Chamberlin relates that, together with five other American geologists, he attempted to estimate how long a time would be required before the carbon dioxide of the air would be consumed by the weathering of rocks. Their various estimates yielded figures ranging from 5000 to 18,000 years, with a probable average of 10,000 years. The loss of carbonic acid by the formation of peat may be estimated at the same figure. The production of carbonic acid by the combustion of coal would therefore suffice to cover the loss of carbonic acid by weathering and by peat formation seven times over. Those are the two chief factors deciding the consumption of carbonic acid, and we thus recognize that the percentage of carbonic acid in the air must be increasing at a constant rate as long as the consumption of coal, petroleum, etc., is maintained at its present figure, and at a still more rapid rate if this consumption should continue to increase as it does now. This consideration enables us to picture to ourselves the possibility of the enormous plant-growth which must have characterized certain geological periods of our earth—for instance, the carboniferous period. This period is known to us from the extraordinarily large number of plants which we find embedded in the clay of the swamps of those days. Those plants were slowly carbonized afterwards, and their carbon is in our age returned to its original place in the household of nature in the shape of carbonic acid. A great portion of the carbonic acid has disappeared from the atmosphere of the earth, and has been stored up as coal, lignite, peat, petroleum, or asphalt in the sedimentary strata. Oxygen was liberated at the same time, and passed into our atmospheric sea. It has been calculated that the amount of oxygen in the air—1216 billion tons—approximately corresponds to the mass of fossil coal which is stored up in the sedimentary strata. The supposition appears natural, therefore, that all the oxygen of the air may have been formed at the expense of the carbonic acid in the air. This view was first advanced by Kœhne, of Brussels, in 1856, and later discussions have strengthened its probability. Part of the oxygen is certainly consumed by weathering processes, and absorbed—_e.g._, by sulphides and by ferro-salts; without this oxidation the actual quantity of oxygen in the air would be greater. On the other hand, there are in the sedimentary strata many oxidizable compounds—_e. g._, especially iron sulphides—which have probably been reduced by the interaction of carbon (by organic compounds). A large number of the substances which consume oxygen during their decomposition and decay have also been produced by the intermediation of the coal which had previously been deposited under liberation of oxygen, so that these substances are, by their oxidation, restored to their original state. We may hence take it as established that the masses of free oxygen in the air