The Evolution of the Sciences 



is revealed by them, just as the incandescent 

 metallic vapours were revealed by their emission 

 spectrum. Thus the atmospheric gases and 

 water vapour, when in thick layers, are each 

 characterised by numerous black lines. 



For a long time no relation was suspected 

 between the emission and the absorption-spectra ; 

 their study formed two separate chapters of 

 spectroscopy , when the investigations of Foucault 

 and of Kirchhoff established between them the 

 following relation. The radiations emitted by 

 an incandescent vapour are absorbed by the same 

 vapour at a less heat . It is impossible to examine 

 here the problems raised by this property. We 

 shall limit ourselves to making its signification 

 clear by recalling the characteristic experiment 

 by means of which it is usually demonstrated. 

 If we examine through the spectroscope the 

 intensely bright yellow light obtained by volatil- 

 ising a grain of sodium in the extremely hot 

 flame of an enameller's lamp, it appears con- 

 stituted, as we have already stated, by two 

 yellow lines situated very close to each other. 

 If we now interpose between this flame and the 



spectroscope an alcohol flame, less hot than the 



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