OF AGKICULTURE. 21 



genous matters may yield fat, with other products, 

 such as urea which passes off by the urine, and 

 carbonic acid which passes off by respiration. 



The non-nitrogenous constituents of the food on 

 the other hand — the fatty matters, and the so-called 

 carbohydrates — starch, sugar, cellulose, &c, are sup- 

 posed to be the chief sources of the carbonic acid 

 respired, and hence have sometimes been classed as 

 the respiratory constituents of food. 



The questions arise — Is the non-nitrogenous sub- 

 stance fat, which is stored up in the feeding of the 

 herbivora, derived chiefly from the nitrogenous 

 matters of the organs, or in the circulation, or from 

 the non-nitrogenous matters of the food, the carbo- 

 hydrates 1 Then, again, in the exercise of force, is 

 the increased amount of the products of oxidation 

 eliminated from the bodv chieflv due to an increased 

 transformation of the nitrogenous substance of the 

 tissues and fluids, or of the non-nitrogenous con- 

 stituents — the fat of the body, or of the food, and 

 the carbohydrates \ 



In reference to the sources of the fat of the animal 

 body, Liebig maintained that the vegetable food 

 consumed by herbivora did not contain anything 

 like the amount of fat which they stored up in 

 their bodies ; and he showed how nearly the com- 

 position of fat was obtained by the simple elimina- 

 tion of so much oxygen, or of oxygen and a little 

 carbonic acid, from the various carbohydrates of the 

 vegetable food — starch, sugar, &c. Much less oxygen 

 would be required to be eliminated from the nitro- 

 genous constituents, such as fibrine, &c, than from 

 a quantity of carbohydrate containing an equal 

 amount of carbon. The formation of fatty matter 

 in plants was of the same kind ; it was the result of 



