THE VALUE OF SCIENCE 343 



the inverse phenomenon should be equally so, and one should be able 

 to reascend the course of time. Now, 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 take the inverse route and to reestablish differences of tem- 

 perature which have been effaced. Motion can be wholly dissipated 

 and transformed into heat by friction; the contrary transformation 

 can never be made except partially. 



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 random, they end by blending. For an eye which should distin- 

 guish 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 inti- 

 mate, our gross senses perceive only the uniformity. This is why, 

 for example, temperatures tend to a level, without the possibility 

 of going backwards. 



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

 law of the internal motion of the liquid, we shall soon see it colored 

 of a uniform rosy tint, and however much from this moment one 

 may shake it afterwards, the wine and the water do not seem capable 

 of again separating. Here we have the type of the irreversible phys- 

 ical phenomenon: to hide a grain of barley in a heap of wheat, this 

 is easy; afterwards to find it again and get it out, this is practically 

 impossible. All this Maxwell and Boltzmann have explained; but 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, Carnot's principle 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 distinguish the 

 elements of the blend; it is because our hands are too gross that we 

 can not 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 wait a long time for the concourse of circumstances which 

 would permit a retrogradation ; but sooner or later they will occur, 

 after years whose number it would take millions of figures to write. 

 These reservations, however, all remained theoretic; they were not 

 very disquieting, and Carnot's principle retained all its principal value. 

 But here the scene changes. The biologist, armed with his microscope, 

 long ago noticed in his preparations irregular movements of little 



