CONTEMPORARY ADVANCES IN PHYSICS 323 



to consist of nothing but alpha-particles — three and four respectively — • 

 and they are among the most stable and abundant varieties which 

 there are. One begins already to guess that nuclear theory is not easy. 



Mass-number 9 is represented by the principal isotope of beryllium, 

 mass 9.0155 ± .0006 (Bainbridge). Beryllium is one of the elements 

 which pour out neutrons most lavishly when assailed by alpha- 

 particles, and one would like to infer that the Be^ nucleus is a cluster 

 of two alpha-particles and a neutron. Formerly the accepted model 

 consisted of two alpha-particles, a loose proton and a loose electron, 

 though this picture made it difficult to understand why beryllium is one 

 of the few light elements which yield up few or no protons when alpha- 

 particles bombard them. On forming the difference of the nuclear 

 masses Be^ and 2He*, we find 1.011, which is a very disconcerting 

 figure, as it is greater than either the accepted value of the mass of 

 the neutron or the sum of those of proton and electron. The excess is 

 very small in each case, so small that without the present-day technique 

 of measurement it would remain undetected; perhaps it is uncertain 

 even yet; but unless and until someone proves that actually there is a 

 deficiency instead of an excess with at least one of the two models, 

 it will be questionable whether the Be^ nucleus comprises two perfected 

 alpha-particles. 



Mass-numbers 10 and 11 are represented by isotopes of boron, 

 masses given by Aston as 10.0135 and 11.0110 with maximum un- 

 certainties of ± .0015. I will use these in explaining how the mass of 

 the neutron is estimated. When bombarded by alpha-particles, boron 

 emits neutrons. Again it is uncertain which isotope emits them; 

 but if we write equations similar to (2), with allowance for kinetic 

 energies : 



BIO + He* + To = a; -I- w -I- Tu B" + He" + To = 3' + w + T,, (4) 



we see that x and y would have to be isotopes of nitrogen, of mass- 

 numbers 13 and 14 respectively. No atom N^^ is known, but N^* is 

 the principal isotope of nitrogen. These facts speak strongly in favor 

 of the second of equations (4) , and so does the fact that when nitrogen 

 gas is bombarded by neutrons there are transmutations in which alpha- 

 particles appear — evidently the converse of the process which that 

 equation was first written down to describe. 



If now in the second of equations (4) we put the nuclear mass of W* 

 for y, and then insert Aston's values for N^* and B^^ and He", we get: 



mass of neutron = (1.0051 ± .005) + {To - Ti). (5) 



