THE CHEMICAL COMPOSITION OF FOODS 15 



heat-producers than carbo-hydrates, and are therefore 

 extremely valuable for the production of energy or work. 

 Any excess over that used in producing heat or work is 

 stored in the form of animal fat, and so is available in 

 case of a subsequent shortage of food. The fats are 

 present in largest amount in such seeds as soya- beans, 

 cotton-seed, and linseed, also to a less extent in the 

 cereal grains, like oats and barley, and occur even in the 

 straw crops and roots in small amount. 



Oxidation. — Before considering the remaining proxi- 

 mate principles, it is necessary to explain as clearly as 

 possible what is meant by oxidation. Many of the 

 vegetable oils or fats are used in commerce for the 

 purposes of heating or lighting, and the main features 

 of the reaction are so familiar as to serve to illustrate 

 the oxidation of food substances in the body. The 

 process of combustion or burning is really a common 

 example of chemical action taking place between two 

 bodies, one of which is oxygen. During the process 

 heat is generated, as in the oxidation or burning of coal 

 and wood, and at the end of the reaction, in place of the 

 original substances, there are left several new substances, 

 in the ashes, as gases in the smoke, and in the soot. 

 Oxidation or combustion may take place suddenly, as in 

 an explosion ; or more slowly in an ordinary fire in which, 

 however, the production of heat is quite evident ; or still 

 more slowly and quietly, as in the union of the food 

 substances in the blood-stream with the oxygen taken 

 in by breathing. A point of great importance, however, 

 is that whether the combustion be instantaneous, as in 

 an explosion, or slow and free from violence, as in the 

 body, whenever the same quantities of the same sub- 

 stances are completely saturated with or united with 



