2. The planets exterior to Mars, Jupiter, and Saturn especially, as the best known of them, appear, by the best judgment which we can form, to be spheres of water, and of aqueous vapor, combined, it may be, with atmospheric air, in which their cloudy belts float over their deep oceans. Mars seems to have some portion at least of aqueous atmosphere; the earth, we know, has a considerable atmosphere of air, and of vapor; but the Moon, so near to her mistress, has none. On Venus and Mercury, we see nothing of a gaseous or aqueous atmosphere; and they, and Mars, do not differ much in their density from the Earth. Now, does not this look as if the water and the vapor, which belong to the solar system, were driven off into the outer regions of its vast circuit; while the solid masses which are nearest to the focus of heat, are all approximately of the same nature? And if this be so, what is the peculiar physical condition which we are led to ascribe to the Earth? Plainly this: that she is situated just in that region of the system, where the existence of matter, both in a solid, a fluid, and a gaseous condition, is possible. Outside the Earth's orbit, or at least outside Mars and the small Planetoids, there is, in the planets, apparently, no solid matter; or rather, if there be, there is a vast preponderance of watery and vaporous matter. Inside the Earth's orbit, we see, in the planets, no traces of water or vapor, or gas; but solid matter, about the density of terrestrial matter. The Earth, alone, is placed at the border where the conditions of life are combined; ground to stand upon; air to breathe; water to nourish vegetables, and thus, animals; and solid matter to supply the materials for their more solid parts; and with this, a due supply of light and heat, a due energy of the force of weight. All these conditions are, in our conception, requisite for life: that all these conditions meet, elsewhere than in the neighborhood of the Earth's orbit, we see strong reasons to disbelieve. The Earth, then, it would seem, is the abode of life, not because all the globes which revolve round the Sun may be assumed to be the abodes of life; but because the Earth is fitted to be so, by a curious and complex combination of properties and relations, which do not at all apply to the others. That the Earth is inhabited, is not a reason for believing that the other Planets are so, but for believing that they are not so.