Geological llistoru of the Atmosphere. 91 



silicates are produced), the reaction in both cases being 

 accompanied by the evolution of carbonic acid. Another 

 way in which carbonic acid may be evolved through volcanic 

 action, is by heating rocks containing oxide of iron in pre- 

 sence of coal or carbonaceous matter to such an extent that 

 metallic iron is produced, and carbonic acid and carbonic 

 oxide are evolved. 



Now it is highly probable that throughout geological 

 history, volcanic action has varied considerably in its degree 

 of activity. At any rate, we know for a certainty that it has 

 varied to a very great extent in special areas or localities ; 

 and it is quite probable that on the earth, taken as a whole, 

 volcanic activity has varied very considerably in degree in 

 different geological epochs. Even if the amount of volcanic 

 action as inferred from the production of volcanic rocks and 

 other heating effects should have been fairly constant on the 

 earth as a whole, yet the amount of carbonic acid produced 

 or evolved may have varied very considerably according to 

 variations in the composition and distribution of the rocks 

 >ubjeeted to heating. The question therefore naturally arises 

 as to whether such variations may have taken place to such 

 an extent as to affect materially the amount and percentage 

 of atmospheric carbonic acid. 



In order to approach this question, it is obviouslv necessarv 

 that we should ascertain with a sufficient degree of approxi- 

 mation (1) the amount of carbonic acid at present existing in 

 the atmosphere ; (2) the amount of carbonic acid annually 

 produced or evolved into the atmosphere by the principal 

 agencies which have this effect: (3) the amount which is 

 annually decomposed or removed from the atmosphere by other 

 agencies: (4) the possible and probable variations that take 

 place in the degree of activity of these various agencies : and 

 (5) the methods in and extent to which these agencies react 

 on each other. 



As regards the amount of carbonic acid in the atmosphere, 

 this may be taken in round figures as 2*2 x 10 12 tons (rather 

 more than two million million tons). The most recent and 

 reliable determinations of atmospheric carbonic acid give a 

 figure considerably lower than those which formerly were 

 accepted. Prof. Letts, of Belfast, investigated the whole 

 >ubject very carefully a few years ago. and found that the 

 average amount of carbonic acid in the air at Queen's ( lollege, 

 Belfast, was only '!'{)?> volumes in 10,000 volumes of air, or 

 0*0293 per cent, by volume; and this figure is more likely 

 to be over than under the average for the whole earth, as the^ 

 air of cities i< richer in carbonic acid than ordinary sea and 



