272 BELL SYSTEM TECHNICAL JOURNAL 



Listen now to men of science having a revolutionary and true idea, 

 and expressing it in a befitting way : 



"Now we must speak of some more recent investigations, which we 

 publish only with hesitation because of the strange results. . . . We 

 come to the conclusion: our 'radium isotopes' have the properties of 

 barium ; as chemists, we really ought to say that these new substances 

 are barium, not radium. ... As chemists, we ought to use the symbols 

 Ba and La and Ce where we have been using Ra and Ac and Th. But 

 as 'nuclear chemists' closely associated with physics, we cannot yet 

 bring ourselves to make this leap, in contradiction to all previous 

 lessons of nuclear physics. Perhaps, after all, our results have been 

 rendered deceptive by a series of strange accidents. ..." 



Here I ought to mention another famous group of nuclear physicists 

 who at an earlier date might have taken the leap, but recoiled before 

 it so vehemently that they could not even bring themselves to mention 

 in print the possibility of making it. These were the physicists of the 

 Institut du Radium at Paris: Irene Curie and Savitch discovered in 

 early 1938 that one of the products of U{n) was indistinguishable, by 

 all the tests that they applied, from the rare-earth element lanthanum 

 (Z = 57). Later on they said that at a certain moment they had 

 envisaged what we now call the fission of the uranium nucleus, but 

 had preferred to believe that they had before them one of the trans- 

 uranic elements resembling lanthanum more closely than anyone as 

 yet had foreseen. 



Now we come on to the middle of January 1939, and I must introduce 

 the grand idea which with the force and suddenness of revelation burst 

 upon several people far apart in the world, as soon as they heard of the 

 experiments of Hahn and Strassmann and of the leap which these two 

 had dared to envisage and publish if not quite to take. 



Let us be audacious enough to take the leap, and let us further 

 imagine that after the entry of the neutron the nucleus divides itself 

 into two pieces or "fragments" of which one shall be barium. I must 

 say directly that this assumption is more specific than need be, and 

 that the same conclusions would be reached if we assumed that one of 

 the fragments is some other element close to barium in the Periodic 

 Table. It will be simpler, however, to be definite: let us assume bar- 

 ium, and for still greater definiteness let us suppose that the isotopes 

 concerned are 238 of uranium and 139 of barium. The neutron, then 

 supposedly enters a nucleus 92U^*^ and with it forms the transitory 

 "compound nucleus" 92U^*^, and from this there splits off a nucleus 

 seBa^^'. What is left behind must be (if in a single piece) the nucleus 



