TRANSMUTATION OF MATTER — RUTHERFORD 203 



explosion. These remarkable devices have played an indispensable 

 part in the rapid growth of knowledge during the last few years. It 

 is to be emphasized that progress in scientific discovery is greatly 

 influenced by the development of new technical methods and of new 

 devices for measurement. With the growing complexity of science, 

 the development of special techniq ues is of ever-increasing importance 

 for the advance of knowledge. 



Up to the year 1932, experiments on transmutation were confined 

 to the use of a-particles for bombarding purposes. It became clear 

 that the process of transformation was in most cases complex, since 

 groups of protons with different but characteristic energies were ob- 

 served when a single element was bombarded. This led to the con- 

 ception that discrete energy levels existed within a nucleus, and that 

 under some conditions part of the excess energy was sometimes released 

 in the form of a quantum of high-frequency radiation. 



The stage was now set for a great advance, and four new discoveries 

 of outstanding importance were made in rapid succession in the period 

 1931-33. I refer to the discovery of the positive electron by Anderson 

 in 1931, of the neutron by Chad wick in 1932, of artificial radioactivity 

 by M. and Mme. Curie-Joliot in 1933 and of the transmutation of the 

 elements by purely artificial methods first shown by Cockcroft and 

 Walton in 1932. 



The discovery of the neutron — that uncharged particle of mass 

 nearly 1 — was the result of a close study of the effects produced in the 

 light element beryllium when bombarded by a-particles. It is note- 

 worthy that the proton and neutron, which are now believed to be the 

 essential units with which all atomic nuclei are built up, owe their 

 recognition to a study of the transmutation of matter by a-particles. 



Before the discovery of the neutron, it had been perforce assumed 

 that nuclei must in some way be built up of massive protons and light 

 negative electrons. Theories of nuclear structure became much more 

 amenable to calculation when the nucleus was considered to be an 

 aggregate of parts like the proton and neutron which have nearly the 

 same mass. There was no longer any need to assume that either the 

 positive or the negative electron has an independent existence in the 

 nuclear structure. We are still uncertain of the exact relation, if any, 

 between the neutron and the proton. The neutron appears to be 

 slightly more massive than the proton, but it is generally believed, 

 although no definite proof is available, that the proton and neutron 

 within a nucleus are mutually convertible under certain conditions. 

 For example, the change of a proton into a neutron within the nucleus 

 should lead to the appearance of a free positive electron, while con- 

 versely the change of a neutron into a proton gives rise to a free 

 negative electron. In this way it appears possible to account for the 



