240 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



The theory appeared to be so perfect that the goal of physical 

 research seemed in sight. But theoretical minds were not content to 

 accept the proton and negative electron simply as brute facts. They 

 asked: Why not an elementary particle without any charge at all? 

 Why not an electron with a positive charge? Why not a proton 

 with a negative charge? Moreover, a theory elaborated by Professor 

 Dirac, which had won the admiration of the whole scientific world 

 by the ease with which it accounted for several " brute facts ", 

 seemed to demand the existence of positive electrons. 



What the theoretical mind demanded has at last been found by 

 the empiricist. No negative proton has yet been discovered, but the 

 existence of an uncharged elementary particle, or neutron, and of a 

 positively charged electron, or positron, has now been placed beyond 

 question. The isolation of the former was the work of Bothe in 

 Germany, Curie and Joliot in Paris, and Dr. Chadwick at Cam- 

 bridge; that of the latter was achieved by Anderson in the United 

 States and Mr. P. M. S. Blackett and Dr. Occhialini at Cambridge, 

 and in the last few weeks M. Jean Thibaud, of Paris, has confirmed 

 their work by a new method. It was suggested that the neutron 

 was composed of a proton and negative electron, but the view now 

 generally held is that the neutron is sui generis. The mass of the 

 neutron is certainly not far removed from that of the proton. It 

 is believed that the neutron plays no small part in the constitution 

 of atomic nuclei. The positive electron, on the other hand, does 

 not appear to form part of the nuclear structure. Positive electrons 

 seem to be generated as twins with negative electrons near, but not 

 in, the nucleus during atomic collisions. A reason has to be given 

 why neutrons and positive electrons are so rare in nature. In the 

 case of positive electrons, the answer was given by Professor Dirac 

 when as yet they were undiscovered ; he showed that almost as soon 

 as they are formed they must be destroyed. But they live long 

 enough for their tracks to be photographed by Mr. Blackett. 



Now that so much is known about the ultimate structure of mat- 

 ter, the dream of the transmutation of the elements has been realized. 

 The history of alchemy affords not a little justification to the Hege- 

 lian belief that history is a process of affirmation, denial, and syn- 

 thesis. Many of the best minds of the Middle Ages favored the 

 idea that other metals could be transmuted into gold, an idea most 

 elaborately j)resented in the Summa Perfectionis of Geber. St. Al- 

 bertus Magnus did not disbelieve it, though he characteristically 

 thought that gold so produced was different from gold won from 

 the earth. The " experimentis of alconomye " of which Piers Plow- 

 man spoke in the next century were universally practiced, and faith 

 in them survived even the upheaval of the Renaissance and Reforma- 

 tion. The immortal Sir Isaac Newton believed in transmutation. 



