92 LIFE AND DEATH. 



possibility. It is subject, in fact, to all sorts of 

 restrictions, of which the most important are due to 

 the second principle. 



§ 8. Carnot's Principle. Its Generality. 



The second fundamental principle is that of the 

 transformations of equilibrium, or of the conditions 

 of reversibility, or again, Carnot's principle. This 

 principle, which first assumed a concrete form in 

 thermodynamics, has been very widely extended. It 

 has reached a degree of generality such that contem- 

 porary theoretical physicists such as Lord Kelvin, Le 

 Chatelier, etc., consider it the universal law of 

 physical, mechanical, and chemical equilibrium. 



Carnot's principle contains, as was shown by G. 

 Robin, d'Alembert's principle of virtual velocities, 

 and according to physicists of to-day, as we have just 

 remarked, it contains the laws peculiar to physico- 

 chemical equilibrium. The application of this prin- 

 ciple gives us the differential equations from which 

 are derived numerical relations between the different 

 energies, or the different modalities of universal 

 energy. 



Its Character. — It is very remarkable that we can- 

 not give a general enunciation of this principle which 

 by its revealing power has changed the face of 

 physics. This is because it is less a law, properly so 

 called, than a method or manner of interpreting the 

 relations of the different forms of energy, and par- 

 ticularly the relations of heat and mechanical energy. 



Conversion of Work into Heat and Vice-versa. — The 

 conversion of work into heat is accomplished without 

 difficulty. For example, the hammering of a piece of 



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