310 PHYSIOLOGY. BOOK II. 



good sense, " that chemistry has hardly advanced the art of 

 agriculture a single step, but that the latter remains, after all 

 the investigations of the chemists, a mere empirical art." 



All that chemistry can be said to have ascertained with 

 regard to the general properties of manures amounts to this, 

 that those are the best which part with carbonic acid most 

 slowly and steadily. If carbonic acid is at first evolved 

 rapidly, plants become gorged, as it were, and are stimulated ; 

 and if, as is usual in such cases, the carbonic acid is afterwards 

 liberated very slowly, they become starved. This is especially 

 the case with animal matter. But, according to Payen, the 

 addition of animal charcoal to such matter moderates its de- 

 composition : at first this mixture yields but little carbonic 

 acid; but, as the charcoal becomes saturated with the pro- 

 ducts furnished by the alteration of the decaying matter, the 

 decomposition of the latter is accelerated, and corresponds 

 with the progress of vegetation. 



Those who wish to understand the modern opinions con- 

 cerning the action of manures (properly so called) should 

 consult De Candolle's " Physiologic," p. 1278., and some 

 papers by Payen in the " Annales des Sciences naturelles," 

 vol. XXX., 8cc. 



