ADVANCE OF ASTRONOMY. 215 



bright lines in the spectra of various flames are due 

 to the presence of definite glowing vapours in these 

 flames. In other words the presence of certain 

 lines in the spectrum is a sure index of the presence 

 of certain elements in the luminous body. 



In a famous experiment, Kirchhoff and Bunsen 

 interposed the flame of a spirit lamp, on whose wick 

 some salt had been sprinkled, in the line of the rays 

 from a lime-light, and found that on what would 

 have been a continuous spectrum there were two dark 

 sodium lines — the phenomenon of " reversal." Yet 

 when the salted flame of a Bunsen burner was sub- 

 stituted for that of the spirit lamp, the " reversal " 

 phenomenon did not occur, but a bright yellow pair 

 of lines was superposed on the lime-light spectrum. 

 Thence they inferred that to effect " reversal " the 

 temperature of the vapour through which the light 

 passes must be less than that of the radiating source 

 — a conclusion afterwards developed by Balfour 

 Stewart, and of great importance in the study of the 

 solar spectrum. For it led investigators to recog- 

 nise that the appearance of dark lines in the spec- 

 trum of the sun implies that the gases in the sun's 

 atmosphere must be at a lower temperature than those 

 in the photosphere behind. 



Kirchhoff's Law. — The experiment of the reversal 

 of the lines was the concrete proof of what Kirchhoff 

 had reached mathematically — ^the law of selective 

 absorption — ^which was also approached by Ang- 

 strom and Balfour Stewart. 



** The law states that the ratio between the emissive 

 power and the absorptive power is the same for all sub- 

 stances at the same temperature for rays of the same 

 wave-length. From this it follows that all opaque sub- 

 stances begin to glow at the same temperature — that is, 



