* * * * * (The atmospheres might be formed of gases which would make the radiation almost zero and would produce a relatively high temperature. This certainly happens in the case of Mars, whose temperature is not lower than that of our globe, where the polar snows are less dense than ours and melt more completely in the course of the summer.) * * * * * But another peculiarity presents itself which may reverse all our ideas relative to the planets from Jupiter outwards. Certain observations tend to show that Jupiter is not yet cooled down, that its real nucleus is still fused or perhaps even in the gaseous state, and that it has not yet formed a solid crust upon which life might develop. Besides the phenomena directly observed or revealed in the spectroscope, the very small specific gravity of these planets also supports that hypothesis--the density of Saturn is about the same as that of cork. It is very difficult to form an exact idea of the constitution of these planets. On the other hand, it is possible to believe that one of the extreme planets, Uranus for instance, is sufficiently cooled to possess at least a liquid surface which, on account of the internal heat of the planet, may have preserved for a certain period a temperature sufficient to entertain life even after the solar heat is no longer sufficiently powerful But these are only hypotheses. * * * * * Let us summarise in a few words the results of this chapter. On the Moon no organic matter can exist; we may suppose living beings to exist in a small zone of Mercury; the surface of Venus is very probably habitable in most of its regions; Mars is certainly habitable and probably under such conditions that certain species of our plants and animals could, if transported to that planet, continue to live there. For the other planets the possibility of habitation cannot be entirely denied, but the existence of living beings on their soil is not probable. Let us add, for the sake of completeness, that on the Sun, which radiates the most intense heat, and on those innumerable other suns which we call stars and which the telescope reveals to us, living matter certainly does not exist. It follows that of the millions of stars visible to us in the universe there are only two or three which we could consider with any certainty capable of being inhabited as we conceive it. That seems a rather unsatisfying result, which allows an icy sentiment of loneliness in infinite space to take possession of our souls.