472 CHANGES PRODUCED UPON THE FOOD 



to stand in pools and ferment. Twenty cart-loads of such dung may 

 hasten the growth of the turnip crop in a less degree than half the 

 weight will do, where the liquid manure has been carefully collected 

 and returned upon the heaps — to hasten and complete their fermenta- 

 tion, and to saturate them with enriching matter. 



Since, then, the quality or richness of the dung of the same animal is 

 liable to be affected by so many circumstances — it is obvious that no 

 accurate general conclusions can be drawn in regard to its precise 

 fertilizing virtue when applied to this or to that crop, or to its relative 

 fertilizing value when compared with equal weights of the dung of 

 other animals. The results obtained in one set of analyses, as in that 

 of Boussingault, or in one series of practical experiments, as in that 

 of Hermbstadt (p. 470), will not agree with those obtained in any 

 other — because the substances themselves with which our different 

 experiments are made, though called by the same name, are yet very 

 unlike in their chemical properties and composition. 



§ 16. Of the c flanges which the food undergoes in passing through 

 the bodies of animals. 



It is the result of long experience that vegetable matter is more 

 sensibly active as a manure, after it has passed through the body of an 

 animal, than if applied to the land in its unmasticated and undigested 

 state. In becoming animalized, therefore — as it has been called — 

 vegetable substances have been supposed to undergo some mysteri- 

 ous, because not very obvious or intelligible, internal change, by which 

 this new virtue is imparted to them. Yet the change is very simple, 

 and when explained is not more satisfactory than it is beautiful. 



You will recollect, as I have already stated to you (p. 469), that 

 the weight of dry manure voided by an animal is always considerably 

 less than that of the dry food eaten by it. Upon the nature and 

 amount of this loss which the food undergoes depends the quality of 

 the manure obtained. 



This you will readily comprehend from the following statement : 



1°. Every thing which enters into the body in the form of food must 

 escape from the body in one or other of three different forms. It must 

 be breathed out from the lungs, perspired by the skin, or rejected in the 

 solid or hquid excretions. We have already seen (Lee. VIII., § 3), 

 that the function of the lungs is to give off carbon in the form of car- 

 bonic acid, while they drink in oxygen from the air — and that the quan- 

 tity of carbon thus given off by a healthy man varies from 5 to 13 or 

 more ounces in the 24 hours. From the skin also carbon escapes along 

 with a small and variable proportion of saline matter. The weight 

 of carbon given off by the skin has not been accurately ascertained. 

 Let us leave it out of view for a moment, and consider solely the ef- 

 fect of respiration upon the nature of the solid and liquid excretions. 



Suppose a healthy man, taking a moderate » degree of exercise, to 

 give off from his lungs 6 ounces of carbon in 24 hours, and to eat 

 durmg the same time 2 lbs. ^f potatoes, half a pound of beef, and 

 half a pound of bread. TheLi he has taken in his food — 



