72 CARBONIC ACID. 



in it, possesses this absorbent power in the same 

 degree as a lime soil with a small admixure of clay; 

 but the quantity of humus present will alter the 

 absorbent relation. 



" By a closer examination, we perceive that this 

 power of arable soil differs in proportion to its 

 greater or less porosity ; a dense, heavy clay, and a 

 loose sandy soil possess this absorbent power in the 

 smallest degree." 



Dr. Seller calculated the annual conversion of 

 the carbon of organic matter into inorganic carbonic 

 acid at not less than six hundred millions of tons ; 

 and infers, on the most favorable aspect of the 

 amount of soil over the earth's surface, that such an 

 annual loss could not be withstood beyond six thou- 

 sand years ; and on a less exaggerated assumption 

 of its amount, probably very near the truth, that 

 the waste would absorb the whole of the existing 

 organic matter of the soil in about seven hundred 

 and forty years. Dr. Seller contends that the truth 

 of these conclusions remains unaltered, even if it be 

 considered that much of the carbon of plants is 

 drawn, not from the organic matter of the soil, but 

 from the inorganic carbonic acid of the atmosphere, 

 unless some inorganic source of their hydrogen and 

 oxygen be at the same time admitted. He therefore 

 regards Licbig's view of the inorganic nature of the 

 food of plants as supported not only by many 

 special facts, — such, for example, as that increase 



