As regards Mars, which commences the outer series of planets, we obtain a still more satisfactory result. We can clearly recognise the subdivision of the surface into water and dry land. Its atmosphere has properties which agree with those of our atmosphere. Not only is there certain evidence of the existence of water-vapour, but spectroscopic studies have proved also that the principal components of the atmosphere are the same as those of the Earth’s atmosphere--that is to say that there is oxygen and nitrogen. Sometimes the soil is hidden by groups of clouds, sometimes they disappear to appear again in other places. Its poles are encased in snow and ice, the white area of which varies in extent according to the seasons. Besides a number of enigmatical facts--we need only refer to the canals and their doubling--there are numerous meteorological phenomena on Mars which frequently occur in our own atmosphere. Although the temperature of Mars is, on account of its farther distance from the Sun, sensibly below that of the Earth, the difference is not sufficiently considerable to oppose a serious obstacle to the stable existence of organised matter in the torrid or temperate zone. The torrid zone of Mars must correspond approximately in climate to our temperate zone. We can therefore finally declare with entire conviction that the conditions offered by Mars are suitable for life as we know it on Earth. _Jupiter, Saturn, Uranus, and Neptune_ With the planets Jupiter, Saturn, Uranus, and Neptune we enter a region quite different from the preceding one. These bodies all possess atmospheres of great density in which, on Jupiter for instance, we can observe immense revolutions and violent cataclysms. They also contain water-vapour, but besides that they contain a substance characterised by a strong absorption of certain red rays. This gas is found in a small quantity on Jupiter, but in much greater quantities on Saturn and Uranus. On the latter planet the atmosphere, apart from the water-vapour contained in it, does not seem to have any analogy with ours; strong absorption-bands show in the less refrangible portions of the spectrum. As we have seen that oxygen is not absolutely necessary to organic life, the single fact that the outer planets have an atmosphere may suffice to show that our first condition is fulfilled. The presence of water-vapour in their atmosphere proves the existence of water and fulfils the second condition. As regards the third condition, Jupiter also seems to satisfy that, at least in equatorial regions, especially if we take into account that radiation of heat into space is much limited by the thick atmosphere filled with clouds. The farther we go away from the Sun the more does the third condition become precarious, and while we may have some doubt concerning Saturn, it cannot be denied that on Uranus and Neptune the solar heat is insufficient to support organised life in a durable way.