i86 SCIENCE PROGRESS 



through nitrogen gives rise to other and much more 

 numerous swift atoms which have a range in air of about 

 9 cm, — i.e., much smaller than that of the hydrogen atoms we 

 have hitherto been considering, but larger than that of the 

 original a-particles (7 cm,). It was at first assumed that these 

 were atoms of oxygen or nitrogen carrying a single charge which 

 were set into rapid motion by close collisions with the a-particles; 

 but if this assumption was correct, it was difficult to explain 

 why the range (9 cm,) should be the same for both gases. 

 There remained the alternative and more fascinating possibility 

 that the particles might be fragments of disintegrated atoms. 

 Further experiments, using the magnetic deflexion apparatus, 

 have shown this to be the case, for the mass of the particles 

 is greater than i (thus excluding the possibility of their 

 being hydrogen) and less than . 4 (thus likewise excluding 

 helium). In all probability their mass lies between 3 and 

 3 X roo8. Thus we have obtained a third constituent of 

 the atomic nucleus, and this time one common both to oxygen 

 and nitrogen. 



It would seem that the nucleus of a nitrogen atom does not 

 break down into its constituents of mass i and mass 3 simul- 

 taneously, for the latter component is produced 5 to 10 times 

 more frequently than the former. Considering, too, the relative 

 infrequency of collisions between a-particles and atoms, it 

 is very improbable that a single atom suffers both types of 

 disintegration. The mass 3 atom carries a double charge, 

 and, when associated with the two electrons required to neutra- 

 lise this charge, should have properties and spectrum very much 

 resembling helium. It is in fact, in all probability, an isotope 

 of that element. 



It is most natural to assume that the nuclei of all 

 atoms are built up of hydrogen nuclei and electrons ; e.g., 

 that the helium nucleus is composed of four hydrogen 

 nuclei and two electrons, giving it a resultant charge + 2 which 

 is neutralised by the two external (or ring) electrons associated 

 with it. The fact that the atomic weight of helium (3*997 

 in terms of = i6) is less than that of four hydrogen atoms 

 (4*032) may be explained as being due to the close interaction 

 of the fields in the nucleus resulting in a smaller electro-magnetic 

 mass than the sum of the masses of the individual components. 

 As Sommerfeld has pointed out, this would make helium (and 

 also its isotope) extremely stable, and here, perhaps, is the 

 explanation of its appearance as a secondary unit in the 

 nuclear structure of the heavy atoms {e.g. those emitting 

 a-particles). 



If the nuclei are built up on this plan, then it is possible 

 that hydrogen has an isotope of nuclear mass 2 and charge i. 



