If Venus should likewise always turn the same side towards the sun, as many astronomers assert, Venus should not have any notable atmosphere, nor clouds either. We know, however, that this planet is surrounded by a very marked developed atmosphere.[5] [Footnote 5: That results from the very strong refraction which light undergoes in the atmosphere of Venus when this planet is seen in front of the sun’s edge during the so-called Venus transits.] And that is the strongest objection to the assumption that Venus follows the example of Mercury as regards the rotation about its own axis. Since, now, warm ages have alternated with glacial periods, even after man appeared on the earth, we have to ask ourselves: Is it probable that we shall in the coming geological ages be visited by a new ice period that will drive us from our temperate countries into the hotter climates of Africa? There does not appear to be much ground for such an apprehension. The enormous combustion of coal by our industrial establishments suffices to increase the percentage of carbon dioxide in the air to a perceptible degree. Volcanism, whose devastations— on Krakatoa (1883) and Martinique (1902)—have been terrible in late years, appears to be growing more intense. It is probable, therefore, that the percentage of carbonic acid increases at a rapid rate. Another circumstance points in the same direction; that is, that the sea seems to withdraw carbonic acid from the air. For the carbonic acid percentage above the sea and on islands is on an average 10 per cent. less than the above continents. [Illustration: Fig. 17.—Photograph of the surface of the moon, in the vicinity of the crater of Copernicus. Taken at the Yerkes Observatory, Chicago, U. S. A. Scale: Diameter of moon, 0.55 m. = 21.7 in. Owing to the absence of an atmosphere and of atmospheric precipitations, the precipitous walls of the crater and other elevations do not indicate any signs of decay.] If the carbonic acid percentage of the air had kept constant for ages, the percentage of the water would have found time to get into equilibrium with it; but the sea actually absorbs carbonic acid from the air. Thus the sea-water must have been in equilibrium with an atmosphere which contained less carbonic acid than the present atmosphere. Hence the carbonic acid percentage has been increasing of late.