CONTEMPORARY ADVANCES IN PHYSICS 297 



it is extremely probable (to say the least) that they are systematic, 

 and are indices of the structures of the nuclei. The choice of a definite 

 standard must therefore be based on expediency or on theory, and 

 none better than the present one has been proposed. 



It would be pleasant to say that this standard is exactly the same 

 as the mass of the proton, and thence to deduce that every nucleus 

 consists of protons entirely. As a matter of fact, there is a difference 

 of about three quarters of one per cent, the standard being lighter 

 than the free proton; but this by itself is no bar to the hypothesis 

 that all nuclei are made up of protons, since it is compatible with the 

 general theory of electricity that charged particles when crowded 

 close together should individually have smaller masses than when 

 they are far apart. It is not, however, admissible to assume that 

 these protons of reduced mass are all that the nucleus comprises. 

 Were this so, the positive charge of a kernel of mass NMs {Ms standing 

 for the standard mass, ^V for any integer) would be + A''^; but it is 

 always (except in the case of hydrogen) observed to be less than this 

 amount — it is equal to Ze, where Z stands for some integer less than N; 

 and one must assume that there are (iV — Z) electrons present to 

 cancel the difference between Ne and the actual charge. As for the 

 alpha-particle, its mass and charge suggest that it consists of four 

 protons and two electrons, and the masses and charges of certain 

 heavier nuclei — carbon and oxygen supply the most vivid examples — - 

 suggest that within them the protons and electrons are united in 

 groups of four and two to constitute alpha-particles, a substructure 

 within the main structure. 



Until a year or two ago, models of nuclei were constructed exclusively 

 out of protons and electrons, sometimes grouped into alpha-particles 

 and sometimes not. The discovery of the three new particles put an 

 end to this era. The interlopers were not entirely welcome; deficient 

 as the prevailing models had proved to be in many ways, people had 

 become accustomed to them, and various eminent physicists were 

 quoted as deploring — in informal and jocular words — the necessity of 

 tearing them down and rebuilding with the new bricks among the old. 

 Nevertheless, neutrons have been observed to spring out of nuclei, and 

 positive electrons have been observed wandering about in space, 

 sometimes among what seem to be the fragments of a kernel ruined 

 by an impact so violent as to provoke an internal explosion. The 

 new kind of hydrogen nucleus is sufficiently low in mass to suggest 

 that it may be a building-stone in the construction of kernels heavier 

 than itself. 



The histories of the discoveries of these three particles have not 



