276 ANNUAL BEPORT SMITHSONIAN INSTITUTION, 19 3 3 



bility of success lay in bringing the largest possible electrical forces 

 to act on the nucleus, he first found that radioactive substance which 

 shot out alpha particles with the highest speed, and then he let them 

 shoot at the nuclei of light atoms, such as nitrogen and aluminum. 

 He chose these because of the relatively small electric charges of their 

 nuclei which repelled the oncoming alpha particles less strongly and 

 therefore permitted them to come closer than the nuclei of heavy 

 atoms would have done. 



Under this vigorous electrical bombardment, some of the nuclei 

 gave out protons. These were detected by the sparks of light which 

 they produced on strildng glass plates coated with special fluorescent 

 materials. Their speed and their identification as protons were de- 

 termined by measuring how far they would shoot through air and 

 how much their paths were curved in a magnetic field. These pro- 

 tons may have been literally knocked out of the nuclei by the im- 

 pinging alpha particles, but from some nuclei, as for example alumi- 

 num, the protons were shot out with much greater speeds than they 

 could possibly have acquired from such impacts. It, therefore, 

 appears that the bombarding alpha particle distorts the structure of 

 the nucleus, which settles down into a new state of stability, 

 shooting out the proton in the process. The alpha particle, there- 

 fore, serves as a sort of key to unlock the nucleus and release some 

 of its energy. Ah, here we would seem to have achieved our goal, 

 but no, the process is hopelessly inefficient as a practical source of 

 energy. Only about 1 alpha particle in 600,000 happens to strike 

 a nucleus in such a way as to produce a transmutation. The other 

 599,999 are simply scattered without apparently exerting any perma- 

 nent effect on the nuclei with which they come in contact. 



The second authentic type of transmutation is associated with the 

 discovery of the neutron by Chadwick of Cambridge less than a year 

 ago. For many years physicists have been led by logic to search for 

 this neutron, and they have predicted some of its properties. For 

 example, we have atoms of atomic numbers from 92 down to 6, 5, 4, 3, 

 2, 1 — uranium to carbon, boron, beryllium, lithium, helium, hydro- 

 gen — whose nuclei have positive electric charges of 92 down to 6, 5, 

 4, 3, 2, and 1 units, respectively. Why should there not exist an 

 atom of atomic number zero, with no charge on its nucleus? Such 

 an atom would have no extranuclear electrons, and its nucleus would 

 consist of equal numbers of protons and electrons (probably one of 

 each) packed very closely together. 



This atom would have no chemical properties and no physical 

 properties of the usual type, which depend principally upon the 

 electric field of the extranuclear electron. It would obviously be 

 hard to detect, would penetrate easily through even the densest 



