CONTEMPORARY ADVANCES IN PHYSICS 151 



developments which that discovery has hastened shall have brought 

 about the generation of streams of artificial alpha-particles more 

 numerous by far than the natural ones. Meanwhile we must be con- 

 tent with scanty data and with fragmentary tests of the important 

 question already mentioned: whether the energy transformed from 

 rest-mass to vis viva or reversely — the quantity here denoted by 

 {Ti — To), elsewhere commonly by Q, designated in German as the 

 Tbnung of the process — is a definite and characteristic quantity. 



Certainly about resonance is essential to these tests ; for if resonance 

 exists, we have to correlate the energy of a group of protons with that 

 particular energy of the alpha-particles which evokes the group; but if 

 resonance does not occur, then probably the best we can do is to cor- 

 relate the energy of the fastest of the ejected protons of a group with 

 that of the fastest of the impinging particles — and if we make the latter 

 guess w^hen it ought not to be made, there will be trouble! Perhaps 

 the most impressive evidence is that available for aluminium. Chad- 

 wick and Constable evaluated {Ti — To) for all of their eight groups: 

 the si.x for which they demonstrated resonance, and the two which were 

 evoked by alpha-particles of a limited interval of energies extending up 

 to the highest which they used, which was 5.3 MEV. They find that 

 (Ti — To) has a common value of +2.3 MEV for four of their groups — 

 to wit, the longer-range members of their four pairs — and a common 

 value of zero for the other four. Haxel plotted the integral distribu- 

 tion-in-range curves for the protons ejected by alpha-particles of sev- 

 eral yet higher energies, running up almost to 9 MEV; he detected two 

 groups; they did not display resonance, but he correlated the highest 

 energy represented in each wdth the highest represented among the 

 impinging particles, and he too found +2.3 MEV and zero for {Ti — To) 

 in the two cases ! ^^ Blackett analyzed eight examples of transmutation 

 of nitrogen observed with the cloud-chamber (here he had the unique 

 advantage of being able to observe the track of the residual nucleus 

 and estimate its energy) and he reported for {Ti — To) a mean value of 

 — 1.27 MEV with a mean deviation of 0.42 from the mean. Future 

 confirmation awaited this work also: Pollard, analyzing his integral 

 distribution-in-range curves, made a computation of (Ti — To) for the 

 6-cm. group which exhibits resonance, and another for the 17.5-cm. 

 group which does not, correlating in this latter case the energy of the 

 fastest protons with that of the fastest alpha-particles; the results were 

 -1.32 and -1.26 MEV. 



'^ The precision of these values can hardly be estimated from what Chadwick and 

 Constable say, but some idea of it can be gained from a graph in Haxel's article, 

 ZS.f. Phys. 83, p. 335 (1933), and loc. cit. footnote 27. 



