48 



SCIENTIFIC THOUGHT. 



34. 



Gustav 

 Kirchhoff. 



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 flame. 1 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 1 8 5 9 : 2 "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 sufficient 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 



1 From this he inferred that the 

 presence of sodium vapour in the at- 

 mosphere of the sun would 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 on this subject in 

 the ' Gesammelte Abhandlungen ' of 



Kirchhoff, 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. 



2 See the translations of Fou- 

 cault's and Kirchhoff's memoirs 

 sent by Sir G. Stokes to the 

 ' Philosophical Magazine ' of March. 

 1860, p. 194 sqq. 



