CHAP. XIII.] ALIMENTATION. 149 



produced whenever two molecules of hydrogen unite with one 

 of oxygen. Next to air, water is the most necessary principle 

 of life. It forms about seventy per cent of the entire bodily 

 weight. It is an essential constituent of all the tissues, as 

 well as forming the chief part of all the fluids of the body. 

 It acts as a solvent upon various ingredients of the food, lique- 

 fying them and rendering them capable of absorption. Most 

 of the water of the body is taken into it from without, but it is 

 also formed within the body by the union of hydrogen and 

 oxygen in the tissues. 



Mineral salts. The mineral substances chiefly necessary for 

 nutrition are : 



Chloride i 



Phosphate 



^ f [ot soda and potash. 



Sulphate 



Carbonate J 



Phosphate ~i _ , . 



~ , \ of lime and magnesia. 



Carbonate J 



Of these substances, chloride of soda, sodium chloride or common 

 salt, is the most important mineral ingredient of food. It is 

 contained in nearly everything we eat, but usually not in suffi- 

 cient quantity to supply all the needs of the body, and we 

 therefore add it as a separate article of diet. It is present in 

 most of the fluids of the body, notably in the blood, the fluidity 

 of which it helps to maintain. The rest of the mineral sub- 

 stances are usually contained in sufficient quantity in an ordi- 

 nary diet, though occasionally it becomes necessary to supply 

 them independently. Of all the mineral salts, phosphate of lime 

 exists in the largest quantity in the body ; it enters largely into 

 the composition of the bones, teeth, and cartilages, and gives 

 firmness and solidity to the tissues. It is present in very 

 small quantities in the bodily fluids, with the exception of 

 milk, which contains a notable amount of phosphate of lime, 

 and which is plainly provided for the ossification of the grow- 

 ing bones of infants and young children. 



Chemical composition of the body. Professor Atwater gives 

 the following average composition of the body of man, weigh- 

 ing 148 pounds : 



