50 SOURCE AND ASSIMILATION OF NITROGEN. 



ces, they must contain less nitrogen than the food. When, 

 therefore, a field is manured with animal excrements, a smaller 

 quantity of matter containing nitrogen is added to it than has 

 been taken from it in the form of grass, herbs, or seeds. There- 

 fore, it follows that the favorable activity of such manure cannot 

 be due to its nitrogen. 



The liquid manure of animals must, on the other hand, be of 

 the highest value with respect to nitrogen : because it contains 

 all or nearly all the nitrogen originally present in the food con- 

 sumed. In order to comprehend more clearly the importance 

 of liquid excrements, it is necessary to consider the manner in 

 which they are formed. 



It is well known that the body of an adult man, or of ah animal 

 in a state of health, remains constantly the same, and neither 

 diminishes nor increases in weight to any appreciable extent. 

 In youth the case is different ; for an increase of the body is 

 occasioned. The same is the case in the artificial process of 

 fattening. The body of the old man, on the other hand, gra- 

 dually diminishes in size. 



The quantity of nitrogen and of other constituents in the body 

 cannot therefore increase, although the animal always receives 

 in his food a considerable quantity of that element. From this 

 it follows, that the quantity of nitrogen expelled from the body 

 must be the same as that taken in the food by an animal in a 

 state of nature, freely exposed to exercise ; for if this were not 

 the case, the body must acquire a larger proportion of nitrogen, 

 which we know it does not. 



When an individual is deprived of food and in the progress of 

 starvation, his body diminishes in weight, in Such a manner that 

 all parts, except the membranes and bones, participate in the 

 loss. By what means has the nitrogen of those tissues been 

 expelled from the system 1 



The emaciation which occurs proves that during every moment 

 in the life of an animal, part of its structure loses its vitality, and 

 assumes the form of dead matter. This, after suffering certain 

 changes, is finally separated from the system by the organs of 

 secretion, namely, the skin, lungs, and kidneys. The daily loss 

 thus experienced is restored by food. 



