108 Professors Kirchhoff and Bunscn on Chemical 



the Relation between the Coefficients of Emission and Absorp- 

 tion of Bodies for Heat and Light/' it was proved from theo- 

 retical considerations that the spectrum of an incandescent gas 

 becomes reversed (that is, that 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. From this we may conclude that the solar spectrum, with 

 its dark lines, is nothing else than the reverse of the spectrum 

 which the sun's atmosphere alone would produce. Hence, in 

 order to effect the chemical analysis of the solar atmosphere, all 

 that we require is to discover those substances which, when 

 brought into the flame, produce bright lines coinciding with the 

 dark ones of the solar spectrum. 



In the paper above referred to, the following experimental 

 facts are given in confirmation of the preceding theoretical con- 

 clusion. 



The bright red line produced in the spectrum of a gas-flame 

 by the presence of a bead of chloride of lithium, is changed into 

 a'dark one when direct sunlight is allowed to pass through the 

 flame. When the bead of lithium is replaced by one of chloride 

 of sodium, the dark double line D (coincident with the yellow 

 sodium line) appears with uncommon distinctness. 



The dark double line D also appears when the rays of a Drum- 

 mond's light are passed through the flame of aqueous alcohol 

 into which chloride of sodium is thrown*. 



It appeared of interest to obtain still further confirmation of 

 this important theoretical conclusion ; the following experiments 

 answered this purpose : — 



We ignited a thick platinum wire in the flame, and then by 

 means of an electric current, heated it to a temperature approach- 

 ing its melting-point. The wire gave a bright spectrum, in 

 which no trace of either dark or bright lines was seen. A flame 

 of weak aqueous alcohol, in which common salt was dissolved, 

 on being brought between the wire and the slit of the apparatus, 

 gave the dark line D most distinctly. 



In the Philosophical Magazine for March 1860, Prof. Stokes calls atten- 

 tion to the fact that in the year 1849 Foucault made an observation very 

 similar to the above. In the examination of the spectrum produced by the 

 electric arc between carbon points, Foucault noticed that bright lines occur 

 where the double line D of the solar spectrum is found, and that this dark 

 line D is produced or made more intense when the rays of the sun, or those 

 from one of the incandescent carbon poles, are passed through the luminous 

 arc. The observation mentioned in the text affords an explanation of this 

 interesting phenomenon observed by Foucault eleven years ago, proving 

 that it is not occasioned by the properties of the electric light, which in 

 many respects is still so enigmatical, but that it arises from a compound 

 of sodium contained in the pole, and converted into incandescent gas by 

 the current. 



