344 ELEMENTS OF GENERAL SCIENCE 



351. Food and body temperature. In the chapter on me- 

 chanical energy we learned that when work is done, a part 

 of the mechanical energy is converted into heat. This is 

 true also of the chemical energy of the cell. When food 

 combines with oxygen to release energy to do work, a large 

 amount of heat is produced. This is the heat which keeps 

 us warm. Often in summer the heat produced is more than 

 we need, especially if we are working hard. The body gets 

 rid of this heat by evaporating a large amount of water 

 from the skin (perspiration). In winter, on the other hand, 

 we would often be cold if we did not put on more clothing 

 to prevent the loss of heat, or exercise vigorously so that 

 more heat will be liberated. 



352. Food and repair. Another use wliich the body makes 

 of food is for repair. Changes take place in all the cells of 

 the body just because they are alive, and we need a constant 

 small allowance of many kinds of repair material. We also 

 need material for body-building during the process of growth. 

 A growing child is constantly forming more bone, more mus- 

 cle, more blood, more nervous tissue. For these processes 

 we use the same name which we use in describing similar 

 processes in the plant cell assimilation. Nearly everyone 

 knows that we must assimilate some protein in both repair 

 and growth, but we need many other things also. We need 

 lime and phosphorus for every cell and especially for bone 

 formation. A small amount of iron is needed for every cell 

 and to make the haemoglobin of the blood. 



It would make a long story to describe all the materials 

 which are needed for repair and growth, but it is not diffi- 

 cult to give a few rules by which food may be chosen to 

 contain all the necessary substances. Most foods which we 

 eat are very complex mixtures and serve several purposes 

 in the body. It is therefore important that we study the 

 kinds of foods and determine the sources from which they 

 may be secured most economically. 



