v.J HIS LATER 



" Imagine a body of very high temperature, in whose spectrum 

 the double line D does not appear, surrounded by a gaseous atmosphere 

 of somewhat lower temperature. If sodium be present in the latter, 

 the spectrum of the whole system so constituted will contain the 

 double line D. From the occurrence of these lines the presence of 

 sodium in the atmosphere may therefore be concluded. Now, the sun 

 is undoubtedly a body of this description ; and therefore, from the 

 occurrence of the lines D in the solar spectrum, the presence of 

 sodium in the sun's atmosphere may be concluded." 



"What has been stated concerning sodium is equally true of 

 every other substance which, when placed in a flame of any sort, 

 produces bright lines in its spectrum. If these lines coincide with 

 the dark lines of the solar spectrum, the presence in the sun's 

 atmosphere of the substances which produce them must be concluded, 

 provided always that the lines in question cannot have their origin 

 in the atmosphere of the earth. In this way means are afforded 

 of determining the chemical constitution of the sun's atmosphere ; 

 and the same method even promises some information concerning 

 the constitution of the brighter fixed stars." 



In a subsequent paper * describing the result of the combined 

 work of Kirchhoff and Bunsen, it is stated that, to obtain further 

 confirmation of the observations recorded in Kirehhoffs first 

 paper, the authors produced a bright continuous spectrum by 

 the intense ignition of a platinum wire, and interposed between 

 it and the slit a flame of weak alcohol containing common salt. 

 The dark D line was then seen most distinctly. 



They also found that the dark D line could be produced 

 by the interposition, between the incandescent wire and the slit, 

 of sodium amalgam heated to boiling, and they pointed out the 

 special importance of this in showing that sodium vapour at 

 a temperature much below that at which it becomes luminous, 

 exerts its absorptive power at exactly the same point of the 

 spectrum as it does at the highest temperatures which we can 

 produce, or at the temperatures existing in the solar atmosphere. 



1 Phil, Mag., Fourth Series, vol. xx. p. 108, August, 1860. 



