MEMOIR OF KIRCHHOPF. 535 



tiiiuoiis way from one to another; the second consist of different bright 

 lines separated by daik si)aoes, whi(;h bright lines l)ave not only char- 

 acteristic colorings, but are placed in particular positions andatdetinite 

 intervals. Just as we recognize the constellation by their configurations 

 and different brightnesses of their stars so do we distinguish the spec- 

 trum of iron from the spectrum of co{)per by the respective distances and 

 coloring of their lines. We could even do without colors; it would be 

 sufficient to measure by means of a seale the intervals between different 

 lines in order to recognize by means of Kirchhoff and Bunsen's tables the 

 element we have before us. It may seem amazing — but it is true — that 

 a color-blind man could know with absolute certainty what colors a 

 tlame emits ! The greatest advantage of a method in natural science, its 

 independence of all subjective jiulgment, was bestowed on spectrum an- 

 alysis by its discoverers. The main part of Kirchhoff and Bunsen's 

 work and their chief merit is however the demonstration of the validity 

 of the method, viz, that the configuration of lines dejiends only on the 

 chemical nature of the luminous incandescent vapor, not on its temper- 

 ature or other elements with which it is combined, and not on the nature 

 of the ilame in w^hich it glows or other accessory circumstances. Of 

 this a carefully worked out exi)erimental proof was given, and Bnnseu 

 was accordingly able long ago to make the perfectly safe assertion that 

 he discovered by means of his si)ectrum analysis a new element, because 

 the salt from a certain mineral spring showed unknown lines. Nowa- 

 days spectrum analysis is the most sensitive chemical method of de- 

 composition. And nevertheless, what is still more astonishing is the 

 further discovery made by Kirchhoff, by means of this method discov- 

 ered jointly with Bunsen. Kirchhoff happened to let a solar ra.y pass 

 through a Hame colored with sodium and then through a ])rism,so that 

 the spectrum of tlie sun and of the tlame fell one upon another. It 

 was to be expected that the well known yellow line of sodium would 

 come out in the solar spectrum; but it was just the opposite that 

 took place. On the spot where the bright line ought to have shown 

 itself there ai)peared a dark line. To Kirchhoff' this reversal of the 

 sodium line appeared at once in the highest degree remarkable, and 

 he suspected immediately that some fundamental law was lurking there. 

 The fact had been noticed by otliers (as was proved afterwards), and 

 that by men of the highest renown. It was reserved however to 

 Kirchhoff's genius to detect and to pick up the treasure of new truths 

 that lay hidden thei'e. Already on the day following the ex])eriment 

 he was able to deduce and to explain the phenomenon from a more 

 general principle which, strange to say, belonged not to optics but to 

 tlie theory of heat. From a proposition, very lemote in appearance, 

 that heat passes ordy fjom a body of a higher temperature to one of 

 a lower and not inversely, he deduced by dint of purely logical infer- 

 ences the fact of the reversal of the sodium line. Tlie nuddle term in 

 the syllogism was given by the celebrated Kirchhoff's law on the emis- 



