372 GUSTAV ROBERT KIRCHHOFF. 



of the atmosphere of the sun, and perhaps also of the stars. " I con- 

 clude from these observations that colored flames, showing sharp, bright 

 lines in their spectra, so weaken rays of the same color which are sent 

 through them that dark lines take the place of the bright ones, if a 

 sufficiently strong light, deficient in these bright lines, is placed behind 

 the flame. Furthermore, I conclude that the dark lines of the solar 

 spectrum, which are not produced by the earth's atmosphere, are evoked 

 by the presence of such substances as would in the spectrum of a flame 

 exhibit bright lines in the same places." 



Again he says : " I take this opportunity of stating a conclusion 

 which I have reached since my earlier communication. According to 

 the investigations of Wheatstone, Masson, Angstrom, and others, we 

 know that in the spectrum of the electric spark bright lines appear, 

 depending on the nature of the metals between which the spark occurs, 

 and we may suppose that these lines coincide with those which would 

 exist in the spectrum of a flame of very high temperature if we brought 

 into it the same metal in a suitable form. I have examined the green 

 portion of the spectrum of the electric spark between electrodes of 

 iron, and have found in it a great number of bright lines, which seem to 

 coincide with dark lines of the solar spectrum. In single lines the 

 coincidence is hardly established securely, but I think that I have seen 

 it in many groups, the brighter lines in the sj:>ark-spectrum corre- 

 sponding to the darker lines in the sun's spectrum : I venture to con- 

 clude that these coincidences are not merely apparent. If the spark is 

 taken from other metals, for example, from copper electrodes, these 

 bright lines are wanting. I feel justified in concluding that among the 

 ingredients of the glowing atmosphere of the sun iron is found : a con- 

 clusion which otherwise comes very close when the frequent occurrence 

 of iron in the earth and in meteoric stones is considered." 



This paper was followed, two months later, by another on the rela- 

 tion between the emission and absorption of light and heat. From the 

 mechanical theory of heat Kirchhoff demonstrated mathematically the 

 law that the proportion between the powers of emission and absorption 

 is the same in all bodies at the same temperature, and for waves of the 

 same length. 



In 1860 Kirchhoff and Bunsen published a long paper under the title 

 of" Chemical Analysis of Substances by Observations on their Spectra.*' 

 This paper was illustrated by two plates; one representing the arrange- 

 ment of the apparatus employed, and the other showing the spectra of 

 six substances in juxtaposition with the solar spectrum. Of this work 

 Kirchhoff writes : " From this comprehensive and prolonged investiga- 



