i8 CONTEMPORARY SCIENCE 



going to make the observed inertia of the hydrogen nu- 

 cleus all electrical, it must possess an even denser charge 

 than that on the negative. 



This brings me to the sixth of our discoveries, namely 

 the discovery of the nucleus atom. Let me give you just 

 a brief statement of how we know that the atom is some- 

 what like a miniature solar system, with an extraordinarily 

 minute nucleus, the size of which is never more than one- 

 one hundred thousandth part of the diameter of the atom 

 and with a certain number of subsidiary bodies nega- 

 tive electrons which we may liken to the planets, and 

 which are somewhere around the outside. How do we 

 know 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 interferring with the structure of 

 the molecules of the glass. I can show you photographs 

 that make the thing so clear that the man in the street can 

 see it. This obviously means that the positive nucleus 

 itself mu'st be extraordinarily 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 do we say there is a 

 nucleus there? Because direct experiment says there is. 

 There is a certain portion of the atom which the alpha 

 particle itself cannot penetrate. If the impact is head 

 on, the alpha particle goes right up to the atom and then 

 it backs out again. Or if it comes up to the atom at an 

 angle, it glances off that way. Rutherford, Geiger and 

 Marsden counted the percentage of alpha particles which 

 go straight on, and the percentage which go off at a 



