EVOLUTION AND MUTABILITY OF MATTER. 263 



Kinetic Conception of Molecular Motion. The idea 

 of this peculiar form of motion is by no means new to 

 us. We were familiarized with it in scientific theories 

 during our school days. The atomic theory teaches 

 us that matter behaves, from a chemical point of 

 view, as if it were divided into molecules and atoms. 

 The kinetic theory explains the constitution of gases 

 and the effects of heat by supposing that these 

 particles are endowed with movements of rotation 

 and displacement The wave theory explains photic 

 phenomena by supposing peculiar vibratory move- 

 ments in a special medium the ether. But these 

 are merely hypotheses which are not at all necessary; 

 they are the images of things, not the things them- 

 selves, 



Reality of tJi Motion of Particles. Here there is 

 no question of hypotheses. This internal agitation, 

 this interior labour, this incessant activity of matter 

 are positive facts, an objective reality. It is true that 

 when the chemical or mechanical equilibrium of 

 bodies is disturbed it is only" restored more or less 

 slowly. Sometimes days and years are required 

 before it is regained. Scarcely do they attain this 

 relative repose when they are again disturbed, for the 

 environment itself is not fixed; it experiences varia- 

 tions which react in their turn upon the body under 

 consideration ; and it is only at the end of these 

 variations, at the end of their respective periods, that 

 they will attain together, in a universal uniformity, an 

 eternal repose. 



We shall see that metallic alloys undergo con- 

 tinual physical and chemical changes. They are 

 always seeking a more or less elusive equilibrium. 

 Physicists in modern times have given their attention 



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