INTRODUCTION. 17 



heat. Or, again, the heat set free under the 

 locomotive boiler is converted by machinery into 

 the motion of the locomotive. By still different 

 mechanism it may be converted into electric 

 force. All forms of motion are readily con- 

 vertible 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 

 energy which may be liberated by breaking the 

 molecule to pieces. The total sum of the active 

 and potential energy in the universe is thus at 

 all times the same. 



This magnificent conception has become the 

 cornerstone of modern science. As soon as 

 conceived it brought at once within its grasp 

 B 



