26 REPOET— 1881. 



and which may hereafter be employed for useful purposes.' Nay, not 

 only can bodies thus be more readily discriminated, but, as we now 

 know, the presence of extremely minute portions can be detected, the 

 •gT^ ooooo o^ ^ grain being in some cases easily perceptible. 



It is also easy to see that the presence of any new simple sub- 

 stance might be detected, and in this manner already several new elements 

 have been discovered, as I shall mention when we come to Chemistry. 



But spectrum analysis has led to even grander and more unexpected 

 triumphs. Fraunhofer himself noticed the coincidence between the 

 double dark line D of the solar spectrum and a double line which he 

 observed in the spectra of ordinary flames, while Stokes pointed out 

 to Sir W. Thomson, who taught it in his lectures, that in both cases 

 these lines were due to the presence of sodium. To Kirchhoff and 

 Bunsen, however, is due the independent conception and the credit of 

 having first systematically investigated the relation which exists between 

 Fraunhofer's lines and the bright lines in the spectra of incandescent 

 metals. In order to get some fixed measure by which they might 

 determine and record the lines characterising any given substance, it 

 occurred to them that they might use for comparison the spectrum of 

 the sun. They accordingly arranged their spectroscope so that one-half 

 of the slit was lighted by the sun, and the other by the luminous gases 

 they proposed to examine. It immediately struck them that the bright 

 lines in the one corresponded with the dark lines in the other — the 

 bright line of sodium, for instance, with the line or rather lines D in 

 the sun's spectrum. The conclusion was obvious. There was sodium 

 in the sun. It must indeed have been a glorious moment when that 

 thought flashed across them, and even by itself well worth all their 

 labour. 



But why is the bright line of a sodium flame represented by a black 

 one in the spectrum of the sun ? To Angstrom is due the theory that a 

 vapour or gas can absorb luminous rays of the same refrangibility only 

 which it emits when highly heated ; while Balfour Stewart independently 

 discovered the same law with reference to radiant heat. 



This is the basis of Kirchhoff's theory of the origin of Fraunhofer's 

 lines. In the atmosphere of the sun the vapours of various metals are 

 present, each of which would give its characteristic lines, but within 

 this atmospheric envelope is the still more intensely heated nucleus 

 of the sun, which emits a brilliant continuous spectrum, containing rays 

 of all degi-ees of refrangibility. When the light of this intensely heated 

 nucleus is transmitted through the surrounding atmosphere, the bright 

 lines which would be produced by this atmosphere are seen as dark 

 ones. 



Kirchhoff and Bunsen thus proved the existence in the sun of hydro- 

 gen, sodium, magnesium, calcium, iron, nickel, chromium, manganese, 

 titanium, and cobalt ; since which Angstrom, Thalen, and Lockyer have 

 considerably increased the list. 



