PRESENT CONCEPTION OF AN ATOM MILLIKAN 189 



Now the arguments for the " loafer electron " theory, as I have 

 called it, are two in number. The first is that such activity as the 

 physicist postulates would soon wear away 'all the energy possessed 

 by the electrons — that is, they would tire themselves out and quit 

 their play. 



There is no answer to this argument. They would indeed tire 

 themselves out, provided the classical electromagnetic laws are uni- 

 \ersally applicable — even in the hearts of atoms. And the physicist's 

 only answer to this argument is, " God did not make electrons that 

 way. Why assume that the electromagnetic laws are universally 

 valid when this is the first chance w^e have h'ad to test them out in 

 the region of the infinitely small ?" 



The second argument which has been advanced for the " loafer 

 electron " theory is the existence of localized valences in chemistry. 

 Now, that these localized valences exist is admitted on all hands ; but 

 it is simply due to 'a misunderstanding that this argument was ever 

 used against the orbit theory. For no physicist — and I wish to empha- 

 size this fact — has ever advanced the theory that the electrons all ro- 

 tate in coplanar orbits. Localized valences probably are just as com- 

 patible with the orbit theory when the orbits are properly distributed 

 in space as with the stationary electron conception. All this I pointed 

 out in 1916,^ trying thereby to clear the misconception which existed 

 in the minds of chemists as to the way in which physicists were 

 thinking. 



Let me pass now to the arguments in favor of the orbit theory. 

 They are all of them definite quantitative arguments in which purely 

 theoretical considerations lead to exact nimierical predictions which 

 can be subjected to the test of experiment. 



The first was the exact prediction with the aid of orMt equations 

 of the so-called Rydberg spectroscopic constant which is in agree- 

 ment, with an accuracy of one part in five hundred, with the directly 

 measured value. 



The second quantitative 'argument comes from the prediction of a 

 difference between the positions in two spectral lines, one due to 

 helium, the other to hydrogen, which two lines should theoretically 

 be one and the same line, if it were not for the fact that the helium 

 nucleus is four times as massive as the hydrogen nucleus. 



To make clear the difference which this causes let me ask you to 

 reflect that when an electron revolves around the nucleus of an atom 

 of hydrogen, the real thing that happens is that the two bodies re- 

 volve about their common center of gravity, but, since the nucleus 

 is 2,000 times heavier than the electron, this center is exceedingly 

 close to the hydrogen nucleus. If now the hydrogen nucleus is re- 



=■ Phys. Rev., May, 1917 ; presented before the American Physical Society, Dec. 1. 1916. 

 20397— 2.J 14 



