48 



SCIENTIFIC THOUGHT. 



34. 

 Gustav 

 Kirchhofl. 



flame which emits the two bright lines in its own spectrum 



destroys them (replacing them by two dark lines) in the 



spectrum of a ray of light which passes through the 



sodium tiame.^ Foucault had in 1849 already shown 



the direct reversal of the sodium line in the spectrum of 



the electric arc. These earlier anticipations remained 



partly unnoticed, partly unknown, or were looked upon 



as isolated cases, and it was reserved for Gustav Kirch- 



hoff to put this remarkable property of emission and 



absorption of special colours by coloured flames into 



practical language, and express it in a general way. He 



wrote in 1859:^ "I conclude that coloured flames in 



the spectra of which bright lines present themselves, so 



weaken rays of the colour of these lines, when such rays 



pass through them, that in place of the bright lines, dark 



ones appear as soon as there is brought behind the flame 



a source of light of sufflcient intensity, in which these 



lines are otherwise wanting." And when he concluded 



further that the dark lines of the solar spectrum which 



are not evoked by the atmosphere of the earth, exist 



in consequence of the presence in the sun's atmosphere 



of those substances which in the spectrum of a flame 



produce bright lines at the same place, " he at once gave 



^ From this lie inferred that the 

 presence of sodium vapour in the at- 

 mosphere of the sun wt)uld explain 

 by absorption the two dark lines in 

 the solar spectrum. Lord Kelvin 

 reports that in consequence of this 

 observation of Stokes he regularly 

 taught his Glasgow students that 

 sodium must be in the sun's at- 

 mosphere. See the reprint of the 

 correspondence ou this subject in 

 the ' Gesammelte Abhandlungen ' of 



Kirehhoff, 1882, p. 639, where it 

 will also be seen that Sir W. 

 Crookes claimed a similar anticipa- 

 tion for Millar in 1846. See also 

 Sir W. Thomson's ninth Baltimore 

 Lecture. 



" See the translations of Fou- 

 cault's and Kirchhoif's memoirs 

 sent by Sir G. Stokes to the 

 ' Philosophical Magazine ' of March. 

 1860, p. 194 sqq. 



