PRINCIPLES OF MATHEMATICAL PHYSICS 609 



But it is not so in nature, and this is precisely what the principle 

 of Carnot teaches us; heat can pass from the warm body to the cold 

 body; it is impossible afterwards to make it reascend the inverse 

 way and reestablish differences of temperature which have been 

 effaced. 



Motion can be wholly dissipated and transformed into heat by 

 friction; the contrary transformation can never be made except in 

 a partial manner. 



We have striven to reconcile this apparent contradiction. If the 

 world tends toward uniformity, this is not because its ultimate parts, 

 at first unlike, tend to become less and less different, it is because, 

 shifting at hazard, they end by blending. For an eye which should 

 distinguish all the elements, the variety would remain always as 

 great, each grain of this dust preserves its originality and does not 

 model itself on its neighbors; but as the blend becomes more and 

 more intimate, our gross senses perceive no more than the uniform- 

 ity. Behold why, for example, temperatures tend to a level, without 

 the possibility of turning backwards. 



A drop of wine falls into a glass of water; whatever may be the 

 law of the internal movements of the liquid, we soon see it colored 

 to a uniform rosy tint, and from this moment, however well we 

 may shake the vase, the wine and the water do not seem capable of 

 further separation. Observe what would be the type of the reversible 

 physical phenomenon: to hide a grain of barley in a cup of wheat 

 is easy; afterwards to find it again and get it out is practically im- 

 possible. 



All this Maxwell and Boltzmann have explained ; the one who has 

 seen it most clearly, in a book too little read because it is a little 

 difficult to read, is Gibbs, in his Elementary Principles of Statistical 

 Mechanics. 



For those who take this point of view, the principle of Carnot is 

 only an imperfect principle, a sort of concession to the infirmity of 

 our senses; it is because our eyes are too gross that we do not dis- 

 tinguish the elements of the blend; it is because our hands are too 

 gross that we cannot force them to separate; the imaginary demon 

 of Maxwell, who is able to sort the molecules one by one, could well 

 constrain the world to return backward. Can it return of itself? That 

 is not impossible; that is only infinitely improbable. 



The chances are that we should long await the concourse of cir- 

 cumstances which would permit a retrogradation, but soon or late 

 they would be realized, after years whose number it would take 

 millions of figures to write. 



These reservations, however, all remained theoretic and were not 

 very disquieting, and the principle of Carnot retained all its practical 

 value. 



