THE VALVE OF SCIENCE 341 



almost all the parts are profoundly hidden from us; but in observing 

 the motion of those that we can see, we are able, by the aid of this 

 principle, to draw conclusions which remain true whatever may be the 

 details of the invisible mechanism which animates them. 



The principle of the conservation of energy, or Mayer's principle, 

 is certainly the most important, but it is not the only one; there are 

 others from which we can derive the same advantage. These are : 



Carnot's principle, or the principle of the degradation of energy. 



Newton's principle, or the principle of the equality of action and 

 reaction. 



The principle of relativity, according to which the laws of physical 

 phenomena must be the same for a stationary observer as for an ob- 

 server carried along in a uniform motion of translation ; so that we have 

 not and can not have any means of discerning whether or not we are 

 carried along in such a motion. 



The principle of the conservation of mass, or Lavoisier's principle. 



I will add the principle of least action. 



The application of these five or six general principles to the differ- 

 ent physical phenomena is sufficient for our learning of them all that 

 we could reasonably hope to know of them. The most remarkable 

 example of this new mathematical physics is, beyond question, Max- 

 well's electromagnetic theory of light. 



We know nothing as to what the ether is, how its molecules are 

 disposed, whether they attract or repel each other; but we know that 

 this medium transmits at the same time the optical perturbations and 

 the electrical perturbations; we know that this transmission must take 

 place in conformity with the general principles of mechanics, and that 

 suffices us for the establishment of the equations of the electromagnetic 

 field. 



These principles are results of experiments boldly generalized; but 

 they seem to derive from their very generality a high degree of cer- 

 tainty. In fact, the more general they are, the more frequent are the 

 opportunities to check them, and the verifications multiplying, taking 

 the most varied, the most unexpected forms, end by no longer leaving 

 place for doubt. 



Utility of the Old Physics. — Such is the second phase of the his- 

 tory of mathematical physics and we have not yet emerged from it. 

 Shall we say that the first has been useless? that during fifty years 

 science went the wrong way, and that there is nothing left but to for- 

 get so many accumulated efforts that a vicious conception condemned 

 in advance to failure ? Not the least in the world. Do you think the 

 second phase could have come into existence without the first? The 

 hypothesis of central forces contained all the principles; it involved 

 them as necessary consequences; it involved both the conservation of 



