ABSORPTION SPECTRA. 321 



to the absorption of certain kinds of rays by the transparent 

 medium to a greater extent than that taking place with other rays. 



The examination of the absorption spectrum thus produced by 

 coloured transparent media (i.e., the determination of the kinds of 

 light specially absorbed thereby) is employed in the analytical dis- 

 crimination of different kinds of colouring matters which otherwise 

 resemble one another. Thus the colouring matter of blood, that 

 of genuine red grape wine, and those of various red coal tar dye- 

 stuffs, can be sharply distinguished from one another by noticing 

 which particular rays are most readily absorbed; instruments for this 

 purpose are termed spectroscopes, and are also employed, to discrimi- 

 nate the nature of bodies that become luminous when sufficiently 

 heated, by examining the spectrum afforded by the light emitted. 



Very few artificial sources of light produce exactly the same 

 coloured light as that which would be developed by the sun were 

 no intervening atmosphere present containing gases, watery 

 vapour, &c., possessing the power of absorbing certain rays more 

 than others. In foggy weather the sun looks red; a partial 

 analogous extinction by absorption of the more refrangible rays 

 takes place by the action of aqueous vapour in the air even in tropical 

 climates and when the sun is viewed from a high elevation (such 

 as a mountain top), so as to diminish as far as possible the 

 quantity of moisture in the air traversed by its beams. In conse- 

 quence the electric arc light always possesses a violet or bluish cast 

 as compared with sunlight, owing to the presence of a larger relative 

 proportion of highly refrangible visible rays, these not having been 

 absorbed during passage through a long column of atmosphere ; in 

 other words, if the electric arc light be regarded as the standard 

 of perfect whiteness, sunlight, as it practically reaches the 

 surface of the earth, is more or less slightly yellowish in tint. 

 Artificial sources of light, such as the lime light, the analogous 

 Welsbach light (a kind of curtain of earthy substances heated to 

 luminosity in an atmospheric gas burner something like a Bunsen 

 burner), incandescent electric lamps (not arc lights), and more 

 especially ordinary gas, oil, and petroleum lamps, candles, <kc., are 

 still yellower in shade, but for a different reason, viz., that the 

 temperature is not so high as in the electric arc, and consequently 

 rays of the highest refrangibility are not so freely emitted.* 



* On gradually heating an infusible solid at first, it only emits invisible 

 heat rays ; by and by it emits visible rays of the lowest refrangibility and is 

 then red hot ; as it gets hotter it emits more refrangible rays, and the colour 

 of the light approaches nearer and nearer to whiteness as it becomes white hot, 

 but the less refrangible rays are always more or less in excess, as compared 

 with the electric arc. 



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