110 THE AIM AND ACHIEVEMENTS OF SCIENTIFIC METHOD. 



conserved: meaning by this statement that whenever a 

 particle returns to a position which it has temporarily left) 

 in an otherwise constant configuration, its vis viva will regain 

 its old value. This somewhat narrow principle is extended in 

 the next section* to the case when the particle, still under 

 the action of central forces, has not yet returned to its 

 original position. In this case, by distinguishing a new 

 quantity (to which Eankin gave the name "potential 

 energy " f while calling Helmholtz's vis viva " kinetic 

 energy ") he was able to show that the sum (as we should 

 call it) of the kinetic and potential energies was constant. 

 In this way Helmholtz reached the Principle of the 

 'Conservation of "Force" [Energy] for the case of particles 

 under the action of central forces. The next two sections 

 of the work are devoted to the consideration first of the cases 

 in which the principle of conservation is already implicitly 

 known to apply, and next to the most interesting casesj (such as 

 inelastic collision and friction) which have hitherto been thought 

 to be cases of an absolute loss of " force " [energy], It is in this 

 chapter that Helmholtz brings into line with his own systematic 

 mathematical treatment the experiments in which Joule has 

 shortly before obtained evidence of an " equivalence " between 

 work and heat ; and so, for the first time, makes it appear 

 possible that the hitherto autonomous province of the science 

 of Heat will be annexed by the expansion of the classical 

 Newtonian Mechanics. 



51. 



But the doctrine of the Conservation of Energy, although it 

 formulates conveniently a connection between dynamical events 

 and temperature changes, does not of itself effect the reduction 

 we are seeking. It does not, that is, enable us to describe 



* Op. cit., pp. 13-19. 



t See Merz, op. cit., ii, p. 139. 



| See Merz, op. cit. t pp. 25-6. 



