It is with these words that Kant in his _Natural History of the Heavens_ terminates the last chapter, which treats of the habitability of the planets. They come from the soul of every thinking man who has preserved the spark of the ideal. The aspect of the starry sky awakens all those who are not among these vulgar spirits the same sensations which manifest themselves in various ways according to the education of him who experiences them and according to his momentary disposition. The astronomer also is delighted by this spectacle in spite of his regular observation of the sky. The astronomer sees farther and clearer into the celestial spaces than the ordinary man, his knowledge leads him by rapid deductions to vast considerations until he suddenly reaches a point where, for the time being at all events, an insuperable barrier presents itself to the human spirit, and where an imperious halt reminds him of the truth that all science is incomplete. The ordinary man looks upon the sky in quite a different way, and so does a woman. To both, the spectacle of the starry sky offers an enjoyment independent of all research and all preoccupation. A purely æsthetic pleasure. Are these stars inhabited? Are they inhabited by thinking beings? Do love and hate reign there as they do here? Such are the questions which occur at once. Afterwards, in hours when the need for hope and consolation is felt, the desire arises to contemplate some day with our own eyes the splendours of those other worlds and to be able to soar up to them. The most brilliant stars are the special objects of such wishes, and if they could be fulfilled a population of loving souls would reside on these same stars: Sirius, Vega, Venus, and Jupiter. The question of the population of the celestial bodies is as ancient as the discovery of the individual existence. It has always occupied thinking humanity, and many have sought to raise the veil which hides the answer. We, also, should like to take a step towards this solution, and it is with that object that we propose to quote here the actual facts bearing upon the conditions of habitability of the celestial bodies and to draw the most probable conclusions. We can combine the astronomical data with the most recent use of physics, and, thanks to the progress recorded by these two sciences in the last ten years, we may be able to draw some new conclusions. (Here the author passes in review the older writers, Huygens, Kircher, and Fontenelle, which I have summarised in my two books called _The Plurality of Inhabited Worlds and Imaginary Worlds_. He arrives eventually at the nineteenth century and at Gruithuisen, whom my readers know less.) HISTORICAL