Prof. Tyndall on the Physical Basis of Solar Chemistry. 155 



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 the 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 Kirch- 

 hoff, Professor of Natural Philosophy in the University of Heidelberg ; 

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

 ments. Mr. Talbot observed the bright lines in the spectra of coloured 

 flames. Sixteen years ago Dr. Miller gave drawings and descriptions 

 of the spectra of various coloured flames. Wheatstone, with his accus- 

 tomed ingenuity, analysed 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 essay 

 on these bands ; Van der Willigen, and more recently Pliicker, have 

 given us beautiful drawings of the spectra obtained from the dis- 

 charge of Ruhmkorff's coil. But none of these distinguished men 

 betrayed the least knowledge of the connexion between the bright 

 bands of the metals and the dark lines of the solar spectrum. The 

 man who came nearest to the philosophy of the subject was Angstrom. 

 In a paper translated from Poggendorff 's Annalcn by myself, and 

 published in the Philosophical Magazine for 1S55, he indicates that 

 the rays which a body absorbs are precisely those which it can emit 

 when rendered luminous. In another place he speaks of one of his 

 spectra giving the general impression of reversal of the solar spec- 

 trum. Foucault, Stokes, and Thomson have all been very close to 

 the discovery; and, for my own part, the examination of the radia- 

 tion and absorption of heat by gases and vapours, some of the results 

 of which I placed before you at the commencement of this discourse, 

 would have led me in 1859 to the law on which all Kirchhoff's spe- 

 culations are founded, had not an accident withdrawn me from the 

 investigation. But Kirchhoff's claims are unaffected by these cir- 

 cumstances. True, much that I have referred to formed the neces- 

 sary basis of his discovery ; so did the laws of Kepler furnish to 

 Newton the basis of the theory of gravitation. But what Kirchhoff 

 has done carries us far beyond all that had before been accomplished. 

 He has introduced the order of law amid a vast assemblage of empi- 



