242 CREATIVE EVOLUTION [chap. 



is to say the sum of its kinetic and potential energy, re- 

 mains constant. Now, if there were only kinetic energy 

 in the world, or even if there were, besides kinetic energy, 

 only one single kind of potential energy, but no more, the 

 artifice of measurement would not make the law artificial. 

 The law of the conservation of energy would express indeed 

 that something is preserved in constant quantity. But 

 there are, in fact, energies of various kinds, 1 and the meas- 

 urement of each of them has evidently been so chosen as 

 to justify the principle of conservation of energy. Con- 

 vention, therefore, plays a large part in this principle, 

 although there is undoubtedly, between the variations 

 of the different energies composing one and the same 

 system, a mutual dependence which is just what has 

 made the extension of the principle possible by measure- 

 ments suitably chosen. If, therefore, the philosopher 

 applies this principle to the solar system complete, he 

 must at least soften its outlines. The law of the con- 

 servation of energy cannot here express the objective 

 permanence of a certain quantity of a certain thing, 

 but rather the necessity for every change that is brought 

 about to be counterbalanced in some way by a change 

 in an opposite direction. That is to say, even if it governs 

 the whole of our solar system, the law of the conservation 

 of energy is concerned with the relationship of a fragment 

 of this world to another fragment rather than with the 

 nature of the whole- 

 It is otherwise with the second principle of thermo- 

 dynamics. The law of the degradation of energy does 

 not bear essentially on magnitudes. No doubt the first 

 idea of it arose, in the thought of Carnot, out of cer- 

 tain quantitative considerations on the yield of thermic 



1 On these differences of quality see the work of Duhem, L'Evolution 

 de la mecanique, Paris, 1905, pp. 197 ff. 



