HERMANN VON HELMHOLTZ 



into another is equal to the work done on the 

 system by the external forces. If these forces 

 depend upon positions and mutual distances of the 

 particles, the system, after a change from one con- 

 figuration into another, and a return to its original 

 condition, will suffer no change in kinetic energy. 

 Such a system is described as conservative. When 

 a conservative system, in its change from one con- 

 figuration into another, does a certain amount of 

 work, then its power of doing work in virtue of 

 its configuration, or, in other words, its potential 

 energy, is greater in the first position than in the 

 second. 



In this way we arrive at a statement of the law of 

 the constancy of energy, which asserts that, in the 

 motions of any conservative system, the sum of the 

 potential and of the kinetic energy is unchangeable. 

 This, of course, implies the impossibility of the per- 

 petual motion, or the production of work from 

 nothing, because if it (the perpetual motion) were 

 possible, we might gain work by changing the 

 system one way and then back again by a different 

 route to its original condition. This principle of 

 conservation was long recognised in a limited sense, 

 but it did not seem to apply to all kinds of forces. 

 Thus, if a system moves backwards and forwards 

 on the same track, first without and afterwards with 

 friction, then the friction in the latter case will 

 diminish the velocity, and there will be a loss of 

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