56 THE CHEMISTRY OF THE SUN. [CHAP. 



KirchhofFs work. He insisted that the absorber must be cooler 

 than the radiator. 



Immediately after the publication of this note of KirchhofFs, 

 Stewart 1 explained, in extension of his former work on the theory 

 of exchanges, why it was that a salt flame of lower temperature 

 was required to darken the D lines, pointing out that it was a 

 phenomenon analogous to that presented when a piece of ruby 

 glass is heated in the fire. So long as the ruby glass is cooler 

 than the coals behind it the light given out is red, because the 

 ruby glass stops the green ; the green light is therefore analogous 

 to the line D which is given out by an alcohol flame into which 

 salt has been put. Should, however, this ruby glass be of a 

 much higher temperature than the coals behind it, the greenish 

 light which it radiates overpowers the red which it transmits, so 

 that the light which reaches the eye is more green than red. 

 This is precisely analogous to what is observed when a Bunsen's 

 gas flame with a little salt is -placed in front of the Drummond 

 light, when the line D is no longer dark but bright. 



In a paper dated Heidelberg, January, 1860, and published in 

 Poggendorffs Annalen? Kirchhoff proves from theoretical con- 

 siderations that the spectrum of an incandescent gas must 

 become reversed (that is, the bright lines become changed into 

 dark ones) when a source of light of sufficient intensity, giving 

 a continuous spectrum, is placed behind the luminous gas. 



He also describes an extended series of observations similar 

 to those recorded in his first paper, and in a note states that in 

 conjunction with Professor Bunsen he had succeeded in reversing 

 the brighter lines of potassium, calcium, strontium, and barium, 

 by exploding, before the slit, mixtures of their chlorates with 

 milk sugar during the passage of the sun's rays. 



In discussing the bearing of his observations made up to that 

 time on the constitution of the sun he says : 



1 " On the Theory of Exchanges and its Eecent Extension." B.A. Reports, 1861. 



2 Translated by Mr. F. Guthrie, in Phil. Mag. vol. xx. July, I860, p. 1. 



