These successive augmentations and diminutions of luminous brilliance are not uncommon in the stars with which we are acquainted. According to M. Flammarion,[25] star ο of the _Whale_ varies very much in luminous intensity and the constellation itself frequently disappears. Star χ of the _Swan_ passes from the fifth to the tenth size under our eyes, the thirtieth star of the _Hydra_, which is of the fourth size, almost always disappears at intervals of 500 days. These variations must, as M. Flammarion observes, produce strange results. To-day, the radiant star is shedding floods of light and fire upon the planets which it governs, and the soul of that planet is warmed by its burning rays. A few months later, without the least cloud in the sky, the shining of the sun becomes fainter, and then, by degrees, the obscurity increases, until at length the planet is plunged into thick darkness. When the diminution of the light of the sun is periodical, this universal night lasts for a fixed time, at the end of which the light returns, if not, the darkness is dispersed after varying periods. The light grows, little by little, until at length the radiant star reappears in all its primitive brightness. The fine days, the glorious light returns, until the moment when the same fading recommences and the darkness sets in once more. Can we picture to ourselves the strange alterations which nature undergoes in regions which are subjected to torrid heat and glacial cold by turns? I am convinced that the _glacial period_ which geologists have defined in the history of our globe, during which an extraordinary and sudden lowering of the temperature caused the death of multitudes of living beings, and covered Europe with glaciers from the mountains--was caused by a momentary weakening of the intensity of the sun's light. When it resumed its ordinary brightness, the sun dispersed the ice which had covered the earth with a death mantle.[26]