A.— MATHEMATICAL AND PHYSICAL SCIENCES 35 



falling on another atom, may make more probable its transition to a higher 

 energy-state. It may be shown that such a theory involves a contradic- 

 tion of the conservation law in single atomic processes, and experiments 

 carried out to test the theory were best explained on the assumption of 

 conservation. 



Recently the supposition of conservation which, as we have seen in 

 the Compton effect, was invoked to explain the changes of frequency 

 involved in the impact of a light quantum and an electron, has again been 

 called into question as a result of experiments made, using modern 

 counting apparatus, on the scattering of y-rays. 



If we apply the conservation laws to nuclear transformations involving 

 protons and neutrons we find that energy is conserved quantitatively, the 

 kinetic energy liberated in a reaction being accurately accounted for by 

 the disappearance of mass which occurs. It is different when we consider 

 atomic processes which involve high speed particles — electrons, say, 

 moving with velocities comparable with that of light. Such processes 

 are not in agreement with the conservation principle, and to pull them 

 into line a new particle, the neutrino, has been introduced, possessing 

 no charge and, if Fermi be right, a negligible mass. Such a particle is 

 not likely to be detected by direct experiment ; its principal function is to 

 ' explain ' continuous (3-ray spectra. 



Obviously we have a considerable range of choice in our atomic build- 

 ing materials, and the supposition that the nucleus is composed of 

 protons and electrons in suitable numbers may need modification. The 

 a-particle, long described as made up of four protons and two electrons, 

 may also be considered as composed of two protons and two neutrons, 

 and there are good reasons for this supposition. But whether the neutron 

 is an elementary particle and the proton may be written as neutron + 

 position, or whether we have more justification for considering the 

 neutron as proton + electron are matters which cannot be discussed in 

 detail here. 



One of the most remarkable of the discoveries of recent years has 

 been that of artificial radio-activity. Rutherford's fundamental discovery 

 of 191 9 was that transmutations may result from bombardment by 

 a-particles. Thus, for example, the bombardment of nitrogen by 

 a-particles results in the transmutation described by the nuclear equation 



N^ + He^CV'+JV 



[Read : The nitrogen nucleus of atomic mass-number 14 and atomic 

 number 7 when disintegrated by an a-particle yields the isotope of 

 oxygen of atomic mass-number 17 and atomic number 8 together with 

 a proton.] 



Radio-active bodies, on the other hand, are bodies that break down 

 spontaneously. We have various particles at hand with which to effect 

 transformations by bombardment of nuclei, and for the most part the 

 products resulting from such transmutations are stable. It might, however, 

 happen that a product is produced which spontaneously disintegrates, 

 and we then have the phenomena of artificial radio-activity. The 

 bombardment (e.g.) of aluminium with a-particles resulted in the 



