Our knowledge of the physical constitution of Mercury is very slight. It seems to be surrounded by a light atmosphere which contains water-vapour. Since the solar heat on Mercury is about seven times stronger than on us, the extreme limits of temperature above indicated must here be very considerably surpassed, and water can hardly exist except in the form of vapour. But the argument is entirely changed if we accept the recent discovery of Schiaparelli, according to which the duration of rotation of Mercury would be identical with its period of revolution, so that, like the Moon to the Earth, Mercury would always turn the same face to the Sun. On that side the temperature would naturally be very high, while the most intense frost would reign on the other side. But between these two extremes there ought to be a mixed zone in which our three conditions might possibly be realised, so that we may assume for Mercury a limited habitability. _Venus_ On Venus the Sun’s heat is still very considerable, and in the torrid zone it would be insupportable to us. But, on the other hand, this planet is surrounded by a dense atmosphere, which on account of the presence of water-vapour proves the existence of water on the planet. The higher regions of the atmosphere are occupied by a thick layer of clouds which hardly ever allows our gaze to reach the ground, but which is equally opposed to the passage of the Sun’s rays. Much more than half the solar radiation is reflected by that layer of cloud, and we can suppose, on the whole, that the upper limit of a tolerable temperature is not passed at the surface of this planet. And as we have already said, since there is water and an atmosphere, we have no reason to doubt the habitability of Venus. _Mars_