As regards the other large planets, the temperatures which we have calculated for them are very low. This calculation is, however, rather illusory, because these planets probably do not possess any solid or liquid surface, but consist altogether of gases. Their densities, at least, point in this direction. In the case of the inner planets, Mars and our moon included, the density is rather less than that of the earth. Mercury stands last among them, with its specific gravity of 0.564. There follows a great drop in the specific gravities of the outer large planets. Saturn, with a density of 0.116, is last in this order; the densities of the two outermost planets lie somewhat higher—by 0.3 or 0.4 about—but these last data are very uncertain. Yet these figures are of the same order of magnitude as that assumed for the sun—0.25—and we believe that the sun, apart from the small clouds, is wholly a gaseous body. It is therefore probable that the outer planets, including Jupiter, will also be gaseous and be surrounded by dense veils of clouds which prevent our looking down into their interior. That view would contend against the idea that these planets can harbor any living beings. We could rather imagine their moons to be inhabited. If these moons received no heat from their planets, they would assume the above-stated temperatures of their central bodies. Looked at from our moon, the earth appears under a visual angle, 3.7 times as large as that of the sun. As the temperature of the sun has, from its radiation, been estimated at 6200° Cent., or 6500° absolute, the moon would receive as much heat from the earth as from the sun, if the earth had a temperature of about 3100° Cent., or 3380° absolute. When the first clouds of water vapor were being formed in the terrestrial atmosphere, the earth’s temperature was about 360°, and the radiation from the earth to the moon only about 1.25-thousandth of that of the sun. The present radiation from the earth does not even attain one-twentieth of this value. It is thus manifest that the radiation from the earth does not play any part in the thermal household of the moon.