Page:Popular Science Monthly Volume 76.djvu/582

578 with this gas, notwithstanding the fact that great coal beds and many great limestones had not yet been formed.

Geikie recognizes a continual abstraction of carbon dioxide since land plants began to live, but only allows that the amount in the air in Paleozoic time may have been somewhat greater than now. Davis thus expresses the view which has been long current:

These views, it will be seen, follow naturally upon the nebular hypothesis, with its mass of heated gases undergoing consolidation.

We find under discussion at the present time the view that the atmosphere is not greatly different from that of early geological periods, but has been subject to important fluctuations in the amounts and proportions of its constituents. These changes are believed to be due to many causes, some effecting loss and some bringing about renewal. Various interchanges are postulated, on the one hand, between the earth and outside spaces, and on the other between the atmosphere and the crust or the interior of the globe. This line of investigation has been recently pursued by Chamberlin and others, particularly with reference to its bearing on glacial climates, and has involved new conceptions of the origin of the earth. But entirely apart from the possible validity of these reasonings, the researches have value in setting forth the changes to which the atmosphere is subject. These changes have so much to do with the earth's crust that they are germane to our theme.

It is shown that the atmosphere loses carbon dioxide in several ways; as through carbonation, that is, by the decomposition of silicates and the formation of carbonates of calcium and magnesium, in limestone and dolomites of great extent. This decomposition is extensive in times of elevation of the lands, such as have occurred widely in some geological periods. When the lands are high, the water table is farther below the surface, and the air pierces deeply, with its chemical activities, and the ground waters also have much more vigorous circulation. The carbon dioxide thus employed in making limestone is extracted from the atmosphere.

There is loss of carbon dioxide through fixation in coal, oil and in all organic matter, diffused through the sedimentary rocks. There is temporary loss of this gas through the ordinary feeding of plants, and the view has been held, that plant growth would exhaust the CO2 of the atmosphere in one hundred years, but for the renewal through