PHYSICS MILLIKAN. 175 



this : The progress of science is almost never by the process of 

 revolution. You see a great deal in your newspaper headings about 

 revolutionary discoveries. They almost never happen. Thus when 

 the atom was found not to be an ultimate but a divisible thing, there 

 was no revolution, there was not a single law that had to be given 

 up. We had simply opened up a new field, tapped a new lead, found 

 an unexplored region, a sub-atomic region, and all that was above 

 it remained just exactly as it had been, and no chemist had any 

 occasion to be disturbed, for the chemists' laws were just as precise 

 as they had been before. Sometimes we do indeed find that we 

 have generalized too far, and that some law which we had sup- 

 posed to be of universal application is limited in its scope, but this 

 does not alter the fact that the growth of science is in general by a 

 process of accretion, almost never by that of revolution. Once in 

 a while we have something revolutionary but not often. 



Let me now run over a list of 10 discoveries which I will call 

 the 10 most important advances of the last 20 years. I will not keep 

 you long upon them; I will just touch upon them, because I could 

 spend the whole evening on any one of them. 



We may aptly characterize the physics of the last 20 years as the 

 physics of atomism, and the first discovery on my list of 10 ad- 

 vances is the recent verification of the adumbrations of the Greeks 

 regarding the atomic and the kinetic theories — the proof that, as 

 Democritus had imagined 500 B. C, this world does indeed consist, 

 in every part of it, of matter which is in violent motion. 



Up to within six years there were not a few distinguished scientists 

 who withheld their allegiance even from these atomic and kinetic 

 theories of matter. The most illustrious of them was Prof. Wilhelm 

 Ostwald, but in the preface to a new edition of his Outlines of 

 Chemistry he now says frankly: 



I am convinced that we have recently become possessed of experimental evi- 

 dence of the discrete or grained nature of matter for which the atomic 

 hypothesis sought in vain for hundreds and thousands of years. The isolation 

 and counting of gaseous ions on the one hand * * * and on the other the 

 agreement of the Brownian movements with the kinetic hypothesis * * * 

 justify the most cautious scientist in now speaking of the experimental proof of 

 the atomic theory of matter. The atomic hypothesis is thus raised to the posi- 

 tion of scientifically well-founded theory. 



I think you all know what the Brownian movements are, but I 

 wish especially to call attention to the fact that this advance was 

 made not by a practical man, but by a man who never did any ex- 

 perimental work in his life, Einstein, a mathematician, a man who 

 was capable of analyzing a theory and predicting results, and the 

 experimentalists have checked those results. The results consist in 

 predicting how far a given particle that you can see in an ultra 



