154 THE CHEMISTRY OF THE FARM 



digestibility may, however, be counteracted in great 

 measure by supplying with the potatoes or mangels 

 some nitrogenous food. When this is done the pro- 

 portion of roots or potatoes may be double that jast 

 mentioned without a serious loss of digestibility. 

 Potatoes exercise a greater depressing effect on the 

 digestibility of hay than roots, starch being more 

 potent in this respect than sugar. The cereal grains 

 are rich in starch, but contain also a fair proportion 

 of albuminoids ; they may be added to dry fodder 

 without seriously affecting its digestibility, if the pro- 

 portion of the nitrogenous to the non-nitrogenous 

 constituents of the diet is not less than 1:8. 



Common salt is well known to be a useful addition 

 to the food of animals, but experiments have failed 

 to show that it increases the digestibility of food. 

 When sodium salts are deficient in the food, salt 

 supplies the blood with a necessary constituent. 

 Sodium salts are tolerably abundant in roots and 

 cabbage, and small in quantity in hay and straw ; they 

 are absent in grain of all kinds. According to Bunge, 

 salt is needed only in the case of foods containing a 

 large proportion of potash to soda. Of these the potato 

 is the most prominent example. A small quantity of 

 salt is recommended as a general ingredient of animal 

 diets ; but an excess of salt interferes with nutrition by 

 increasing the quantity of water drunk, and with this 

 the degradation of albumin and the production of urea. 



Comparative Nutritive Value of Foods. — (1) Propor- 

 tion of Digestible Matter. Having made ourselves 

 acquainted both with the composition, and with the 

 degree of digestibility of ordinary cattle foods, we are 



