NUCLEAR FISSION 271 



These so-called "trans-uranic elements" were for several years the 

 principal study of Hahn and Meitner and their colleagues at the great 

 institute in Berlin-Dahlem. The groups at Paris and at Rome con- 

 tributed also — not very much, but enough to signify their full adhesion 

 to these concepts. Other physicists scarcely entered the field, but had 

 the fullest reliance in views sustained by such authorities. Yet now, 

 the trans-uranic elements are gone! This is regrettable, because it 

 was pleasant to think that human artifice had succeeded even in 

 lengthening the list of the elements. It is regrettable for the chemists 

 especially, because they were looking forward to getting information 

 about the chemical properties of elements beyond 92. Whether on 

 balance there is regret among physicists I doubt, because the knowledge 

 that has replaced the trans-uranic elements seems even more spec- 

 tacular than they did. Let us see how this knowledge was attained. 



Some time in 1938, Hahn observed that three of the radioactive 

 substances resulting from the exposure of uranium to neutrons had 

 some of the chemical properties of barium — enough to follow barium 

 in certain of the distinctive precipitations which are known to chemists. 

 Now this is a statement which is true of radium. Hahn assumed that 

 he had three new isotopes of radium, and this was entirely natural, for 

 two reasons. First, radium and its isotopes already known are all 

 radioactive, suggesting that any which remained to be discovered 

 should also be so; and second, the atomic number of radium is 88, so 

 that radium isotopes could conceivably come into being through the 

 reaction U{n, a) followed by the spontaneous emission of an alpha- 

 particle from the residue. Yet (and this is the fact which came out 

 on the 6th of January 1939) these substances were much too much 

 like barium ! When Hahn and Strassmann used some of the pro- 

 cedures which separate radium from barium, the novel substances 

 declined to be separated. In a typical experiment, one of them to- 

 gether with some well-known isotope of radium would be introduced 

 into a solution of some salt of barium. Fractional crystallization being 

 performed, it was found as usual that the relative concentration of the 

 radium isotope was greatly changed in the first-to-be-formed of the 

 crystals; but not the relative concentration of the new substance, 

 which entered into the crystals in just the same proportion as the 

 barium itself.^ 



There are people who have revolutionary and false ideas about 

 questions of science, and who irritate the scientists by their overconfi- 

 dent, their often arrogant ways of offering those ideas to the world. 



''The salts were the bromide and the chromate (perhaps also the chloride and 

 carbonate) of barium; the isotopes of radium were ThX and MsThi. 



