HERMANN VON HELMHOLTZ 



' What Newton really wanted then was to know 

 what becomes of work that is spent in friction.' 



Descartes affirmed the doctrine of the constancy of 

 the quantity of motion, that is of momentum, in the 

 world. Leibnitz, in whose dynamical views of nature 

 force was the ultimate reality, contended that Des- 

 cartes's statement, that motion is measured by velocity, 

 should be abandoned for the conception of a vis 

 matrix^ a moving force measured by the square of the 

 velocity. He enunciated in 1686 the principle of 

 the conservation of vis viva^ and came near a full 

 mathematical expression of the law of the conserva- 

 tion of energy. Both Descartes and Leibnitz were 

 correct in their contentions, and the principle of 

 Descartes may be called the conservation of momen- 

 tum, while that of Leibnitz is a partial statement 

 of the conservation of energy. 1 



The discussion is closely connected with the views 

 held by thinkers regarding the nature of heat. Thus 

 Bacon wrote : < Heat is a motion, expansive, re- 

 strained, and acting in its strife upon the smaller 

 particles of matter.' 2 



John Locke, by a priori reasoning, had also made a 

 happy guess, that heat was not matter but motion, 

 that it 'was a brisk agitation of the particles of 

 matter,' but the statement was unsupported by 

 experimental evidence. 



1 Sorley, Art. Leibnitz in Encycl. Britann., vol. xiv., p. 422. 



2 Bacon, Speddin^i Translation, vol. iv. 



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