342 On the Aqueous Lines of the Solar Spectrum. 



flint-glass prisms. These sulphide-of-carbon prisms are very- 

 variable in their action ; but under the best conditions they 

 might show the D line as in fig. 3, when with the flint-glass 

 prisms it would appear as in fig. 2. 



The facts stated in this paper fully account for the discrepan- 

 cies in the representations which different observers have given 

 of the I) line. Some time since Mr. Gassiot of London gave in 

 the ' Chemical News ' a representation of the D line as seen with 

 his instrument, showing several lines in addition to those seen 

 by myself and other observers. On visiting the Kew Observa- 

 tory in the summer of 1864, I was surprised to find that this 

 instrument was less powerful than the one I was then using, and 

 I also learned that these lines were only seen on a single occa- 

 sion. The moist climate of England is the evident explanation 

 of the additional lines. 



As I stated at the beginning of this paper, the D line has been 

 selected simply to illustrate a general truth. The development 

 of aqueous lines in contiguous portions of the spectrum is even 

 more marked than in the exceedingly limited portion here repre- 

 sented. Indeed, as has been already intimated, the number of 

 these lines seen in the yellow region of the spectrum, on the 

 17th of November, was at least ten times as great as that of the 

 true solar lines. That part of the yellow of the spectrum which 

 lies on the more refrangible side of the D line, and in which, 

 during dry weather, only a comparatively few lines can be dis- 

 tinguished, was then as thickly crowded with lines as the blue 

 or the violet; but the lines were of course far less intense. 



Professor Tyndall of London has shown, by a remarkable 

 series of experiments with the thermo-multiplier, not only that 

 aqueous vapour powerfully absorbs the obscure thermal rays, 

 but also that the elementary gases of the atmosphere exert little 

 or no action upon them. I have endeavoured to establish in this 

 paper, from direct observations with the spectroscope, a similar 

 truth in regard to the luminous rays. It has been estimated by 

 Pouillet and others, that about one-third of the solar rays inter- 

 cepted by the earth are absorbed in passing through the atmo- 

 sphere; and it now appears that aqueous vapour is the most im- 

 portant, if not the chief, agent in producing this result. It is 

 impossible, however, from any data we yet possess, to determine 

 how great a power of absorption is exerted by the oxygen and 

 nitrogen gases which constitute the great mass of our atmo- 

 sphere. 1 have shown that a very great many, and I have no 

 doubt that almost all the lines hitherto distinguished as air-lines 

 are simply aqueous lines ; but it is very difficult to distinguish 

 atmospheric lines from the true solar lines, and our knowledge of 

 the first is as yet very incomplete. It still remains to make 



