iv.] GENERAL AND SELECTIVE ABSORPTION. 39 



to the eye is made to pass among the molecules of more or less 

 transparent substances. The phenomena were so definite that 

 the experiments did not go on very long before the conviction 

 was forced on Brewster that a new method of chemical analysis 

 was possible by these means, and that such a spectrum analysis 

 was destined to become a chemical agent of great value. In his 

 paper he says, " the first and principal object of my inquiries " 

 was " the discovery of a general principle of chemical analysis, 

 in which simple or compound bodies might be characterised by 

 their action on definite parts of the spectrum ; " and he showed 

 how he had already found it possible, by means of the variety 

 and constancy of the effects thus observed, to distinguish the 

 coloured juices of plants, solutions of salts, minerals, &c., " by 

 merely looking through them at a well-formed spectrum." 



These investigations of Brewster and Herschel can be easily 

 and cheaply imitated by a few very simple additions to th 

 improvised spectroscope described in Chapter III. To study 

 the absorption of coloured glasses, or other solids, it is only 

 necessary to interpose them between the candle and the slit, 

 and to observe the spectrum as before. 



The observation of a great many substances, in this manner, 

 has intensified the interesting fact first revealed by Brewster, 

 that while some bodies absorb equally all the prismatic rays, 

 and so give rise to a general darkening of the spectrum, other 

 bodies act locally, their absorption being confined to special 

 regions. So that we have general and selective absorption just 

 as we have general radiation, giving a continuous spectrum, and 

 selective radiation, giving lines here and there. 



To observe the effect of general absorption it is only neces- 

 sary to employ a piece of smoked, or still better, neutral tinted 

 glass, the only effect of which will be to deaden the spectrum 

 along its whole length. 



A piece of the ruby-red glass commonly used for photo- 

 graphic purposes will cut off nearly all but the red light, 



