CONSTITUTION OF THE SUN. 513 



When the coloured incandescent vapour, for instance, 

 of sodium or lithium, is between a white hot body and 

 the slit, as in our first reversion experiments, the bright 

 line of the vapour can only appear comparatively dark 

 if the quantity of light given out by the vapour is 

 much less than the quantity of light of the same colour 

 emitted by the hot body which the vapour absorbs, 

 that is, only when the light of the incandescent solid 

 body far exceeds in intensity that given out by the 

 incandescent vapour. 



There are various methods of producing continuous 

 spectra which exhibit the dark sodium line ; but all 

 agree in this that very intense light, given out by some 

 white hot solid or liquid substance, is transmitted 

 through the much fainter light of sodium vapour. We 

 may conclude from this that the dark line in the solar 

 spectrum which corresponds to the sodium line is 

 produced in the same manner. It is supposed that the 

 Sun is an intensely hot (probably liquid) body, sur- 

 rounded by a vaporous envelope which is less hot. 

 That the heat of the Sun must be enormous follows 

 from the fact that this heat is sensible even at vast 

 distances, and that the envelope of vapour must be less 

 hot is a consequence of the constant radiation of heat 

 from the envelope into the surrounding colder space. 

 If the dark absorption line of sodium in the solar 

 spectrum were the only absorption line, the presence of 

 which would be explained by supposing the Sun to be 

 constituted in the manner stated, doubts might be en- 

 tertained about the correctness of the explanation, 

 although there would be nothing improbable in it. But 

 if the dark lines in the solar spectrum be carefully com- 



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