250 PROGRESS OF SCIENCE. 



ticularly in difficult cases. Quantitative analysis, on the 

 other hand, is carried out by two methods, respectively 

 called analysis by weight and volumetric analysis. The 

 spectroscope, then, as we see, is not a physicist's instrument 

 only, it is one which is just as precious to the astronomer 

 as to the chemist. By means of this unique instrument 

 Kirchhoff and Bunsen solved the PROBLEM OF THE DARK 

 LINES OF THE SOLAR SPECTRUM* which Wollaston and 

 Fraunhofer had detected, and this solution enabled Kirch- 

 hoff to discover the elements constituting heavenly bodies. 

 Kirchhoff found that the black lines are caused by the 

 passage of light through the vapours of bodies which by 

 themselves would give bright lines in the same position 

 in the spectrum when incandescent, and this theory goes 

 far to explain the constitution of meteorites, comets, nebulae 

 and stars, as well as the sun.f Some explanation will not 

 be inappropriate here. The illuminating power of the solar 

 spectrum attains its maximum in the yellow, and decreases 

 on each side of it, and intense yellow, produced by incan- 

 descent sodium, occupies a certain fixed place in the 

 spectrum of bright lines (as distinguished from the continuous 

 spectrum, and from the absorption or dark line spectrum). 

 Kirchhoff showed in the laboratory that when the vapour 

 of a substance intervenes between the observer and the 

 coloured flame produced by the same kind of substance, 

 the colour is extinguished because it is absorbed by the 

 intervening vapour, and instead of a bright line at its fixed 

 place, a dark line appears. In other words, the intervening 

 vapour has screened (because it has absorbed) the colour 

 of the flame beyond, and therefore there is no light, but 

 darkness, at that point of the spectrum that is, a black 

 line. Now this holds good for every substance. This is 

 the key of the problem, and the explanation of the dark 

 lines in the solar spectrum. A dark line is due, then, to 



* Prof. Stokes intimated their significance as early as 1851, and so 

 did Prof. Balfour Stewart, a few years later, in 1858 ; but both failed to 

 demonstrate it fully this was KirchhofFs great work. 



t Kirchhoff read the paper about this immortal discovery on the 2;th 

 of October, 1859. 



