ENERGY IN GENERAL. 67 



geneous among one another, and that their apparent 

 heterogeneity is only the result of the intervention of 

 our sensorium. They differ only in so far as move- 

 ments are capable of differing — that is to say, in 

 velocity, mass, and trajectory. There is something 

 fundamental which is common to them and this quid 

 commune is energy. Thus the idea of energy may be 

 derived from the kinetic conception, and this is the 

 usual method of exposition. 



This method has the great inconvenience of causing 

 an idea which lays claim to reality to depend upon an 

 hypothesis. And besides that, it gives a view of it 

 which may be false. It makes of the different forms 

 of energy something more than varieties which are 

 equivalent to one another. It makes of them one and 

 the same thing. It blends into one the modalities of 

 energy and mechanical energy. For the experimental 

 idea of equivalence, the kinetic theory substitutes the 

 arbitrary idea of the equality, the blending, and the 

 fundamental homogeneity of phenomena. This no 

 doubt is how the founders of energetics, Helmholtz, 

 Clausius, and Lord Kelvin understood things. But a 

 more attentive study and a more scrupulous deter- 

 mination not to go beyond the teaching of experiment 

 should compel us to reform this manner of looking at 

 it. And it is Ostwald's merit that, after Hamilton, he 

 insisted on this truth — that the various kinds of 

 physical magnitudes furnished by the observation of 

 phenomena are different and characteristic. In par- 

 ticular, we may distinguish among them those which 

 belong to the order of scalar magnitudes and others 

 which are of the order of vector magnitudes. 



The Idea of Energy derived from the Connection 

 of Phenomena. — The idea of energy is not absolutely 



