90 Mr. J. Stevenson on the Chemical and 



there was probably a long period during which the forces or 

 agencies which removed carbonic acid from the atmosphere 

 were stronger than those which produced or returned it. 



However, it is quite possible that after a certain point was 

 reached in this abstracting process, the amount of carbonic 

 acid in the atmosphere may have become variable; that is to 

 say, the quantit}^ may have alternately increased and dimi- 

 nished according to the relative activity of the forces or 

 reactions which produce or evolve carbonic acid, and the 

 counteracting forces or reactions which decompose carbonic 

 acid or otherwise remove it from the atmosphere. 



The view that the amount of atmospheric carbonic acid 

 varies or may vary within very wide limits is not a new one ; 

 at least the view that the percentage was at one tiem 

 very different from what it is now is by no means new. 

 Hugh Miller and probably most of his contemporaries were 

 of opinion that the amount of carbonic acid in the atmosphere 

 at the beginning of the Carboniferous epoch was very much 

 greater than it is now, the difference being represented 

 roughly by the amount of coal deposited during and since 

 that period, and therefore amounting to many times, possibly 

 one hundred or even several hundred times, the amount of 

 our present atmospheric carbonic acid. Probably, however, 

 the view 7 usually taken by the earlier geologists on the 

 question was not so much that the amount of atmospheric 

 carbonic acid varies from time to time, as simply that the 

 amount was very much greater in very early times than it is 

 now. That is not quite the view taken in the present article. 

 We propose to give reasons in support of the view that 

 the amount of carbonic acid in the atmosphere has varied 

 very considerably by increasing at certain times as w r ell as 

 by decreasing at others, during a very large part of geolo- 

 gical history — possibly enough in early epochs such as the 

 Cambrian and Silurian, as well as in more recent times. 



It is obvious that among the forces which produce or 

 evolve carbonic acid a prominent place is occupied by 

 volcanic action. It has long been known that carbonic acid 

 is evolved from volcanoes and also from the ground in 

 volcanic districts even where the volcanoes are no longer 

 active. This volcanic or telluric carbonic acid (as it was 

 called by Prof. Dittmar) is probably for the most part pro- 

 duced by the heating of rocks containing limestone or other 

 carbonates along with sandstone (silica), or with rocks con- 

 taining a high percentage of silica to such an extent that 

 the carbonates are decomposed and silicates are produced (or 

 silicates with a larger percentage of base than the original 



