46 APPLIED BIOLOGY 



The value of the food of the frog or of the coal depends 

 upon the amount of stored or potential energy which each 

 contains. Thus a ton of one kind of coal may run a certain 

 large engine eight hours, while another quality from another 

 mine may make the engine work just as hard for ten 

 hours. Likewise, the amount of energy in foods is highly 

 variable. Chemists have methods of burning samples of 

 foods and fuels and determining how much energy they are 

 capable of furnishing ; that is, how much stored or potential 

 energy they contain. Such tests of energy value are espe- 

 cially interesting and important in connection with human 

 food, and will be studied later. 



In addition to supplying energy for the life-activities, we 

 should note that the food of the frog also supplies the materials 

 for repair and growth of the frog's body, while the coal only 

 furnishes energy and is powerless to repair the continual wear 

 of the engine. Here is the great difference between a living 

 machine, such as the frog's body, and the lifeless engine. 

 Only the living machine, the animal's body, has the power of 

 using food in order to repair the parts wasted by activity. 

 When the animal's body grows old, it loses the power of 

 using food to make repairs, and some of its essential parts 

 wear out and stop the activity of the living machine. 



Summary : Uses of Food. We can now give a complete 

 answer to the question, " Why does the frog need food? " 

 by saying (1) food supplies energy, (2) food supplies the mate- 

 rials for repairing the waste of the living animal body, and 

 (3) food supplies the materials for increase in size (growth), 

 especially in the case of young, growing animals. 



43. Changes in Food. Having learned that food supplies 

 materials for repair, growth, and energy of the frog's body, 

 we now turn to trace it as it is taken into the body and 

 undergoes the changes which take place internally when it 

 supplies energy, builds up the body, and repairs the wasted 

 body-substances. 



