578 TEE POPULAR SCIENCE MONTHLY 



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 : 



Some of the more volatile mineral substances in the rock-crust of the earth 

 presumably at an early time made a part of the atmosphere, but all these have 

 long ago left it. Nearly all of the water that must have once been boiled off in 

 the steaming atmosphere of early times has now condensed upon the cooled 

 surface of the earth, forming the deep oceans. Some of the gases themselves, 

 particularly the oxygen of the air, must have been much diminished by com- 

 bining with the surface rocks of the earth's crust and rusting them. 2 



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 con- 

 ceptions of the origin of the earth. But entirely apart from the pos- 

 sible 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 lime- 

 stone 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 circu- 

 lation. The carbon dioxide thus employed in making limestone is ex- 

 tracted 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 C0 2 of 

 the atmosphere in one hundred years, but for the renewal through 



3 " Elementary Meteorology," W. M. Davis, p. 3. 



