CLIMATE AND CARBONIC ACID. 245 



content of carbonic acid have been the direct cause of variations of 

 climate. It is necessary, therefore, to assign agencies adequate to bring 

 about such alternations of poverty and wealth of carbonic acid. The 

 agencies must operate to produce great cycles of climatic change which 

 are recognized through study of the geologic record. Among others, 

 these comprise an ancient event of extensive glaciation in India, 

 Australia and South Africa, closely following the period of mild 

 climate during which the Coal Measure flora flourished. The agencies 

 must further promote subsidiary action by which minor oscillations of 

 climate may be explained, since within the latest, the Pleistocene, 

 period of glaciation, at least five, and probably more, advances of the 

 ice occurred in alternation with intervals of comparative mildness, dur- 

 ing which the ice retreated notably. Depletion and enrichment of the 

 atmosphere must furthermore occur within reasonable limits of geologic 

 periods. And cause must be shown why the atmospheric changes pro- 

 moted glaciation about peculiarly local centers. In searching the 

 sources of carbonic acid, Chamberlin has been led to reconsider the 

 original constitution of the atmosphere, and thus also theories of the 

 origin of the earth, including the nebular hypothesis. Thither this 

 review may not follow him, but it will be of interest to advert to his 

 views as to the conditions affecting biologic evolution, which are also 

 causally related to variations of the carbonic acid contained in the air. 



Carbonic acid, or as it is more accurately called, carbon dioxide, 

 COo, occurs in many relations and plays many parts in the economy 

 of the world. In some of these activities it enters into permanent com- 

 binations and is lost to the atmosphere. In others it passes through 

 cycles of combination and release by which it is temporarily with- 

 drawn from and subsequently returned to the air. If the atmosphere's 

 resources in CO2 be compared with a bank account, we may suppose 

 that the balance follows one or the other of two familiar cases. In the 

 one example there may have been originally a definite though possibly 

 large deposit, which has not since been added to, but upon which many 

 drafts have been and are being drawn. Under this assumption, how- 

 ever rich the atmosphere once was, it is now by comparison poverty 

 stricken. On the other hand, there is reason to believe that the original 

 capital in CO2 was not materially greater than it is now, but that 

 losses have been nearly balanced by gains. The first example represents 

 a view held by geologists who believe that the atmosphere was exceed- 

 ingly dense, moist and charged with carbon dioxide in early ages of the 

 earth's history; the second illustrates the conceptions based on modern 

 advances of biology and geology, and its acceptance is essential to the 

 hypothesis of glaciation here discussed. 



The sources from which fresh contributions may be made to the 

 atmosphere are suggested by the occurrence of carbon dioxide among 



