Geological History of the Atmosphere. 95 



not usually regarded as being volcanic. It is well known 

 that '• ground air/ 3 that is. air lying close to the ground or 



enclosed within the upper layers of the soil, is usually richer 

 in carbonic acid than the atmosphere in general. This higher 

 percentage of carbonic acid in ground air is probably for the 

 most part due to eremacausis or oxidation of organic matter 

 within the soil : but possibly it may not be all due to this cause, 

 some of it may come from a deeper source. The carbonic 

 acid that occurs so frequently in wells and pit-workings is not 

 likely to be all produced by eremacausis in the upper layers 

 of the soil, it is more likely to have a deeper source even 

 though the source should not be of a volcanic nature. 



There are still one or two other ways deserving of notice 

 in which carbonic acid is produced and added to the atmo- 

 spheric supply. The process of fermentation (natural and 

 artificial) is one of these, and possibly it is of some quanti- 

 tative importance: but still it may be regarded as a phase of 

 eremacausis, or at le;ist as a stage in the oxidation of certain 

 substances, and therefore it hardly needs to be discussed 

 separately. The combustion of ordinary coal should also be 

 mentioned, as coal is now mined and consumed on a very large 

 scale, the amount now raised being about 800,000,000 tons 

 per annum. Taking the average amount of carbon in coal as 

 80 per cent., the burning of the above quantity of coal will 

 cause the addition of about 1700 x 10 6 tons carbonic acid to 

 the atmosphere annually. This amount is large enough to 

 have an important bearing on the question as to whether the 

 percentage of carbonic acid in the atmosphere is increasing 

 at present or not : but still it is obvious that the whole period 

 during which coal has been mined on a large scale is not 

 very great (from a geological standpoint) ; and we may quite 

 well leave the raising and burning of coal out of consideration 

 for the present as a noteworthy factor in the history of the 

 atmosphere. Jt is therefore evident that we must look 

 upon the carbonic acid of telluric or subterranean origin as 

 being sufficient not only to make good the loss of atmo- 

 spheric carbonic acid which is caused by the weathering of 

 rocks, but also to make good the difference between the 

 amount that is decomposed by the growth of vegetation and 

 the amount that i- restored to the atmosphere by the oxidation 

 of vegetable and other organic remains. It is somewhat 

 unfortunate that there ir- so little information regarding the 

 amount of telluric carbonic acid annually evolved, but there i> 

 no difficulty in understanding that it maybe or could be vory 

 large when we consider the amount of limestone and other 

 carbonate- exi-ting in the earth, and the way in which the) 



