156 AJ^raTrAL report Smithsonian institution, 194o 



are produced with neutrons as the projectiles and uranium ^ as the 

 target, and they therefore belong in the fifth stage of the chronology. 

 But they also depend on the first and the second stages, for neutrons 

 are always obtained by bombarding various targets with projectiles of 

 the first three kinds ; and of course they depend on the third, because 

 if the neutron had not been recognized it would hardly now be in 

 use as a tool. Moreover, they depend upon the fourth; the phenom- 

 ena of fission were first detected because the new-born elements re- 

 sulting from it are radioactive, and to this day they are often, though 

 not always, observed through this radioactivity. Next it will be 

 noticed that instead of putting fission into the fifth stage, I gave 

 it a line and a stage to itself, and said "recognition of fission" instead 

 of "discovery of fission." This was not in order to compose a three- 

 word poem, but because the phenomena were detected about 4 years 

 before they were properly analyzed ; a strange and interesting story, 

 for which, however, there is not space. 



Now I make final use of the table in speaking about energy. Every- 

 one has heard so much about the gigantic energies and the huge 

 voltages required for transmutation, that anyone may be pardoned 

 for thinking that transmutation is a process which swallows up 

 enormous quantities of energy — which is strongly endothermic, to 

 use the chemical word. Well, there are many transmutations that 

 swallow energy up without restoring it, but many of them give back 

 much more than they receive. I mean by this simply that whenever 

 a projectile makes a "lucky" hit on a target the total energy of motion 

 of the new-bom nuclei is greater than that of the projectile. On 

 balance the experimenter does spend much more energy than is re- 

 leased, because of the amount wliich he is obliged to squander on 

 projectiles which never make lucky hits; but if one considers only 

 those which do transmute, then their energy may well be smaller and 

 even very much smaller than that which appears on the new-born 

 nuclei. This is what I mean to suggest by using the name "energy of 

 activation" for the energy which hydrogen or helium nuclei must have 

 in order to make them efficient projectiles. It is, however, the release 

 of energy which is one of the spectacular features of fission. 



This release of energy is indeed amazing. Wlien the process occurs 

 in any single nucleus there is released — in the form of kinetic 

 energy of the new-born particles — the appalling amount of 175,000,000 

 electron-volts. To get a notion of what this figure means, remember 

 that in the synthesis of hydrogen and oxygen into water — perhaps 

 the most terrific explosion of all of chemistry — there is released 



* The lecture was confined to the fission of uranium by slow neutrons. "Fast" neutrons 

 (of energies amounting to a million or millions of electron-volts) produce fission of a differ- 

 ent Isotope of uranium, and also of thorium and of protactinium. 



