POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 667 



lines may be ascribed to the absorption of certain rays of light in its 

 passage from the luminous mass of the sun to the earth. The remark- 

 able progress made in all spectroscopic research dates from the in- 

 vestigations made by Kirchhoff'(l859)on. the relation between absorption 

 spectra and the spectra of luminous incandescent gases. It had already 

 been observed long before (by Fraunhofer, Foucault, Angstrom) that 

 the bright spectrum of the sodium flame gives two bright lines which 

 are in exactly the same position as two black lines known as D in 

 the solar spectrum, which evidently belong to an absorption spectrum. 

 When Kirchhoff caused diffused sunlight to fall upon the slit of a 

 spectroscope, and placed a sodium flame before it, a perfect super- 

 position was observed the bright sodium lines completely covered 

 the black lines D of the solar spectrum. When further the continuous 

 spectrum of a Drummond light showed the black line D on placing 

 a sodium flame between it and the slit of the spectroscope that 

 is, when the Fraunhofer line of the solar spectrum was artificially pro- 

 duced then there was no doubt that its appearance in the solar spectrum 

 was due to the light passing somewhere through incandescent vapours 

 of sodium. Hence a new theory of reversed spectra 29 arose that is, 



chromatic photography, the spectra of blood, chlorophyll (the green constituent of leaves), 

 and other similar substances, all the more carefully 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 appliaaces on 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 extensive but .as yet little studied field, both 



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



for the general theory of spectroscopy, and for gaming 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 a definite 

 number of wave-lengths. 



29 A number of methods have been invented to demonstrate the reversibility of 

 spectra; among these methods we will cite two which are very easily carried out. In 

 Bunsen's method sodium chloride is put into an apparatus for evolving hydrogen (the 

 epray of the salt is then carried off by the hydrogen and colours the flame with the 



