66 THE CHEMISTKY OF THE SUN. [CHAP. v. 



The result of this comparison was, that there was no accord- 

 ance between the bright lines of the gases and the atmospheric 

 lines, H alone being inconclusive. 



From the variability of the appearance of the atmospheric lines 

 from time to time when the sun is on the horizon, and from 

 the fact that one of the lines had been observed during a shower, 

 and the most prominent lines during a fog, they had been 

 referred to aqueous vapour, but Dr. Gladstone considered this 

 could scarcely be, for they are not exhibited by the sun's rays 

 passing through the edge of a cloud unless near the horizon, 

 and they appeared near sunset when, in frosty weather, the 

 aqueous vapour is reduced to a minimum, though they are not 

 seen when the sun is higher up in the heavens on a warm day. 



In 1864, Janssen, imitating Gladstone's method of work, 

 observed the spectrum of a large bonfire through a thickness of 

 atmosphere of 21,000 metres over the Lake of Geneva, and in 

 this experiment was more fortunate than Dr. Gladstone, for he 

 saw many lines, though close to the fire there was no absorption 

 whatever. 



By a subsequent experiment he proved that the lines were 

 really due to aqueous vapour. In this he used a long length 

 of gas main which the Paris Gas Company had placed at his 

 disposal, and filled it with steam, taking precautions to keep 

 the temperature high and the glass ends transparent. At one 

 end of this he placed a bright flame, at the other he observed 

 the light by means of a spectroscope after it had traversed the 

 whole length of tube. He thus obtained a spectrum which was 

 the exact equivalent of that which is superadded on to the true 

 solar one, and which becomes most marked when, the sun being 

 low, there is the greatest possible thickness of our atmosphere 

 and its contained aqueous vapour, to give rise to a larger amount 

 of absorption. 



In 1867, Angstrom, 1 referring to this note of Janssen's, stated 



1 Phil. Mag., 1867, p. 76. 



