FlCBRUARY 1, lba3,] 



KNOWLEDGE. 



stances is furnished by a solution of sulph:ite of quinine. 

 To a superficial observer the solution appears clear and 

 inactically colourless. If, however, the eye be placed 

 nearly on a level with the surface of the liquid on wliicli a 

 ray of white light falls, and a black screen be placed behind 

 the vessel which contains the liquid, a bright blue colour 

 is seen on the surface, and for a short distance below the 

 surface of the liquid. The singular thing about the phe- 

 nomenon is that this blue colour is not due to the 

 absorption and emission of blue rays, but of rays of another 

 colour, viz., the more refrangible violet rays. This can 

 be shown by examining with the spectroscope the light 

 transmitted by the solution. It is found that it is 

 not the blue but the violet rays which arc wanting 

 from the spectrum. If the white light of the sun be 

 caused to pass through yellow glass the emergent light 

 produces no fluorescence in a solution of sulphate of 

 quinine, the active violet rays having been removed by the 

 yellow glass. Sunlight may, however, be passed through 

 a violet glass without diminishing its power of causing a 

 fluorescence in the quinine. It is possible to quench 

 entirely a beam of white light by interposing first a violet- 

 tinted glass which absorbs all rays but the violet, and then 

 a j-ellow glass which quenches these violet rays. Such a 

 combination is impervious to white light, but if a vessel 

 containing a solution of sulphate of quinine be introduced 

 between the two glasses, light at once shines through. 

 This is because the violet rays falling on the sulphate of 

 quinine are changed to blue rays, which the yellow glass 

 has no power to absorb. 



The properties of fluorescent substances have been 

 applied to the important problem of mapping the ultra- 

 violet portion of the solar and other spectra. The radiation 

 of hot bodies, as has long been known, is not confined to 

 the visible rays of the ordinary spectrum as perceived by 

 the eye when light is passed through a prism. A 

 thermometer, or a thermopile, placed beyond the visible 

 red is still affected by the radiation wliich is not visible to 

 the eye. The heating effect of the " ultra-red " rays is, in 

 fact, very great. In the same way it has been found that 

 there is radiation beyond the limit of tlie deepest violet 

 which is visible to the eye. The ultra-violet radiation has 

 but little heating effect, but is potent in producing chemical 

 change. This portion of the spectrum of the sun can be 

 made visible and " mapped " by the use of a fluorescent 

 substance. The fluorescent substance placed in the 

 invisible ultra-violet shines with a blue light readily 

 perceived by the eye. The band of blue light is crossed 

 by dark lines corresponding to the dark lines of the ultra- 

 violet part of the solar spectrum. The dark lines are due 

 to absorption of particular rays in the solar atmosphere. 

 Where these lines occur there is no radiation, no ultra- 

 violet ray, and consequently nothing to cause fluorescence. 

 The blue band is therefore crossed b)- dark lines, the 

 position of which can be measured as in ordinai-y 

 spectroscopic work. 



In most cases fluorescence lasts only so long as the 

 substance is exposed to the radiation, but in some sub- 

 stances the emission of light continues after the exciting 

 radiation has ceased to act. This phenomenon of 

 persistent fluorescence is termed phosphorescence. It is 

 well seen in the behaviour of the sulphides of the alkaline 

 earths, which, after having been exposed to light, continue 

 to shine for a long time in a darkened room. Many sub- 

 stances, however, which shine in the dark are not truly 

 phosphorescent in the sense defined above, their "glow" 

 depending on some slow chemical action, as in the case 

 of common yellow phosphorus (ride Knowledge for June, 

 1892, " Pho.tphorus niirfihilis"). 



It remains to enquire into the mechanism of the phe- 

 nomena of absorption, fluorescence and phosphorescence. 



According to the undulatory theory, light is the eft'ect of 

 a wave motion in a fluid medium. The character of the 

 wave motion is not greatly dift'erent from that of waves in 

 water. According to the molecular theory of the constitu- 

 tion of matter, substances are made up of small particles, 

 termed molecules, which are in a constant state of vibratory 

 and other motion. The mass of the molecules and the 

 period of vibratio '. is different in the case of dift'erent 

 substances. We have to explain what takes place when 

 the wave motion which constitutes light falls on a body 

 composed of such vibrating molecules. Sir G. Stokes has 

 furnished the explanation in the form of a simple analogy. 

 Suppose a fleet of ships of different sizes to be lying at 

 rest on a calm sea. Suppose a series of waves to pass 

 over the surface of the sea without wind. The waves may 

 be supposed to be the efl'ect of a distant storm. Each ship 

 will begin to oscillate. The time of oscillation, swing, or 

 vibration will depend upon the size and mass of each ship, 

 and may, or may not, in any particular case be the same 

 as the periodic time of the waves themselves. The dura- 

 tion of a single oscillation of a ship may be the same as 

 that of the wave, or it may be greater. In no case can it 

 be less. The oscillating ships become themselves centres 

 of disturbance from which waves are propagated over the 

 sea. The periods of these waves may be the same as or 

 greater than those of the original waves, but cannot be 

 less. 



The waves from the distant storm correspond to the 

 light waves from a luminous body. The ships of various 

 tonnage corieipond to the molecules of difl'erent substances. 

 Those ships wliich vibrate in a slower period than that of 

 the original waves, and themselves cause fresh waves of 

 the slower period, correspond to the molecules of a fluo- 

 rescent substance. They act like the molecules of sulphate 

 of quinine, which, when agitated by the rapid wave of 

 violet light, take up themselves a slower period of vibration, 

 and set in motion the surrounding ether waves of this 

 slower period, which affect the eye with the sensation of 

 blue light. 



Properly speaking, these ships correspond more closely 

 to the molecules of a phosphorescent body, since they would 

 continue to vibrate for some time after the subsidence of 

 I the disturbing waves. 



[ If wo suppose that there are ships in our imaginary fleet 



! which tak ■ up the same period of vibration as that of the 



j disturbing waves, and if we suppose that these ships come 



! to rest directly the disturbing waves cease, then we have 



I the analogues of the molecules in the film of gold of which 



we spoke at the commencement of this article, for the 



molecules at the surface of the film of gold reflect the 



waves of a particular period, being set in motion in that 



] period themselves, as is indicated by the fact that the 



j light which passes through the film is robbed of just those 



rays which are reflected at the surface. 



THE TEL-EL-AMARNA TABLETS. 



By J. H. MiTCHiNEE, F.E.A.8. 



TEL-EL-AMARNA is the Arabic name for a village 

 in Egypt situated on the east bank of the Nile, 

 halfway between Minieh and Assiout, and about 

 180 English miles south of the ancient city of 

 j\Iemphis. It comprises a number of straggling 

 Arab dweUings erected in a wilderness of sand and ancient 

 ruins. About five years ago, some peasant women, search- 

 ing, it is said, for fire-wood, but more probably for inscribed 



