ENERGY IN GENERAL. gi 



in the impact of elastic bodies. No doubt the kinetic 

 theory affords us a very striking image of these truths 

 which are independent of it; but it may be false: and 

 the theory of energy which assumes the minimum of 

 possible hypotheses would yet be true. 



It contains a great man^ other Principles. — 

 The principle of the conservation of energy contains 

 a large number of the most general principles of 

 science. It may be shown without much difficulty 

 that, for example, it contains the principle of the 

 inertia of matter, laid down by Galileo and Descartes; 

 that of the equality of action and reaction, due to 

 Newton ; and even that of the conserv^ation of matter, 

 or rather of mass, due to Lavoisier. And finally, it 

 contains the experimental law of equivalence con- 

 nected with the name of the English physicist Joule, 

 from which may be derived the Law of Hess and the 

 principle of the initial and final states which we owe 

 to Berthelot. 



// iyivolves the Laiv of Equivalence. — Here we may 

 be content with noticing that the law of the conserva- 

 tion of energy involves the existence of relations of 

 equivalence between the different varieties. A certain 

 quantity of a given energy, measured, as we have 

 seen, by the product of two factors, is equivalent to a 

 certain fixed quantity of quite a different form of 

 energy into which it may be converted. The laws 

 which govern energetic transformations therefore con- 

 tain, from both the qualitative and the quantitative 

 points of view, all the connections of the phenomena 

 of the universe. To study these laws in their detail is 

 the task that physics must take upon itself. 



The conversion one into the other of the different 

 forms of energy by means of equivalents is only a 



