ATOMIC NUCLEI HARNWELL 199 



and proton numbers each increase by two, leading to boron of 

 mass 10. The excess energy left with this nucleus, however, does 

 not allow it to settle down stably, but a neutron emerges which 

 would produce an unknown species of boron of mass 9. This disin- 

 tegrates by emission of a positive electron into the familiar stable 

 beryllium isotope. If heavy hydrogen is driven at a high speed 

 into a target of lithium, a light and hitherto unknown beryllium of 

 mass 8 would be formed. As we might expect, this is not stable and 

 a proton emerges, leaving the well-known lithium atom of mass 7. 

 It is apparently possible that this lithium atom is not formed in 

 its ordinary configuration but is in a somewhat disorganized state 

 from which it returns to normal with the emission of gamma radia- 

 tion, for this is observed and can only be attributed to a process of 

 this sort. Finally, if neutrons are used as projectiles, lithium of 

 mass 7 results, but this is not the ordinary stable lithium of mass 7 

 but again a disorganized type of greater energy. "We find that this 

 can settle into a stable configuration in any one of three possible 

 ways. One is by the emission of a gamma ray leaving lithium 7. 

 As we find helium nuclei thrown off, this represents a second mode 

 of distintegration, one product of which is the heavy variety of 

 hydrogen of mass 3. As mentioned previously, this is not found in 

 nature, for it in all likelihood gives off a neutron at some later time 

 and becomes ordinary heavy hydrogen.^ Finally, the lithium may, 

 however, give off a proton, resulting in another unknown element, 

 helium of mass 6. This exhibits a new phenomenon, for it disin- 

 tegrates slowly as radioactive atoms do, a neutron changing into a 

 proton, which is evidenced by the external appearance of an electron. 

 The mean life of this type of helium is of the order of a fraction 

 of a second before this shift occurs and lithium of mass 6 again 

 results. 



A study of other types of atoms when subjected to bombardment 

 by these heavy-velocity projectiles yields further information of this 

 general type but with the appearance in certain instances of positive 

 electrons and other less well understood phenomena. Also, if our 

 ideas are correct, the incidence of gamma rays on lithium of mass 7 

 should result in the emission of a neutron and the production of 

 lithium of mass 6. As yet a suitable source of gamma rays is not 

 available to test this hypothesis. This simple discussion of nuclear 

 phenomena by means of our diagram resembles to some extent 

 a type of chess. One has certain squares representing stable 

 elements from which we must begin and on which we must probably 

 end. The possible initial moves are determined by the type of pro- 

 jectile, for if electric charge and large units of mass are conserved, 



' Since the preparation of this article it has been found that helium of mass 3 is stable 

 and hydrogen of mass 3 is unstable, decaying into this light variety of helium. 



