206 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1925 



Frankly, electrons and protons are monads. Leibnits thus lives 

 again in modern science as Lucretius did in the science of Dalton. 



I beg you see no blame in what I have just said. It is not neces- 

 sary, it is perhaps impossible, that we should understand every- 

 thing. The penetration of human insight possibly lias limits. We 

 should not expect present science to explain everything. Knowl- 

 edge we should have. It should allow us to predict. Our ideas as 

 to what is comprehensible or absurd are doubtless hereditary habits 

 of our intellect. It is fortunate that science does not halt because 

 of such considerations; it would then repulse as incomprehensible 

 Newton's theories of actions at a distance, as the Cartesians did. 

 Doubtless, the mathematician would never have had imaginary 

 quantities which, being inconceivable, would be neither more nor 

 less than absurd. 



In the evolution of scientific theories there are surprising enough 

 changes in the point of view. Take, for instance, Fresnel's optics 

 where the light phenomena were pictured as analogous to the mo- 

 tions of the pendulum. The model expresses laws the phenomena 

 show. The aptness of the explanation resulted from the closeness 

 of the analogy. Imagination was satisfied. Then there is the 

 reality which we were prone to accord to the ether of physics. But 

 the younger school of physicists are against the ether because at the 

 same time it must be more fluid than an}' gas and more rigid than 

 steel. They look upon it as a resurrection into modern science of a 

 fossil remain of the ancient fluids. They prefer to replace it by the 

 equations of Maxwell or something as subjective. I wish that we 

 might thus gain in reality what we lose in simplicity, but we must 

 confess that there is nothing to be hoped for from the change. 



As to the atom, have we attained something real or only a model 

 symbolic of observed phenomena? It is difficult to say. The 

 model which Bohr lias elaborated from that of Rutherford shocks 

 our former beliefs. For the electricity which circulates in the 

 form of exterior electrons in that atom, does not obey the ordinary 

 experimental laws of electricity in motion. We had learned that 

 a displacement of an electrical charge is accompanied by radiation. 

 But Bohr postulates privileged orbits wherein the exterior electrons 

 may circulate without radiating. 



Bohr, in making this hypothesis, breaks with a tradition of which 

 no theorist has dared to neglect the laws. His immediate prede- 

 cessor, Rutherford, had allowed the electrons to radiate since it was 

 supposed the model must obey the standard laws. Bohr's models 

 have their own laws with no necessary relation to the laws of ordi- 

 nary experiments, and he thus turned scientific thought into a new 

 and unknown direction. The models lose the explanatory charac- 



