256 M. G. KirchhofiP on the History of Spectrum Analysis 



ness of the lines varies with the proportion of carbon to hydro- 

 gen in the substance which is burned, being greatest where 



there is most carbon The absolute identity which is thus 



shown to exist between the spectra of dissimilar carbohydrogen 

 compounds is not a little remarkable. It proves, 1st, that the 

 position of the lines in the spectrum does not vary with the 

 proportion of carbon and hydrogen in the burning body — 

 as when we compare the spectra of light carburetted hydrogen 

 C H 2 , olefiant gas C 2 H 2 , and oil of turpentine, C 10 H 8 ; and 

 2ndly, that the presence of oxygen does not alter the character 

 of the spectrum ; thus ether, C 4 H 5 0, and wood spirit, C 2 H 4 O 2 , 

 give spectra which are identical with those of paraffin, C 20 H 20 , 

 and oil of turpentine, C 20 H 8 . 



u In certain cases, at least, the mechanical admixture of other 

 substances with the carbohydrogen compound does not affect 

 the lines of the spectrum. Thus, I have found that a mixture 

 of alcohol and chloroform burns with a flame having a very 

 luminous green envelope — an appearance characteristic of the 

 presence of chlorine — and no lines are visible in the spectrum. 

 When, however, the flame is urged by the blowpipe, the light 

 of the envelope is diminished, and the ordinary lines of the hy- 

 drocarbon spectrum become visible." 



In this research, Swan has made a most valuable contribu- 

 tion towards the solution of the proposed question as to whether 

 the bright lines of a glowing gas are solely dependent upon 

 its chemical constituents; but he did not answer it positively, 

 or in its most general form ; he did not indeed enter upon this 

 question, for he wished to confine his investigation to the spectra 

 of the hydrocarbons, and was only led to the examination of this 

 yellow line by its frequent occurrence in these spectra. 



No one, it appears, had clearly propounded this question 

 before Bunsen and myself; and the chief aim of our common 

 investigation was to decide this point. Experiments which were 

 greatly varied, and were for the most part new, led us to the con- 

 clusion upon which the foundations of the " chemical analysis 

 by spectrum-observations " now rest. 



2. I have likewise a few remarks to make concerning the 

 history of the Chemical Analysis of the Solar Atmosphere. 



The substance of the theory of solar chemistry which I have 

 developed consists of a proposition which may be shortly stated 

 as follows : — The relation between the power of emission and 

 the power of absorption for each kind of rays (heat or light) is 

 the same for all bodies at the same temperature. From this 

 proposition it easily follows that a glowing body which emits only 

 rays of certain wave-lengths, likewise absorbs only rays of the 

 same wave-lengths ; and from this we learn how the dark lines 



