268 BELL SYSTEM TECHNICAL JOURNAL 



commonest kind of oxygen atom) ; charge appears as a subscript before 

 the symbol, mass as a superscript after it: 



TVi232. „,Po231. „„TT234. „T T235 . 11238 



9oin , 9ua , 92U , 92U , 92U 



From this Hst I omit several very unstable isotopes of which we shall 

 probably never be able to assemble enough to observe their fission. 

 Protactinium 231 is itself so rare that only one man in the world (he is 

 von Grosse, of Chicago) ever got enough of it together for this experi- 

 ment. He brought his precious sample — less than 9 mg. — to Dunning 

 at Columbia for the test, and the three of them found fission. I make 

 this allusion at the start, because there will be little further occasion 

 to refer to protactinium, and yet it should not be forgotten. There is 

 danger of forgetting even thorium, since so disproportionately great 

 an amount of study has been lavished on uranium. Neither thorium 

 nor uranium is a very rare element, but more than 99 per cent of any 

 sample of uranium consists of the isotope 238, so that the two other 

 isotopes must also be classed as rare; yet it is believed at present that 

 235 is responsible for some of the most remarkable of the phenomena. 



Now let me indicate two qualities shared by all five of these nucleus- 

 types. First: all are radioactive, that is to say, they are unstable. 

 I must not be too emphatic with this word ; the average lifetime of nuclei 

 of either Th^*^ or U^^^ is hundreds of millions of years, and there are not 

 many organizations which would be considered unstable if they could 

 bank on a probable lifetime of that scale. Still they are, in the physi- 

 cist's sense, unstable; and this suggests that it might be relatively easy 

 to disorganize, to disrupt, to explode them by a fitting agency coming 

 from without. 



Now to any physicist the term " fitting agency coming from without " 

 suggests at once the bombarding particles by which transmutation was 

 first effected: alpha-particles, protons, deuterons — the positively- 

 charged nuclei of the elements helium and hydrogen at the other end 

 of the periodic table from uranium. Should one not project these 

 nuclei against uranium nuclei, and see what happens? Well, it has 

 often been done, and nothing has happened; ^ and an adequate reason 

 is supplied by the second important quality of these five nucleus-types, 

 their greatness of atomic number. All of them are so highly charged 

 with positive electricity, that the proton, the deuteron and the alpha- 

 particle, however fast they are when they start, cannot approach them 

 closely enough to do them any harm. (What with the current progress 

 in cyclotrons that statement may soon be out of date !) Even with our 



' Until in October of this year Gant reported strong indications of fission of uranium 

 produced by very energetic (8-Mev) deuterons. 



