iv.] APPLICATION TO SUN. 43 



lines were truly solar in their origin ; and more than this, he 

 could now explain how the lines came about, for if a tube of 

 nitrous oxide gas gave us lines identical in character with the 

 solar lines as a result of its absorptive properties, what more 

 natural than to suppose that the lines were produced by absorption 

 in the atmosphere of the sun? 



He went further. His observations led him to the conclusion 

 that many of the lines produced by the absorption of the 

 gas in question were identical in their position along the 

 spectrum with several of the Fraunhofer lines themselves ; 

 and he felt himself justified in announcing, on the strength 

 of these coincidences, the discovery that there was nitrous 

 acid gas in the atmosphere of the sun. This is most 

 interesting, for here we have the first chemical touch in 

 solar inquiry. 



Brewster went on to make another important observation : 



" When the sun descends towards the horizon and shines through 

 a rapidly increasing depth of air, certain lines which before were 

 little, if at all, visible, become black and well defined, and dark 

 bands appear even in what were formerly the most luminous parts 

 of the spectrum." 



He states that this effect was observed both at sunrise and 

 sunset, and that it could not be due to any, general obscuration 

 caused by the increasing darkness at sunset, because it was not 

 necessary that the sun should be very low on the horizon in 

 order to produce these effects, but that they were visible while 

 the lines H and K in the violet were still distinct, and therefore 

 before any considerable darkening occurred, which would 

 have obscured this end of the spectrum first. He therefore 

 announced the discovery that the variable bands were "produced 

 by the absorptive effect of the earth's atmosphere," 1 and from 



1 Loc. tit. p. 528. 



