72 SOURCE AND ASSIMILATION 



land there is a certain quantity of nitrogen con- 

 tained both in the corn and fruit which it produces, 

 and in the men and animals which feed upon them, 

 and also in their excrements. We shall suppose 

 this quantity to be known. The land is cultivated 

 without the importation of any foreign substance 

 containing nitrogen. Now, the products of this farm 

 must be exchanged every year for money, and other 

 necessaries of life, for bodies therefore which contain 

 no nitrogen. A certain proportion of nitrogen is 

 exported with corn and cattle ; and this exportation 

 takes place every year, without the smallest com- 

 pensation ; yet after a given number of years, the 

 quantity of nitrogen will be found to have increased. 

 Whence, we may ask, comes this increase of nitro- 

 gen ? The nitrogen in the excrements cannot repro- 

 duce itself, and the earth cannot yield it. Plants, 

 and consequently animals, must, therefore, derive 

 their nitrogen from the atmosphere. 



It will in a subsequent part of this work be shown 

 that the last products of the decay and putrefaction 

 of animal bodies present themselves in two different 

 forms. They are in the form of a combination of 

 hydrogen and nitrogen ammonia, in the temperate 

 and cold climates, and in that of a compound, con- 

 taining oxygen, nitric acid, in the tropics and hot 

 climates. The formation of the latter is preceded 

 by the production of the first. Ammonia is the 

 last product of the putrefaction of animal bodies ; 

 nitric acid is the product of the transformation of 



