64 



NATURE 



\Nov: 2^, 1875 



The phenomena of absorption are treated very fully in 

 this volume. 



A pretty mode is given by the author on p. 177 of 

 showing how the absorption bands yielded by a coloured 

 body, gradually thicken, but are not displaced, when 

 greater depths of colouring matter are used. " To de- 

 monstrate this a number of gelatine discs coloured with 



Fig. 4. — Absorption of colouring matter of litmus. 



litmus may be used, which are placed between two 

 colourless glass plates in a graduated manner. If these 

 be placed before the slit, there will be seen in the aperture 

 (Fig. 4) the graduated amount of absorption correspond- 

 ing to the different thicknesses of the gelatine. In the 

 case of the thinnest layer only a thick dark band is seen 

 in front of D, whilst the thickest layer only permits the 

 red end of the spectrum to be seen. The appearance of 

 this spectrum explains why a layer of litmus gradually 

 increasing in thickness first appears whitish, then blue, 

 then violet, and finally purple red." 



The next diagram (Fig. 5) represents the absorption 



Fig. s — Absorption spectra. 



spectra given by different bodies, the Fraunhofer lines 

 being used as convenient standards of reference. The 

 uppermost band, i, is the spectrum as modified by trans- 

 mission through a solution of permanganate of potash ; 

 the^absorption by^blood diluted with water is shown in 2, 



the violet end of the spectrum being cut out and two 

 broad bands between D and E making their appearance; 

 an alkaline solution of chlorophyll gives the absorption 

 shown in 3 ; glass coloured blue by cobalt is shown in 4, 

 and coloured red by oxide of copper in 5 ; solution of 

 bichromate of potash is given in 6, and of ammoniacal 

 oxide of copper in 7— these two are seen to transmit com- 

 plementary colours, and hence their conjunction cuts off 

 the entire spectrum. When the upper half of the screen 

 is white and the lower half covered with red paper, the 



Fig. 6. — Absorption and fluorescing spectrum of napthalin red. 



effect shown in 9 is obtained. No. 8 shows the absorp- 

 tion lines produced by glass containing didymium in 

 combination ; to the eye such glass appears colourless, 

 but the characteristic bands in the spectrum enable the 

 faintest trace of that metal to be detected. Conversely 

 by heating the oxide of didymium to incandescence, 

 bright lines appear in the spectrum of the emitted light in 

 the place of the dark lines. As is well known, the oxides 

 of didymium and erbium are rare examples of glowing 

 solids giving a linear and not a continuous spectrum. 



Concerning the production of these bright lines. Prof. 

 Lommel remarks further on (p. 250) : " The vibrations 

 which the molecules of solid and fluid [liquid] bodies 

 exhibit under the influence of the force of cohesion, do 

 not prevent the simultaneous occurrence of those vibra- 

 tions within each molecule to which the molecule is 

 attuned owing to its chemical composition. As a general 

 rule the latter are not visible, because the bright lines 

 which correspond to them disappear upon the bright J 

 background of the continuous spectrum. The charac- 

 teristic linear spectrum which discloses to us the che- ! 

 mical quality of a body is much better and more clearly ' 



Pig. 7. — Comparison of prismatic with grating spectnmi. 



seen when its molecules, freed from the chains of cohi 

 sion, enter into the gaseous condition." We ought 

 fact to find the traces of its linear spectrum in a glowir 

 liquid ; if, for example, the metal sodium be heated i 

 incandescence in nitrogen, the yellow portion of the spec 

 trum should be more pronounced than the remaininj 

 regions ; and if the glowing liquid metal could slowly pas 

 into glowing gas, the extinction of the whole spectrum 

 except the characteristic D lines, should proceed impe 



