32 THE STORY OF LIFE'S MECHANISM. 



the machine must be always supplied with an 

 amount of energy equal to that which it gives 

 back in another form. Indeed, a larger amount 

 of energy must be furnished the machine than is 

 expected back, for there is always an actual loss 

 of available energy. In the process of the con- 

 version of one form of energy into another some 

 of the energy, from friction or other cause, takes 

 the form of heat,<and is then radiated into space 

 beyond our reach. It is, of course, not destroyed, 

 for energy cannot be destroyed ; but it has as- 

 sumed a form called radiant heat, which is not 

 available for our uses. A machine thus neither 

 creates nor destroys energy. It receives it in 

 one form and gives it back in another form, 

 with an inevitable loss of a portion of the energy 

 as radiant heat. With this understanding, we 

 may now ask if the living body can be properly 

 compared with a machine. 



A GENERAL COMPARISON OF A BODY AND A 

 MACHINE. 



That the living body exhibits the ordinary 

 types of energy is of course clear enough when 

 we remember that it is always in motion and is 

 always radiating heat two of the most common 

 types of physical energy. That this energy is 

 supplied to the body as it is to other machines, 

 in the form of the energy of chemical composi- 

 tion, will also need no further proof when it is 

 remembered that it is necessary to supply the 

 body with appropriate food in order that it may 

 do work. The food we eat, like coal, represents 





