482 THE POPULAR SCIENCE MONTHLY. 



the sun, might exercise such an action ; and he showed that the solar 

 spectrum contained a whole system of dark and fine lines, comparable 

 to the lines of solar origin, but which were due to the action of our 

 atmosphere. Brewster had already discovered that the solar spectrum 

 was enriched with dark bands at sunrise and sunset, but that in his 

 instrument they wholly disappeared during the day. Both Brewster 

 and Gladstone, his eminent co-laborer, declared in their last memoir 

 on this subject, in 1860, that they could not determine the cause of the 

 phenomenon. 



The absorbing action of our atmosphere was still more plainly 

 demonstrated by an experiment on the Lake of Geneva, in which the 

 absorption rays were obtained with the light of a fire passing over 

 Lake Leman, from a distance of fourteen miles. It was also shown in 

 an experiment made at Villette, with a tube filled with vapor at seven 

 atmospheres of pressure and one hundred and twenty feet long, that 

 the vapor of water has a complete absorption spectrum, and that the 

 largest proportion of the absorption phenomena of our atmosphere 

 should be attributed to it. 



These observations and experiments doubled the field of investiga- 

 tion opened to spectrum analysis. Not only could the incandescent 

 atmospheres of the sun and the stai-s now be made to reveal their 

 nature and their composition to us, but our researches might also be 

 extended to objects having a still greater interest for us. We could 

 at once take our own atmosphere for an object, investigating high and 

 inaccessible regions, and making analyses in them which could not be 

 attempted by any other means. Then, going away from the earth, we 

 could interrogate the planetary atmospheres, and seek in them the 

 vapor of water, and with it one of the first conditions of the develop- 

 ment of terrestrial life. We could also, comparing the composition 

 of the planetary atmospheres with the astronomical facts which per- 

 mit us to judge of the geological conditions of the surfaces of the 

 planets, follow in them the atmospheric evolutions which on the earth 

 belong to the domain of the past or of the future. Finally, the same 

 study of the planetary atmospheres, when it shall have become more 

 complete, will show us whether our atmosphere is a type reproduced 

 everywhere, the composition of which appears from that fact indis- 

 pensable to the existence of living beings, or whether, discovering 

 atmospheres of varied compositions, we shall be led to suppose that 

 life may appear and be developed in media essentially different. The 

 planetary stars are not, however, the only ones that lend themselves 

 to these applications. There are also fixed stars the spectra of which 

 reveal the characteristics of the vapor of water. Now, we know that 

 the atmosphere of a star must be considerably cooled to permit the 

 gases of which water is constituted to combine and generate a vapor. 

 Our sun is still very far from this critical condition. It is also remark- 

 able that the stars presenting these characteristics are generally red or 



