396 BELL SYSTEM TECHNICAL JOURNAL 



falling on carbon is indistinguishable from it). In one of the cases of 

 radioactivity induced by alpha-particle impact, Ellis and Henderson 

 at the Cavendish Laboratory observed a continuous distribution of 

 energies of the positive electrons ranging between 1 and 2.5 MEV. 



In all of these cases of delayed transmutation, nothing is observed 

 of the ultimate disruption excepting the emergence of the electron; 

 the other fragments apparently do not receive energy enough to make 

 a track or reach a detector, and our knowledge is thus forcedly incom- 

 plete as it is with most other examples of transmutation. In respect 

 to the initial process occurring at the collision, the prospect of attaining 

 complete knowledge seems even dimmer. We are not without some 

 guidance, for when alpha-particles impinge on aluminium or boron, 

 certain particles are expelled with apparently no delay, and these may 

 be fragments resulting from that initial process. There is, however, 

 an embarras de choix; both protons and neutrons are expelled in each 

 of these cases; if one is a fragment resulting from the same process of 

 which an unstable nucleus of half-period 3' 15" is another fragment, 

 then the other must be due to something entirely different. Actually 

 Ellis and Henderson inferred from their data that in the case of 

 aluminium, the number of protons produced by a given bombardment 

 is fifty times as great as the number of unstable nuclei which eventually 

 eject orestons. This obliges us to assume that the initial process out 

 of which the delayed transmutation arises is either the one which 

 produces the neutrons, or else some other producing no fast-moving 

 particle at all. 



Decision between these alternatives is made from a most notable 

 experiment of the Joliots, sufficient indeed by itself to settle the nature 

 of the initial process. To introduce it in the way in which it suggested 

 itself to them, I make the tentative assumption that the initial process 

 is a case of what is called ^ "disintegration by capture with emission 

 of a neutron," and that the residue of this process is the unstable 

 nucleus. Embodying this assumption in equations of "nuclear 

 chemistry" written after the fashion of those in the Second Part with 

 atomic number for a subscript preceding the symbol of each element 

 (so that ow and le become the proper symbols for a neutron and an 

 oreston) we have for boron and for aluminium: 



2He + aB = 7N + ow, followed by 7N = eC + le, 

 2He + 13AI = 15P + 0", followed by 15P = uS\ + le. 



The unstable nucleus, if it is surrounded by its proper quota of orbital 

 6 "The Nucleus, Second Part," pp. 147-148, 155. 



