1861.] on the Physical Basis of Solar Chemistry. 395 



liant incandescence. Observe the spectrum. The yellow band is 

 clearly and sharply cut out, and a band of intense obscurity occupies 

 its place. I withdraw the sodium, the brilliant yellow of the spectrum 

 takes its proper place : I reintroduce the sodium and the black band 

 appears. 



Let me be more precise: — The yellow colour of the spectrum 

 extends over a sensible space, blending on one side into orange and on 

 the other into green. The term " yellow band " is therefore somewhat 

 indefinite. I want to show you that it is the precise yellow band 

 emitted by the volatilized sodium which the same substance absorbs. 

 By dipping the coal-point used for the positive electrode into' a solu- 

 tion of common salt, and replacing it in the lamp, I obtain that bright 

 yellow band which you now see drawn across the spectrum. Observe 

 the fate of that band when I interpose my sodium light. It is first 

 obliterated, and instantly that black streak occupies its place. See 

 how it alternately flashes and vanishes as I withdraw and introduce 

 the sodium flame ! 



And supposing that instead of the flame of sodium alone, I intro- 

 duce into the path of the beam a flame in which lithium, strontium, 

 magnesium, calcium, &c., are in a state of volatilization, each metallic 

 vapour would cut out its own system of bands, each corresponding 

 exactly in position with the bright band which that metal itself would 

 cast upon the screen. The light of our electric lamp then shining 

 through such a composite flame would give us a spectrum cut up by 

 dark lines, exactly as the solar spectrum is cut up by the lines of 

 Fraunhofer. 



And hence we infer the constitution of the great centre of our 

 system. The sun consists of a nucleus which is surrounded by a 

 flaming atmosphere. The light of the nucleus would give us a con- 

 tinuous spectrum, as our common coal-points did ; but having to pass 

 through the photosphere, as our beam through fehe flame, those rays of 

 the nucleus which the photosphere can itself emit are absorbed, and 

 shaded spaces, corresponding to the particular rays absorbed, occur in 

 the spectrum. Abolish the solar nucleus, and we should have a 

 spectrum showing a bright band in the place of every dark line of 

 Fraunhofer. These lines are therefore not absolutely dark, but dark 

 by an amount corresponding to the difference between the light of the 

 nucleus intercepted by the photosphere, and the light which issues from^ 

 the latter. 



The man to whom we owe this beautiful generalization is KirchhoflT, 

 Professor of Natural Philosophy in the university of Heidelberg ; 

 but, like every other great discovery, it is compounded of various 

 elements. Mr. Talbot observed the bright lines in the spectra of 

 coloured flames. Sixteen years ago Dr. Miller gave drawings and de- 

 scriptions of the spectra of various coloured flames. Wheatstone, with 

 his accustomed ingenuity, analyzed the light of the electric spark, and 

 showed that the metals between which the spark passed determined the 

 bright bands in the spectrum of the spark. Masson published a prize 



