92 THE USES OF FOODS 



get rid of the waste material, for no engine can run with 

 its fire box clogged with ashes, nor can any human being 

 live long without getting rid of wastes. Neither engine nor 

 body can be overworked without breaking down. Rest 

 has been found to be necessary for metals as well as for 

 man. It is necessary to allow the most smooth-running 



Fuel gives heat and energy, and makes motion possible. (After Hoadley.) 



machine to take a rest now and then if the machinery is 

 to be kept in good condition. 



But the human machine differs from the engine in some 

 respects. It has the remarkable ability to build itself out 

 of the very materials that are put into it as fuel. No 

 engine could do this. Besides, as we have already seen, 

 germs may attack the human body and render it unable to 

 carry on its regular work. Owing to the activity of these 

 germs, we become ill. 



What the body is made of. If we study the figure on the 

 next page, we find that the human body is made up of 

 about 72 % oxygen, 13.5 % carbon, 9 % hydrogen, 2.5 % 

 nitrogen, about 3 % of various mineral salts, including cal- 

 cium, phosphorus, potassium, sulphur, sodium, chlorine, 

 magnesium, iron, and some others. All of these substances 



