358 THE PHILOSOPHY OF BIOLOGY 



of a higher potential is required to force the current 

 through water than to force it through thick copper^wire. 

 In all these cases we might think that energy is lost, but 

 we cannot believe this. The kinetic energy of the flying 

 bullet becomes transformed into the increase of the 

 kinetic energy of the molecules of the metal of which 

 the bullet was composed ; for the latter becomes greatly 

 heated when its flight is arrested ; and this increased 

 heat ought to be equal to the kinetic energy of the 

 bullet in flight. The red-hot piece of iron cools, and 

 the kinetic energy of its molecules becomes less and less, 

 but this does not cease to exist, for the energy is simply 

 transferred by radiation and conduction to the sur- 

 rounding bodies, the temperature of which it raises. 

 The golf-ball driven up the hill comes to rest and loses 

 its kinetic energy. Some of this has been transferred 

 to the air through which it passes, the latter being 

 heated very slightly ; some of it is expended by friction 

 with the grass over which the ball rolls before coming to 

 rest, and this energy is traceable in heat-effects, or in 

 mechanical effects, but the rest of it apparently ceases 

 to exist. But this would be contradictory to the 

 principle of conservation, and so we say that the lost 

 kinetic energy has become potential. The current of 

 electricity may heat the water through which it passes, 

 and some of the energy which seems to disappear is so 

 to be traced, but the greater fraction is apparently lost. 

 A quantity of free hydrogen and oxygen is, however, 

 generated, and we say that the kinetic energy of the 

 moving electrons has become transformed into the 

 potential chemical energy of the gaseous mixture. 



POTENTIAL ENERGY 



Therefore, if energy disappears or appears, we do 

 not say that it is destroyed or is created : we invent 



