molecules of the sensitive bodies, and that these molecules in 

 turn became new centre* of disturbance, from whence emanate in all 

 directions ethereal undulations agreeing in their periodic times, not 

 with the periods of the vibrations which produced the effect in the first 

 instance, but with the periods in which the molecules are disposed to 

 swing. This supposition entails the supposition of a certain <li- 

 in the effect, equal at least to that of a great number of vibrations ; 

 but as many hundred millions of millions of luminous vibrations take 

 place in one second, such a duration may very well exist while the 

 phenomenon is yet as to sense instantan< 



M. Ivlmond Becquerel, who has studied so carefully the closely allied 

 phenomenon of phosphorescence, bos recently devised a very int 

 instrument, which he calls a ph'ajjwroicopc, adapted to the c: 

 phosphorescence of short duration, and to the detection of a Imii.- 

 durotion of the effect in cases of fluorescence ('Annales tlf cliiinu-,' 

 torn. Iv. p. , r .). With this instrument M. Becquerel has suceeeilc I in 

 demonstrating, in the case of crystallised nitrate of uranium ainl 

 other substances remarkable for their powerful fluorescent 

 ence of a powerful phosphorescence of short duration. Although .1 

 finite duration in the luminosity has not yet been detected in the case 

 of any liquid, these researches go far to demonstrate experimentally 

 that there is no definite lino of demarcation between phosphorescence 

 and fluorescence, but that fluorescence is merely phosphorescence of 

 very short duration. 



It is remarkable that in a paper published in the year l-M'J C Phil. 

 Trans.' for 1842, p. 194), Sir John Herschel mentions on extraoi 

 prolongation of the spectrum when received on paper washed with 

 ' of turmeric, which however he suppoeed to be due to the 

 visibility of the highly refrangible rays, 041 tuck. In a paper pub- 

 lished in 1843 ('Annales de Chimie,' torn. ix. p. 320), M. Becquerel 

 :w a phenomenon which he had frequently olwerved in tin- 

 course of his researches on phosphorescence, namely, tint . 

 spectrum was thrown on certain papers, prepared with pl>- 

 substances, the more refrangible and usually invisible part si 

 beyond the violet was rendered visible by the screen 

 as the light fell upon it. This phenomenon, he suggests, may . 

 in a brilliant phosphorescence of short durati 

 connecting it too closely with the previously known 

 phorescence, he failed to perceive its full bearing, and never su-.-. 

 that the blue colour exhibited for instance by a dilute solution oi 

 sulphate of quinine, a fluid whose absorbing action on 

 he studied by means of photography, and whose ilirln -oism i 

 mentions (p. 289), was actually produced by the invisible rays. 



FLUORIDES. (FU-OHINK.] 



FLUOKIN'K (K), a substance which, though long known i:- 

 liination with other bodies, has been only lately procured in .-in in- 

 sulated state, if indeed as much as this can be said, and it* pr 

 in a separate state are consequently very iinperti-ftly known. It was 

 first obtained, or at any rate supposed to be obtained, in a 

 form by Baudrimont, by passing fluoride of lionui --, - r ileutoxide of 

 lead, heated to redness : the gas was received in a dry vessel. Although 

 little is known of fluorine in an elementary condition many of its com- 

 pound* have been studied. Those which it forms with the metals will, 

 if of sufficient importance, be found described under the re*| 



