150 LAW OF EXCHANGES. PART i. 



bright lines on the spectra of volatilized metals could 

 be reversed by a stronger light shining through their 

 flames. Neither of these gentlemen was aware of the 

 importance of a discovery which enabled M. Kirchhoff 

 to apply his delicate and refined analysis of terrestrial 

 matter to the sun and stars. 



He had already determined the coincidence of the 

 double yellow sodium line with Fraunhofer's dark line D, 

 but while looking with a prism at a bright solar beam 

 passing through a yellow sodium flame, he was surprised 

 to see a strong and well-defined double dark line instead 

 of the double yellow sodium line which he expected. He 

 obtained the very same result, more strongly, with 

 Drummond's lime light, which is brighter than the 

 flame of any volatilized metal, and as he found that he 

 could produce the dark and yellow lines alternately, by 

 admitting and shutting out the brighter light, he con- 

 cluded that the sodium flame is subject to the law of ex- 

 change, in consequence of which it absorbs rays of the 

 same refrangibility with those that it emits. In fact, the 

 soda flame is pervious to all the rays in solar light 

 and Drummond's flame, except those of the same re- 

 frangibility with its own ; these it absorbs and it may 

 be supposed changes them into heat. Hence M. Kirch- 

 hoff came finally to the conclusion, that the double dark 

 line in the solar spectrum is the reverse or negative of 

 the double yellow line seen on the spectrum of the sodium 

 flame. 



Quite recently, M. Fizeau has discovered that the 

 spectrum of sodium burning in air is reversed during 

 the combustion. At first it is black, with the usual 

 double yellow line ; at last,- when the light is at its 

 maximum, the double yellow line becomes black on a 

 continuous spectrum with all the seven colours. 



After M. Kirchhoff had ascertained that the bright 

 lines in the spectra of calcium, chromium, magnesium, 



