THE SOURCE AND NATURE OF VITAL ENERGY 293 



is fundamentally the power in the heat that goes into the engine 

 and finally exhibits itself in the motion. In the same way, the 

 electric current, flashing along the electric wire, is energy, 

 since this also, if received by a proper machine, can be made 

 to set machinery in motion and thus accomplish work. Each 

 of these four examples of force clearly comes under the defi- 

 nition given, since they all show the power of doing work. 

 They also have another common characteristic: they all rep- 

 resent motion. The cannon ball and the flywheel are evi- 

 dently in motion, and the physicist has shown that heat 

 and electricity .are also forms of motion. Each of these four 

 examples, then, represents energy in action. An indefinite 

 number of other examples of this same type could be given, 

 for all forms of light, hoat, motion, chemical action, and elec- 

 tricity are examples of energy, and, in one form or another, all 

 represent energy in motion. This general type of energy in mo- 

 tion is active energy or kinetic energy (Gr. kinetos = moving). 



Passive or Potential Energy. Energy is not always active 

 but,, under some circumstances, it assumes a dormant form, 

 which we sped: of as potential energy. By the term "poten- 

 tial" is meant that the energy, though not at the moment 

 active, may at any time be converted into active energy. Fcr 

 example, a heavy stone, poised on the roof of a house, is at 

 rest, exhibiting no active energy; but it has potential energy, 

 in virtue of the fact that it is raised some distance above the 

 ground. The moment it is dislodged it begins to move, falling 

 to the ground by the law of gravitation, and as it falls it de- 

 velops the energy of motion. No energy is put into the stone 

 by simply dislodging it from its position on the roof; hence it 

 follows that the stone contained the energy when it rested 

 upon the roof, only the energy was in a dormant or potential 

 form. When it was dislodged from its position the potential 

 energy began to be active, and when the stone reached the 

 earth it became quiet again, its energy having apparently 

 disappeared. 



