It is therefore probable that Mercury is as devoid of an atmosphere as the Moon. The opposite extreme is represented by Venus, which reflects not less than 59 per cent. of the sunlight received, according to H. N. Russell. Terrestrial clouds were found by Abbot to return 65 per cent. We believe from astronomical observations that the entire surface of Venus is hidden behind a thick opaque cloud-covering. The slight difference between O.65 and O.59 may be due to errors of observation, but also to a small absorption of light in those parts of Venus’s atmosphere which are outside of the clouds. Saturn and Jupiter are very similar to Venus in this regard with 63 and 56 per cent. respectively. The gases above the clouds on these planets extinguish to a considerable extent the sunlight reflected from the clouds, as apparent from their spectra. (Compare Fig. 13.) Hence the value 0.63 given by Russell for Saturn is probably too high. Concerning Jupiter it has been observed that its red light becomes deeper when the sunspots are few, but whiter when the spots are numerous. The sunspots have been found to favour the formation of high clouds, such as cirrus, and this would seem to apply also to Jupiter; when spots are plenty, the clouds are high, and consequently the absorbing layers above, which cause the red colour, are thinner, so that Jupiter will then shine with a whiter--less red--lustre than when the sunspots are rare. The two outmost planets, Uranus and Neptune, return, according to Russell, 63 and 73 per cent. respectively of the sunlight received. These figures are probably too high. They do not agree well with Slipher’s records of their spectra (Fig. 12). There now remains Mars. This planet approaches the Moon inasmuch as it reflects only 15.4 per cent. of the sunlight arriving to the orb. Everything points to the conclusion that the atmosphere of Mars is very thin. Lowell estimates, on somewhat meagre grounds, however, that on each square metre of the planet rests only 22 per cent. of the mass of air supported by each square metre of the Earth’s surface.