﻿Disintegration of Elements by a. Particles. 431 



element boron, all the active elements are " pure " elements, 

 i. e., have no isotopes. This may be of some significance in 

 differentiating between the structure of active and inactive 

 elements. The absence of isotopes indicates that, as regards 

 mass, there is only a narrow range of stability of the nucleus 

 for a given nuclear charge ; the addition or subtraction of 

 an equal number of H nuclei and electrons leads presumably 

 to an instability of the nuclear system. In the case of 

 lithium and of chlorine, which form isotopes, the forces 

 binding the nuclei together may consequently be very 

 different from those in the case of the pure active elements. 

 If there is any significance in this point of view, it would 

 indicate that H satellites are only present in pure odd- 

 numbered elements ; but, as we have seen, boron is an 

 exception to this rule. 



In comparing the phenomena shown by the six active 

 elements, it seems at once clear that nitrogen occupies an 

 exceptional position in the group. Not only is the range of 

 the expelled H nuclei the smallest of all the group, but the 

 ratio of the ranges in the two directions is markedly different 

 from those shown by the other elements. It is natural to 

 connect this anomalous behaviour with the fact that the mass 

 of the nitrogen nucleus is given by 4w + 2, while the rest of 

 the group are of the class 4n + 3. The slower speed of 

 ejection of the particles from nitrogen at first sig;ht suggests 

 that the H satellite is more lightly bound than in the case of 

 the other elements. This suggestion is, however, not borne 

 out by calculation of the distribution of momentum among 

 the three bodies involved in the collision, viz., the a particle, 

 the H satellite, and the residual nucleus. Jn our previous 

 paper, we showed that the distribution of momentum could 

 be calculated on certain assumptions from the observations 

 of the ranges of the expelled nuclei in the forward and 

 reverse directions of the a particle. It was supposed that 

 the law of conservation of momentum holds, and that the sum 

 of the energies of the H particle and the residual nucleus 

 was the same whether the H particle was liberated in the 

 forward or backward direction. It follows from these 

 assumptions that the relative velocity of the H nucleus and 

 the residual nucleus is the same in the two cases. The results 

 of this calculation for the group of active elements are 

 collected in the following table (Table II.). 



