The Evolution of the Sciences 



different luminous sources by the nature of their 

 i adiations . It may not be superfluous to recapitu- 

 late briefly the truths established in the laboratory 

 before applying them to astronomical research. 



Solids or liquids in an incandescent state 

 examined by the spectroscope give a continuous 

 spectrum formed by all the colours of the rain- 

 bow, ranging from red to violet. Of this nature 

 is the light emitted by the carbons of an electric 

 arc, or by a charge of molten steel when poured 

 from the crucible, but we find the same continu- 

 ous spectrums in the flame of a candle, of a lamp, 

 or of coal gas. 



It is important that we should not lose sight 

 of the fact that, in these flames, what gives 

 light and gives this spectrum is a solid, consisting 

 of particles of caibon suspended in the gaseous 

 mass and heated by it. On the other hand, 

 bodies which are really in a state of gas or 

 vapour, free from more condensed elements, 

 are characterised by the poverty of their lumin- 

 ous emission; their spectrum, when they are 

 brought to a high tempei ature, is constituted by 

 a certain number of very fine lines, character- 

 istic of their chemical nature. These lines, 



I/O 



