PHYSICS- — MILLIKAN. 179 



how large it is, provided we define the nucleus as the part of the 

 atom that is impenetrable to the alpha rays of radium. 



This brings me to the sixth of our discoveries — namely, the dis- 

 covery of the nucleus atom. Let me give you just a brief statement 

 of how we know that the atom is somewhat like a miniature solar 

 system, with an extraordinarily minute nucleus, the size of the nu- 

 cleus never being more than one one-hundred thousandth part of the 

 diameter of the atom, with a certain number of subsidiary bodies — 

 negative electrons — which we should liken to the planets, somewhere 

 around the outside. How do we know that that is the case ? We have 

 this direct evidence. Nature takes a helium atom which is going 

 with a speed of 18,000 miles per second, and nature shoots that atom 

 right through a glass wall without leaving any hole behind, and 

 without in any way interfering with the structure of the molecules 

 of the glass. I can show you photographs that make the thing so 

 clear that the wayfaring man can see it; you don't need to be a 

 physicist. I will do so at the end of the hour, if there is time. This 

 obviously means that the positive nucleus itself must be extraordi- 

 narily minute. Indeed the fact that the negative electron actually 

 shoots through those hundreds of thousands of atoms without ever 

 going near enough to any constituent of those atoms to knock any 

 one of them out, and the fact that the positive nucleus of helium, viz, 

 the alpha particle, shoots through even more molecules without being 

 deflected at all from its course, causes one to wonder whether there is 

 anything at all that is impenetrable in the atom. Why then do we say 

 that there is a nucleus there at all? Because direct experiment says 

 there is. There is a certain portion of the atom which the alpha par- 

 ticle itself can not penetrate. If the impact is head on, the alpha par- 

 ticle goes right up to the atom and then it backs straight back again, 

 or if it comes up to the atom at an angle like this it goes off that way. 

 (Illustrating.) It is only rarely that that happens, but Rutherford 

 and Geiger and Marsden counted the percentage of alpha particles 

 which go straight on, and the percentage which go off here, and in 

 that manner, by perfectly simple algebraic analysis that any one of 

 you can understand, without any assumption at all except the law of 

 inverse squares, which can hardly be called an assumption, since we 

 can prove that it holds, at the distance involved, for the attrac- 

 tion between the positive nucleus and the negative electron, we find 

 how big that nucleus is. By the size of the nucleus — I mean the size 

 of that portion of the atom which is impenetrable to the alpha parti- 

 cles. It comes out something like 10~ 13 centimeters. The diameter 

 of the atom is 10~ s centimeters. Furthermore, by counting how the 

 deflections of the alpha particles are distributed around this sphere, 

 which we can do directly with the aid of zinc sulphide spread over 



