374 CELESTIAL CHEMISTRY. 



Saturn, he considered to show evidences of possessing" collec- 

 tively an atmosphere of its own. Recent observations by Air. 

 Slipher of the Flagstaff Observatory, Arizona, have, however, 

 convinced him that, if Saturn's rings do possess any atmosphere 

 at all, it must be one of very much less density than that of the 

 planet itself. Mr. Slipher has made during the last four years 

 ■ — and is still continuing to make — a very interesting series of 

 spectroscopic observations of several of the planets, and 

 towards the close of 1907 he discovered that a number of bands, 

 which do not occur in the spectrum of the sun, are present in 

 the spectra of Jupiter, Saturn, Uranus, and Neptune, and the 

 remarkable and suggestive fact was observed that these new 

 bands and lines increase in intensity as we go outward from 

 the sun, being least noticeable in Jupiter and most distinct in 

 Neptune, the outermost planet of all at present known. Both 

 Neptune and Uranus were thus shown to contain hydrogen 

 in their atmospheres, and to be almost certainly self-luminous : 

 in other words, they are still so intensely heated that they shine 

 partly by their own light. It w^as also seen that substances 

 which abound in Jupiter's atmosphere are rare in that of 

 Saturn, and others again, which are abundant in Saturn's 

 atmosphere do not occur in large proportion in that of Jupiter. 



There is no spectroscopic evidence to show that Saturn 

 possess any water vapour in its atmosphere, but as long ago as 

 1867 Sir William Huggins arrived at the conclusion that the 

 spectrum of Mars showed black absorption bands in the orange 

 part of the field which were due to very appreciable amounts of 

 water vapour: early last year Mr. Slipher obtained by the 

 spectroscope convincing proof of the correctness of Sir 

 William's conclusion, and only eight months ago, Professor 

 Very showed that the intensity of certain water bands in the 

 spectrum of Mars was 22 per cent, stronger than in that of our 

 own atmosphere alone, when viewed through the latter. In the 

 atmosphere of this earth when moisture-laden, these bands 

 become most distinct and they therefore go by the name of 

 " rain bands." The spectrum of the moon shows no absorption 

 bands that give any indication whatever of water vapour in the 

 moon's atmosphere, or indeed, of there being any lunar 

 atmosphere at all. This conclusion is also confirmed bv the 

 great clearness of detail with which the moon's surface can be 

 photographed : such a clearness as that shown in the 

 photographs taken in the Paris Observatory would be quite 

 impracticable if there were any appreciable lunar atmosphere. 

 From all this we see that human beings could not breathe either 

 on the moon, where there is no atmosphere, or on any of the 

 planets from Alars outwards, where the atmospheres are 

 essentially different in chemical composition from that of our 

 earth. 



We pass on from the consideration of the planets to those 

 seemingly erratic, and unquestionably eccentric bodies which 

 we call comets. Of their constitution we know comparatively 

 little, but there is strong probability that the next few months 

 will witness a great increase of knowledge on this point, owing 

 to the near approach of that interesting object. Hallev's comet. 



