39 AGRICULTURAL CHEMISTRY 



grams or more. Meats and nearly all animal foods con- 

 tain about one per cent, of mineral matter of which about 

 half is phosphoric acid. Milk and eggs contain phos- 

 phates and mineral matter in liberal amounts. In a 

 mixed ration of three or more food articles, there is al- 

 ways enough phosphates and mineral matter for purposes 

 of nutrition. A part of the excess of phosphates in a 

 ration is eliminated through the kidneys. The feces also 

 contain phosphoric acid. Inability of the organs to as- 

 similate phosphates, due to malnutrition and lack of 

 available forms of other nutrients, is more frequently a 

 source of trouble than lack of phosphates in the food. 



It is estimated that in the ration, of an adult, about 20- 

 grams per day of sodium chlorid are necessary. This 

 compound takes an important part in nutrition and is a 

 normal constituent of all the fluids of the body. 



512. Digestibility of Foods. The digestibility of 

 foods is a subject which belongs for investigation alike 

 to the chemist, the physiologist, and the bacteriologist. 

 The physiologist considers the structure of the digestive 

 tract and the functions of the various organs ; the chem- 

 ist studies the chemical changes which occur while the 

 food is undergoing digestion, the completeness of the 

 digestion process, and the extent to which the nutrients 

 of the food are made available to the body ; the bacteri- 

 ologist deals with the ferment bodies which assist in the 

 process of digestion. 



513. Digestibility of Heats. The nutrients of meats, 

 particularly the fats and proteids,, are more completely 



