xiii.] BRIGHT LINES. 171 



of brightness as we observe in the similar strata of the sun's 

 atmosphere." l 



So much for the first results obtained in localising the solar 

 chemistry. We pass from a general theory, that the absorption 

 is above the sun, and that the sun consists of such and such 

 chemical substances, to a very much more complete picture, 

 in which we see that the solar absorption is built up by vapours 

 extending to certain definite heights in the sun's atmosphere, and 

 that they are ever changing their heights and distribution at 

 different times. 



It should be seen clearly, from the foregoing statements, 

 how wide, after the accumulation of these facts, was the chasm 

 which separated KirchhofFs view of the construction of the 

 solar atmosphere from the facts as we imagined them in 1873. 



There is, indeed, another point to which a slight reference 

 may be made. It is impossible on KirchhofF s hypothesis to 

 explain the existence of bright lines in the ordinary solar 

 spectrum, and yet that such bright lines do exist is undoubted. 

 They were first noticed in 1869, when a large dispersion was 

 employed, and they have been re-discovered two or three times 

 since. 



The bright lines seen on the disc of the sun also from 

 time to time, which have been previously referred to while 

 discussing the phenomena of the spots, also militate against 

 this hypothesis. 



2. The Chemical Nature of the Sun unlike that of the Earth. 



Let us now leave the distribution of the chemical substances, 

 and deal with the atmosphere of the sun taken as a whole. 

 This is the only way open to us if we wish to consider the 

 chemical structure of the sun itself. If the elements were 

 identical we should not be justified in imagining any great 

 1 Rede Lecture, Cambridge, 1871. 



