NATURE 709 



thought to be abandoned and definitively condemned by experiment, 

 are suddenly revived from their ashes and begin a new life. It is be- 

 cause they expressed true relations, and had not ceased to do so when 

 for some reason or other we felt it necessary to enunciate the same 

 relations in another language. Their life had been latent, as it were. 



Barely fifteen years ago, was there anything more ridiculous, more 

 quaintly old-fashioned, than the fluids of Coulomb? And yet, here 

 they are re-appearing under the name of electrons. In what do 

 these permanently electrified molecules differ from the electric mole- 

 cules of Coulomb? It is true that in the electrons the electricity is 

 supported by a little, a very little matter; in other words, they have 

 mass. Yet Coulomb did not deny mass to his fluids, or if he did, it 

 was with reluctance. It would be rash to affirm that the belief in 

 electrons will not also undergo an eclipse, but it was none the less 

 curious to note this unexpected renaissance. 



But the most striking example is Carnot's principle. Carnot estab- 

 lished it, starting from false hypotheses. When it was found that 

 heat was indestructible, and may be converted into work, his ideas 

 were completely abandoned; later, Clausius returned to them, and 

 to him is due their definitive triumph. In its primitive form, Car- 

 not's theory expressed in addition to true relations, other inexact rela- 

 tions, the debris of old ideas; but the presence of the latter did not 

 alter the reality of the others. Clausius had only to separate them, 

 just as one lops off dead branches. 



The result was the second fundamental law of thermodynamics. 

 The relations were always the same, although they did not hold, at 

 least to all appearance, between the same objects. This was sufficient 

 for the principle to retain its value. Xor have the reasonings of Car- 

 not perished on this arcount ; they were applied to an imperfect con- 

 ception of matter, but their form i.e., the essential part of them, 

 remained correct. What I have just said throws some light at the 

 same time on the role of general principles, such as those of the prin- 

 ciple of least action or of the conservation of energy. These principles 

 are of very jreat value. They were obtained in the search for 

 what there was in common in the enunciation of numerous 

 physical laws: they thus represent the quintessence of innumer- 

 able observations. However, from their very generality results a 

 consequence to which I have called attention in Chapter VIII. 

 namely, that they arc no longer capable of verification. As we 

 cannot give a general definition of energy, the principle of the con- 

 servation of energy simply signifies that there is a something which 

 remains constant. Whatever fresh notions of the world may be given 

 us by future experiments, we are certain beforehand that there is 

 something which remains constant, and which may be called energy. 

 Does this mean that the principle has no meaning and vanishes into 



