THE EVOLUTION OF THE WOKLD. 251 



evolution of substance, in the perpetuum mobile of the 

 universe. 



As a body which has been set in motion continues 

 to move as long as no external agency interferes with 

 it, the idea was conceived long ago of constructing 

 apparatus which should illustrate perpetual motion. 

 The fact was overlooked that every movement meets 

 with external impediments and gradually ceases, 

 unless a new impetus is given to it from without and 

 a new force is introduced to counteract the impedi- 

 ments. Thus, for instance, a pendulum would swing 

 backwards and forwards for an eternity at the same 

 speed if the resistance of the atmosphere and the 

 friction at the point it hangs from did not gradually 

 deprive it of the mechanical kinetic energy of its 

 motion and convert it into heat. We have to furnish 

 it with fresh mechanical energy by a spring (or, as 

 in the pendulum-clock, by the drag of a weight). 

 Hence it is impossible to construct a machine that 

 would produce, without external aid, a surplus of 

 energy by which it could keep itself going. Every 

 attempt to make such a perpetuum mobile must 

 necessarily fail ; the discovery of the law of substance 

 showed, in addition, the theoretical impossibility 

 of it. 



The case is different, however, when we turn to the 

 world at large, the boundless universe that is in 

 eternal movement. The infinite matter, which fills it 

 objectively, is what we call space in our subjective 

 impression of it ; time is our subjective conception 

 of its eternal movement, which is, objectively, a 

 periodic, cyclic evolution. These two " forms of 

 perception " teach us the infinity and eternity of the 

 universe. That is, moreover, equal to saying that 



