BATTLE OF THE ALCHEMISTS — COMPTON 277 



materials, might readily penetrate through even the nuclei of other 

 atoms. The one thing it could do would be to " bump ", for if it 

 happened to strike head-on some other particle, such as a protx)n or 

 an electron, it could deliver momentum to that particle by impact. 

 If such a neutron particle should exist, it would not only be of the 

 utmost interest as a new " building block " of atomic structure, but 

 it would also be a most interesting tool, for it alone of all known 

 particles could penetrate unopposed the sacred structure of nuclei 

 and perhaps knock out a key stone or foundation stone of their 

 structure, causing their collapse. But the neutron would be a most 

 unmanageable tool, since, having no electric charge, we could not 

 speed it up or control it by an electric field as we do electrons, 

 protons, and other ions. We would have to take it as we get it and 

 simply watch to see what it does. 



Well, Chadwick discovered this neutron and found that it con- 

 sisted of 1 electron and 1 proton. It is like a hydrogen atom whose 

 orbital or valence electron has been completely captured by the 

 proton nucleus — a hydrogen atom shrunk down to almost nothing. 

 For the preceding 4 years Bothe and his German colleagues had been 

 playing with neutrons but did not know it, considering them to be 

 photons, i.e., radiations of wave length even shorter than the gamma 

 rays of radium. Chadwick showed that, if the law of conservation 

 of energy is true, they cannot be photons, and that their action on 

 other atoms like nitrogen or argon is exactly what would be ex- 

 pected if they are neutral material particles of mass 1, i.e., neutrons. 

 When these neutrons bump into nitrogen, argon, and other atoms, 

 they knock them forward by just the amounts that would be calcu- 

 lated from the laws of impact of balls of mass 1 against balls of 

 mass 14, 40, etc. 



This is how the neutron was produced. The Kelly Hospital in 

 Baltimore gave Chadwick a lot of old radium emanation tubes 

 which had lost their activity for therapeutic purposes but wdiich 

 contained the radioactive residues. From these tubes Chadwick 

 extracted polonium, an element wliich ejects alpha particles of 

 extremely high speed. This polonium w^as spread over a small plate, 

 which was placed about 2 centimeters away from a plate of beryllium, 

 so that the beryllium was subjected to bombardment by the fast 

 alpha particles from the polonium. It was then found that the 

 beryllium emitted rays of a tremendously penetrating nature, which 

 had the power of ionizing any gas through which they passed and 

 of knocking forward those atomic nuclei which they happened to 

 hit. All this was studied by means of ionization devices known as 

 " Geiger ion counters ", or by scintillations produced on fluorescent 

 screens. These rays are the neutrons. 



