53f> MEMOIR OF KIRCHHOFF. 



8ioii and absorption of light and heat by bodies, wlii(?li says that all 

 bodies absorb chiefly those rays, those colors they emit themselves, and 

 that the ratio of the absorbed and the emitted amoant of light is 

 one and the same in all bodies however different. The paper where 

 this law is proved is the most beautiful Kirchhoff ever composed, 

 although there is the smallest amount of mathematics iu it. The history 

 of this law might serve as a model for the work of a student of nature ; 

 the law is vigorously deduced from well-known general propositions ; 

 but says itself something new; it gives the different particular infer- 

 ences which are to be verified by experiment. It will be the lot of a 

 few only to make such discoveries, but all ought to consider as a model 

 to imitate, the diligence, the conclusiveness, and the care — and not less 

 the great modesty — with which Kirchhoff communicates his discovery to 

 the world : " On the occasion of a research made jointly with Bunsen on 

 the spectra of colored flames, by means of which it became possible to 

 us to recognize the qualitative composition of complex aggregates by 

 inspection of their blow-pipe flames, I made some observations that 

 give an unexpected disclosure as to origin of Fraunhofer's lines, and 

 justify the inference to be drawn from them as to the material consti- 

 tution of the solar atmosphere, and perhaps of those of the brightest 

 stars." These words show that Kirchhoff himself made the wonderful 

 application of his law. The Fraunhofer's lines to which he alludes are, 

 as is well known, fine dark bands that furrow the solar spectrum, such 

 as it is, even without the help of a flame. The nature of these lines was 

 at first very enigmatic. The just described experiment of Kirchhoff 

 shows however that artificial Fraunhofer's lines may be produced by 

 means of a flame. The inference was near that the natural lines are pro- 

 duced by the same cause as the artificial ones, that they are reversed 

 gas spectra, and that the light of the glowing solar body has already 

 traversed somewhere incandescent gases, before it reached the earth. 

 We may go further, however. When the artificial lines coincide with the 

 Fraunhofer's lines, as (for instance), Kirchhoff proved to be the case for 

 iron, sodium, or nickel, one may conclude — taking one's stand on the 

 joint research of Kirchhoff and Bunsen — that these chemical elements 

 are found in those hypothetical incandescent gases. The fact that the 

 sun consists of a glowing liquid nucleus, surrounded by an envelope of 

 luminous vapors, and above all that these vapors contain the terrestrial 

 substances whose line-spectra coincide with Fraunhofer's lines, this fact 

 was inferred, " with as much certainty," says KirchhoftV' '' ♦^s can be 

 attained in natural science." 



It is a characteristic trait of Kirchhoff that he calculated numerically 

 this certainty. It would be possible for the bright lines of iron, for in- 

 stance, to coincide by mere chance with Fraunhofer's lines; but the 

 probability for such an event was found to be equal to T,T)-5Ti7o-DVo-(ro;ooo 

 (one-billionth), an almost evanescent quantity. "There must be a 

 cause that occasions these coincidences," says Kirchhoff. " An adequate 



