8 THE STORY OF THE LIVING MACHINE. 



are convertible into each other in such a way 

 that when one disappears another appears. A 

 cannon ball flying through the air exhibits energy 

 of motion ; but it strikes an obstacle and stops. 

 The motion has apparently stopped, but an ex- 

 amination shows that this is not the case. The 

 cannon ball and the object it strikes have been 

 heated, and thus the motion of the ball has simply 

 been transformed into a different form of motion, 

 which we call heat. Or, again, the heat set free 

 under the locomotive boiler is converted by ma- 

 chinery into the motion of the locomotive. By 

 still different mechanism it may be converted into 

 electric force. All forms of motion are readily 

 convertible into each other, and each form in 

 which energy appears is only a phase of the total 

 energy of nature. 



A second condition of energy is energy at rest, 

 or potential energy. A stone on the roof of a 

 house is at rest, but by virtue of its position it 

 has a certain amount of potential energy, since, if 

 dislodged, it will fall to the ground, and thus de- 

 velop energy of motion. Moreover, it required to 

 raise the stone to the roof the expenditure of an 

 amount of energy exactly equal to that which 

 will reappear if the stone is allowed to fall to the 

 ground. So in a chemical molecule, like fat, there 

 is a store of potential energy which may be made 

 active by simply breaking the molecule to pieces 

 and setting it free. This occurs when the fat 

 burns and the energy is liberated as heat. But it 

 required at some time the expenditure of an equal 

 amount of energy to make the molecule. When 

 the molecule of fat was built in the plant which 

 produced it, there was used in its construction an 

 amount of solar energy exactly equivalent to the 



