POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 559 



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- 

 r>>xti Cations <)<"/< !>;/ Kirchhoff (I&5$) on the relation between absorption 

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

 been long since observed (by Frauenhofer, Foucault, Angstrom) that 

 the bright spectrum of the sodium flame gives exactly the same two 

 bright lines which are known as the line D in the solar spectrum, and 

 expressed by black lines which evidently belong to an absorption spec- 

 trum. When Kirchhoff caused moderated 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 later the continuous 

 spectrum of a Drummond light appeared with the black line D on placing 

 a sodium flame between it and the slit of the spectroscope that is, when 

 the Frauenhofer line of the solar spectrum was artificially produced 

 then there was no doubt that it was seen in the solar spectrum because 

 the light somewhere travelled through incandescent vapours of sodium. 

 Hence a new theory of reversed spectra 2 ^ arose that is, respecting the 

 relation between the waves of light emitted and absorbed by a substance 

 under given conditions of temperature, which is expressed by Kirchhoff's 

 law discovered by a careful analysis of the relations between the lumi- 

 nous rays absorbed and emitted by a substance. This law of the theory 

 of light may be formulated in an elementary form in the following 

 manner : At a given temperature the relation between the intensity 

 of the light emitted (of a definite wave length) and the absorptive 

 capacity with respect to the same colour (of the same wave length) is 

 a constant quantity. 30 As a black dull surface emits a considerable 



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

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

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

 spray of the salt is then carried off by iftie hydrogen and colours the flame with the yellow 

 sodium colour), and the hydrogen is ignited in two burners in one large one with a wide 

 flame giving a bright yellow sodium light, and in another with a small fine orifice whose 

 flame is pale : this flame will throw a dark patch on the large bright flame. In Ladoffsky's 

 method the front tube (p. 552) is unscrewed from a spectroscope directed towards the light 

 of a lamp (a continuous spectrum), and the flame of a spirit lamp coloured by a small 

 quantity of NaCl is placed between the tube and the prism ; a black band corresponding 

 to sodium will then be seen on looking through the ocular tube. This experiment is 

 always successful if there be only the requisite relation between the strength of the light 

 of both lamps. 



50 The absorptive capacity is the relation between the intensity of the light (of a 

 given wave length) falling upon and retained by a substance. Bunsen and Roscoe 

 showed by direct experiment that this ratio is a constant quantity for every substance. 

 If A stand for this ratio for a given substance at a given temperature for instance, for 

 a flame coloured by sodium and E be the intensity of the light of the same wave length 



