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 he 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., &c. 
