54 THE CHEMISTRY OF THE SUN. \CHAF. 



Prevost's law of exchanges, in the case of the heat-rays, and 

 generalising his conclusion for all rays. 1 



In October of the year 1859, Kirchhoff established experi- 

 mentally the same law for light-rays. His first announcement, 

 dated Heidelberg, 20th October, 1859, read before the Berlin 

 Academy on the 27th. 2 must here be given inextenso: 



" On the occasion of an examination of the spectra of coloured 

 flames not yet published, conducted by Bunsen and myself in com- 

 mon, by which it has become possible for us to recognise the quali- 

 tative composition of complicated mixtures from the appearance of 

 the spectrum of their blowpipe flame, I made some observations 

 which disclose an unexpected explanation of the origin of Fraun- 

 hofer's lines, and authorise conclusions therefrom respecting the 

 material constitution of the atmosphere of the sun, and perhaps 

 also of the brighter fixed stars. 



" Fraunhofer had remarked that in the spectrum of the flame of 

 a candle there appear two bright lines which coincide with the two 

 dark lines D of the solar spectrum. The same bright lines are 

 obtained of greater intensity from a, flame into which some common 

 salt is put. I formed a solar spectrum by projection, and allowed 

 the solar rays concerned, before they fell on the slit, to pass through 

 a powerful salt flame. If the sunlight were sufficiently reduced, 

 there appeared in place of the two dark lines D, two bright lines ; 

 if, on the other hand, its intensity surpassed a certain limit, the 

 two dark lines D showed themselves in much greater distinctness 

 than without the employment of the salt flame. 



" The spectrum of the Drummond light contains, as a general 

 rule, the two bright lines of sodium if the luminous spot of the 

 cylinder of lime has not long been exposed to the white heat ; if the 

 cylinder remains unmoved these lines become weaker, and finally 

 vanish altogether. If they have vanished, or only faintly appear, 

 an alcohol flame into which salt has been put, and which is placed 

 between the cylinder of lime and the slit, causes two dark lines of 

 remarkable sharpness and fineness, which in that respect agree with 

 the lines D of the solar spectrum, to show themselves in their stead. 



1 Edinburgh Transactions, 1858-9. 



2 See translation, by Professor Stokes, in the Phil. Mag , Fourth Series, vol. 

 xix. p. 195. 



