HERMANN VON HELMHOLTZ 



turned, with mature powers and with the enormous 

 experience gained in many fields of physiological 

 and physical research, to the problems of his early 

 youth. As already explained, the notion of action 

 at a distance was gradually but completely abandoned, 

 and its place was taken by that of a medium 

 connecting masses of matter with each other, and 

 transmitting force. But if this new conception is 

 still mechanical, if particles of matter are straining 

 upon the invisible ties that bind them together, if 

 all attractions and repulsions occur in the medium 

 known as the ether, what is the simplest expression 

 of the laws that control these dynamical operations ? 

 Can any general principle be established by which, 

 and even without experiment, these dynamical opera- 

 tions can be deductively explained ? It was felt by 

 many that, even with the help of Galileo's notion 

 of inertia and Newton's laws, some mechanical con- 

 ceptions, such as statical equilibrium, or the theory 

 of virtual velocities, were not fully proved ; while, on 

 the other hand, many phenomena, molecular, chemi- 

 cal, electrical, magnetical, were hidden from direct 

 observation and immediate experience. The enuncia- 

 tion of the law of the conservation of energy changed 

 the aspect of affairs, and gave a great impulse to the 

 development of theoretical dynamics. The notion 

 of force fell into the background, while mass and 

 energy became recognised as indestructible quantities. 

 Energy was twofold : kinetic energy depended in all 

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