70 THE CHEMISTRY OF THE FARM. 



4. Excretion. — The products which result from the 

 oxidation of tissue, or of the food consumed, are removed 

 from the body by the lungs, the kidneys, or the skin. 

 The chief products of oxidation in the body are carbonic 

 acid, water, urea, and salts. Carbonic acid is removed 

 through the lungs, and to a smaller extent by the skin ; 

 urea and salts by the kidneys ; water by all the organs of 

 excretion. 



Non-nitrogenous substances, as fat and sugar, when 

 oxidised in the body, yield .simply water and carbonic acid. 

 The nitrogen of the albuminoids, gelatinoids, and amides 

 is not oxidised, but is excreted in the fomi of urea. The 

 sulphur of the albuminoids is apparently oxidised to sul- 

 phuric acid. 



The quantity of nitrogen in the urine is a measure of 

 the albuminoids, gelatiuoids, and amides oxidised in the 

 body. In the urine are also removed all the salts not 

 required for the animal economy ; sodium and potassium 

 salts are generally abundantly present. 



The solid excrement contains the undigested parts of 

 the food, with the residues of the bile, and other secretions 

 of the alimentary canal. 



