558 



PRINCIPLES OF CHEMISTRY 



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

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

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

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



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

 ;uid dilute solutions. 



observed in sunlight, especially in the evening 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 Frauenhofer 



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), or 



PIG. 74. Absorption spectra of nitrogen dioxide oxide and iodine. 



the spectra of colouring matters (Eder and others), and especially those applied to ortho- 

 chromatic photography, or the spectra of blood, chlorophyll (the green constituent of 

 leaves), and other similar substances, all the more as by the aid of their spectra the 

 presence of these substances may be discovered in small quantities (even in microscopical 

 quantities, by the aid of special appliances or the microscope), and the changes they 

 undergo investigated. 



The absorption spectra, obtained at the ordinary temperature and proper to substances 

 in all physical states, offer a most vast but as yet little studied field, both for the theory 

 of all spectroscopy, and for gaining an insight into the structure of substances. The 

 investigation of colouring matters has already shown that in certain cases a definite 

 change of composition and structure entails not only a definite change of the colours but 

 also a displacement of the absorption bands by definite wave lengths. This subject, 

 which has only begun to be worked out, promises great scientific fruition. 



