394 FKAGMENTS OF SCIENCE. 



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 Poggen- 

 dorff 's ' Annalen ' by myself, and published in the 

 6 Philosophical Magazine' for 1855, 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 im- 

 pression of a reversal of the solar spectrum. Foucault, 

 Stokes, and Thomson, have all been very close to the 

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

 the radiation 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 

 speculations are founded, had not an accident withdrawn 

 me from the investigation. But Kirchhoff's claims are 

 unaffected by these circumstances. True, much that I 

 have referred to formed the necessary basis of his dis- 

 covery ; 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 empirical observa- 

 tions, and has ennobled our previous knowledge by 

 showing its relationship to some of the most sublime of 

 natural phenomena. 



