Volcanism is the natural process by which the greatest amount of carbonic acid is supplied to the air. Large quantities of gases originating in the interior of the earth are ejected through the craters of the volcanoes. These gases consist mostly of steam and of carbon dioxide, which have been liberated during the slow cooling of the silicates in the interior of the earth. The volcanic phenomena have been of very unequal intensity in the different phases of the history of the earth, and we have reason to surmise that the percentage of carbon dioxide in the air was considerably greater during periods of strong volcanic activity than it is now, and smaller in quieter periods. Professor Frech, of Breslau, has attempted to demonstrate that this would be in accordance with geological experience, because strongly volcanic periods are distinguished by warm climates, and periods of feeble volcanic intensity by cold climates. The ice age in particular was characterized by a nearly complete cessation of volcanism, and the two periods at the commencement and at the middle of the Tertiary age (Eocene and Miocene) which showed high temperatures were also marked by an extraordinarily developed volcanic activity. This parallelism can be traced back into more remote epochs. It may possibly be a matter of surprise that the percentage of carbon dioxide in the atmosphere should not constantly be increased, since volcanism is always pouring out more carbon dioxide into our atmosphere. There is, however, one factor which always tends to reduce the carbon dioxide of the air, and that is the weathering of minerals. The rocks which were first formed by the congelation of the volcanic masses (the so-called magma) consist of compounds of silicic acid with alumina, lime, magnesia, some iron and sodium. These rocks were gradually decomposed by the carbonic acid contained in the air and in the water, and it was especially the lime, the magnesia, and the alkalies, and, in some measure also the iron, which formed soluble carbonates. These carbonates were carried by the rivers down into the seas. There lime and magnesia were secreted by the animals and by the algæ, and their carbonic acid became stored up in the sedimentary strata. Högbom estimates that the limestones and dolomites contain at least 25,000 times more carbonic acid than our atmosphere. Chamberlin has arrived at nearly the same figure—from 20,000 to 30,000; he does not allow for the precambrian limestones. These estimates are most likely far too low. All the carbonic acid that is stored up in sedimentary strata must have passed through the atmosphere. Another process which withdraws carbonic acid from the air is the assimilation of plants. Plants absorb carbonic acid under secretion of carbon compounds and under exhalation of oxygen. Like the weathering, the assimilation increases with the percentage of carbonic acid. The Polish botanist E. Godlewski showed as early as 1872 that various