NUCLEAR FISSION 279 



of the fragments. The Columbia school has done the same experiment, 

 with Hke results. 



Another way of ascertaining the energy released by fission was 

 adopted by Henderson of Princeton; it is the oldest and most unim- 

 peachable of all the methods of measuring energy, for he determined 

 the rate at which heat was being developed in a uranium target and a 

 container surrounding it while the Assuring was going on. His value 

 was 175 Mev. per fission, with an uncertainty of some ten per cent. 

 As some of this energy belonged not to the fragments but to the elec- 

 trons emitted after the fission, the agreement is better than passable. 



Now we come back to the question of the masses of the initial frag- 

 ment-pair; and I will develop a second consequence of these masses, 

 entirely different from the first. I revert to the use of mass-numbers, 

 since the corrections needed for converting these into actual masses 

 have not the slightest bearing on the point which is now to occupy us. 



If the members of the initial fragment-pair are Ba^^** and Kr^°*', then 

 the second of these two is fourteen units heavier than the heaviest 

 stable isotope of seKr. It is therefore much too massive for its charge. 

 This suggests that it may be able to shed neutrons, and so bring down 

 its weight to the highest value compatible with its charge. But one 

 may also say that Kr^"" is too feebly charged for its mass. One has to 

 go no fewer than six steps along the periodic table — to 42M0 — to find 

 an element with a stable isotope of mass-number 100. Yet there is 

 nothing to prevent us from assuming that the nucleus seKr^"" may 

 shoot out six negative electrons, and so increase its charge to the mini- 

 mum value compatible with its mass. The six might come out seri- 

 atim, in which case there would be a chain of six radioactive substances 

 comprising all the elements from seKr to 4iCb. Again, the nucleus 

 might conceivably eject any number of neutrons under fourteen and 

 some number of negative electrons under six, arriving at a sort of 

 compromise pair of values of mass and charge compatible one with 

 the other. One guesses already a mighty number of possible radio- 

 active bodies resulting from the fission ! 



But now let us discard the assumption that Ba"* and Kr^"" are the 

 actual fragments of the fission, replacing these with any two nuclei 

 which {a) lie in the middle region of the Periodic Table and (6) have 

 atomic numbers adding up to 92 and mass-numbers adding up to 239. 

 What, then, will happen to our two inferences from the masses? Es- 

 sentially, nothing. Whichever such pair we take, one at least of its 

 members must be too heavy for its charge and too feebly charged for 

 its mass. (With most conceivable pairs, this will be true of both the 

 members!) This derives from one of the fundamental facts of nuclear 



