RADIOACTIVITY— RUTHERFORD X73 



graph. The difference between the masses on the left- and the right- 

 hand side of equation (1) is 0.0181 on the atomic scale, correspond- 

 ing to a change of energy of 17.1 million electron-volts. This is in 

 close accord with the accurate measurements of Oliphant and others 

 of the energy of the expelled a-particles, viz, 17.1 million volts. 

 Similarly, kinetic energies of the a-particles liberated in equation 

 (2) are 22.5 million volts, and this is found to agree well with the 

 change of mass of the system. 



As far as our observations have gone, the conservation not only 

 of energy but also of momentum and nuclear charge appears to hold 

 in all nuclear reactions where the energy is liberated in the form 

 of massive particles. In the cases, however, in which either positive 

 or negative electrons are expelled in the transformation, there are 

 certain difficulties in interpretation which have not yet been resolved. 



The application of the law of conservation of energy to nuclear 

 changes promises to give us very accurate and reliable data on the 

 relative masses of the atomic nuclei — probably far more precise than 

 we can hope to obtain by the mass-spectrograph, especially in the 

 case of the heavier elements. The transformation data are in some 

 cases inconsistent with the measurements of the masses by Aston 

 and others, and a new scale of masses was recently suggested by 

 Oliphant and Bethe which fit closely with observation. New measure- 

 ments by Aston and others to fix the masses of the light elements 

 with the greatest possible precision are now in progress, and it 

 seems likely that the new values will be in much closer accord with 

 those deduced from transformation data. It is noteworthy that prac- 

 tically every type of nuclear reaction takes place which is consistent 

 with the laws of conservation, although the probability of the differ- 

 ent reactions may vary widely. This is very well illustrated by the 

 great variety of transformations that have been observed in the light 

 elements like lithium, beryllium, or boron when bombarded by differ- 

 ent types of particle. 



STRUCTURE OF RADIOACTIVE NUCLEI 



The discovery of the neutron has much simplified our conception 

 of the structure of nuclei, which are now believed to consist of neu- 

 trons and protons with probably helium nuclei as secondary units, 

 composed of a very stable combination of two protons and two neu- 

 trons. These particles are contained in a minute nuclear volume with 

 radius of the order SXlQ-^^ cm which is surrounded by a high-po- 

 tential barrier that prevents the escape of the particles. In the case 

 of a heavy nucleus like that of uranium, where the potential barrier 

 is very high, about 20 million volts, the a-particle has not sufficient 

 energy to escape over the barrier. On wave-mechanical principles, 



