155 CARBON PERAIANENTLY WITHDRAWN FROM THE AIR. 



ter, or by the more rapid process of combustion, — the constant addition 

 of carbonic acid derived from volcanoes, and from the combustion of fos- 

 sil coal, should gradually, though slowly, augment the proportion of this 

 gas in the air we breathe ; — unless it be perpetually undergoing a per- 

 manent diminution, to at least an equal extent, from the operation of 

 other causes. In- reference to this point there are three circumstances 

 which are proper to be considered : — 



1®. It has been observed that, as we recede from the land and ap- 

 proach the centre of great lakes, or sail into the open sea, the quantity 

 of carbonic acid in the air gradually diminishes. It is therefore inferred 

 that the sea is constantly, and to a sensible extent, absorbing carbonic 

 acid from the atmosf)here, without afterwards restoring it, so far as is 

 yet known, by any compensating process. 



2°. The waters which flow into the sea or great lakes constantly 

 bear down with them portions of animal and vegetable matter. These 

 fall along with the mud which the waters hold in suspension, and are 

 permanently imbedded in the deposits of clay, silt, and sand, which are 

 continually in the course of formation. 



3°. In many parts of the world, especially in the latitudes north and 

 south of 45°, vegetable matter accumulates in the form of peat, becomes 

 buried beneath clay and sand, and thus is prevented from undergoing 

 the ordinary process of natural decay. 



It is impossible to say how much carbon is permanently withdrawn 

 from the atmosphere by these several agencies. There is reason to be- 

 lieve that it is quite as great as the quantity added to the air by the 

 combustion of coal, and by the evolution of carbonic acid in volcanic 

 districts. Indeed, the supply from these two sources appears to return 

 only a small portion of that carbonic acid which is abstracted from the 

 air by the agencies just stated, and which have been in operation during 

 every geological epoch. 



Conclusions. — The general conclusions, therefore, which we seem jus- 

 tified in drawing in regard to the supply of carbonic acid to the atmos- 

 j)here are as follow : — 



1°. That a large portion of the carbonic acid absorbed by plants is 

 immediately and directly restored to the air by the respiration of the 

 animals which feed upon vegetable productions. 



2°. That a still larger portion is more slowly returned by the gradual 

 re-conversion of vegetable substances into carbonic acid and water dur- 

 ing the process of natural decay. 



3°. That nearly all the remainder is given back in the results of or- 

 dinary combustion. 



4°. That a further portion, which has not previously existed in the 

 atmosphere of our time, is conveyed to it by the burning of fossil fuel, 

 and by the emission of carbonic acid from cracks and fissures in the 

 surface of the earth ; yet that the quantity thus added cannot be sup- 

 posed to exceed that which is constantly and permanently separated 

 from the atmosphere by other causes. 



The balance of all the evidence we possess is probably in favour of 

 the opinion that the carbonic acid in the atmosphere is slowly diminish- 



