CONTEMPORARY ADVANCES IN PHYSICS 155 



which agree in showing a rather sudden rise of the curve from the 

 horizontal axis, then a peak, then a valley and then a sweeping rise. 

 It is hardly likely that the peak and the valley are entirely due to dis- 

 tortion of a truly smoothly-rising curve by the aforesaid agency; and 

 the argument of paragraph (e) of page 149 leads us to infer a group of 

 neutrons displaying resonance, in addition to other neutrons for which 

 perhaps there is no resonance. Curves obtained with thick targets of 

 beryllium or of boron have conspicuous steps, carrying the same im- 

 plication. Those for boron (Chadwick and the Joliots) and some of 

 those for beryllium (Rasetti, Bernardini) suggest but a single group, 

 but there are other curves for beryllium suggesting two (in recent work 

 of Chadwick's) and even four (Kirsch and Slonek). Thus, although 

 the first four tests of resonance w^hich I listed above (page 149) have as 

 yet remained untried for emission of neutrons, the fifth has given some 

 pretty convincing evidence in its favor. 



It is always assumed that transmutation with emission of a neutron 

 is a case of disintegration-by-capture, though no one has proof of this 

 yet. The imagined process may be symbolized thus: 



zM-^ + 2He4 + T, = z+2N^^3 + ^„i j^x, (12) 



Such equations as this are used for evaluating the rest-mass of the 

 neutron, it being assumed that the rest-mass of the residual nucleus 

 z+2^^^^ is identical with that of the nucleus of the atom of mass-number 

 {A -f 3) and atomic number (Z + 2). One encounters at once the 

 difficulty that there are neutrons of a wide range of speeds, and conse- 

 quently a wide range of values of Ti. It is necessary to assume that the 

 slower neutrons leave behind them a nucleus in an excited state 

 (page 138) and that only the very fastest leave behind them the normal 

 nucleus which is to be identified with that of the isotope {A -\- 3) of the 

 element (Z -f- 2). Doing this, Chadwick got consistent values for the 

 mass of the neutron from the observations on boron and on lithium, 

 assuming the nucleus M of equation (12) to be that of B^^ and that of 

 Li'^ respectively.^^ To obtain a consistent value from the neutrons of 

 beryllium, one would have to observe some at least having an energy as 

 great as 12 MEV (when To = 5.3 MEV). Those observed in the earlier 

 work on beryllium were all much too slow. One of the driving motives 

 of recent research has been the desire of finding at least a few^ of ade- 

 quate energy; and it appears that this desire has at last been fulfilled. 



'* Were we to assume B'" and Li'^, the nucleus N would correspond to an isotope as 

 yet unknown; this is a powerful but not an absolutely imperative argument against 

 these choices. There is also the question of whether, if resonance occurs, the right 

 correlation is being made between values of T\ and values of T^ (page 151). — The 

 equation for the transmutation of boron has been worked out in Part I., pp. 323-324. 



