566 



PKINCIPLES OP CHEMISTRY 



reversed spectra, it must be known that when light passes through 

 certain transparent substances these substances retain rays of a certain 

 refrangibility. The colour of solutions is a proof of this. Light which 

 has passed through a yellow solution of a uranium salt contains no 

 violet rays, and after having passed through a red solution of a per- 

 manganate, does not contain many rays in the yellow, blue, and green 

 portions of the spectrum. Solutions of copper salts absorb nearly all 

 red rays. Sometimes colourless solutions also absorb rays of certain 

 definite refractive indexes, and give absorption spectra. Thus solu- 

 tions of salts of didymium absorb rays of a certain refrangibility, 

 and therefore an impression of black lines is received, 28 as shown in 

 fig. 73. Many vapours (iodine) and gases (nitric peroxide) give similar 



750710660 060 610690570 SSO 630 510 490 470 



FIG. 73. Absorption spectrum (Lecoq de Boisbaudran) of salts of aidymium in concentrated 

 and dilute solutions. 



spectra. Light which has passed through a deep layer of aqueous 

 vapour, oxygen, or nitrogen also gives an absorption spectrum. For 

 this reason the peculiar (winter) dark lines discovered by JBrewster are 

 Observed in sunlight, especially in the evenibg and morning, when the 

 sun's rays pass through the atmosphere (containing these substances) 

 by a longer path than at mid- day. It is evident that the Fraunhofer 



general the spectra of metals are simpler than those of the halogens, and the latter are 

 variable ; at an increased pressure all spectral lines become broader. 



28 The method of observing absorption spectra consists iu taking a continuous 

 fepectrunf of white light (one which doe*s not show either dark lines or particularly 

 bright luminous bands for instance, the light of a candle, lamp, or other source). The 

 collimator (that is, the tube with the slit) is directed towards this light, and then all the 

 colours of the spectrum are visible in the ocular tube. A transparent absorptive 

 medium for instance, a solution or tube containing a gas is then placed between the 

 feource of light and the apparatus (or anywhere inside the apparatus itself in the path of 

 the rays). In this case either the entire spectrum is uniformly fainter, or absorption 

 bands appear on the bright field of the continuous spectrum in definite positions along 

 it. These bands have different lengths and positions, and distinctness and intensity of 

 absorption, according to the properties of the absorptive medium. Like the luminous 

 spectra given by incandescent gases and vapours, the absorption spectra of a number of 

 substances have already been studied, and some with great precision as, for example, 

 the spectrum of the brown vapours of nitrogen dioxide by Hasselberg (at Pulkowa), 

 the spectra of colouring matters (Eder and others), especially of those applied to orthol. 



