Thus, it would seem as if the mean temperature of the Earth’s surface hardly had changed to any extent worth mentioning during immense spans of time estimated to about 500 million years. Nevertheless, a slow process of cooling proceeding toward the centre of the planet probably takes place. Ever growing quantities of matter are transported from the interior of the earth through volcanic action. Sedimentary deposits increase continuously while the interior becomes hollow. As a result the crust must gradually settle, causing large cracks in the process. For these weakened places the volcanic products show a special liking and the craters are strung out in lines along such fissures. In other places, where volcanic action is less pronounced, hot springs appear instead, generally emitting carbon dioxide in abundance, occasionally also sulphurous acid and sulphuretted hydrogen. The dislocations in the crust also take place along these cracks accompanied by earthquakes. The study of these various phenomena has enabled us to map out the fissures, which generally radiate in nearly straight lines from one point, the so-called centre of collapse, as the cracks in a pane of glass issue from the point of breakage caused by a swift blow. We shall later see that such breakage lines and centres of collapse are common on all stellar bodies which possess a solid crust and are observable from the Earth. We may now easily form an idea of the general trend in the development of the atmosphere. The gases originally present were all, except the hydrogen, the nitrogen, and the rare gases, strongly absorbent of light and in particular of heat. It is, therefore, natural that the planets which have not formed a solid crust possess a strongly absorbing vapour-shell, as indeed is the case with the large planets (compare Fig. 13). The crust once formed and the air gradually purged of these gases, thanks to the sunlight, so that mainly nitrogen and oxygen, small quantities of the rare gases, and carbon dioxide besides water remained, the temperature fell rather rapidly. Carbon dioxide formed the last effective heat-conserving ingredient. As the crust grew thicker, the supply of this gas diminished and was further used up in the processes of disintegration. As a consequence the temperature slowly decreased, although decided fluctuations occurred with the changing volcanic activity during different periods. Supply and consumption of carbon dioxide fairly balanced as disintegration ran parallel with the proportion of this gas in the air. But evolution on the whole can only proceed in one direction toward a final cooling of the Earth. This must occur if for no other reason because the store of energy in the Sun and therefore its radiation must slowly decrease. With deepening crust and disappearing carbon dioxide vegetation must ebb, and with it the production of oxygen. This gas also partakes in the general disintegration through oxidation of iron protoxides in the