﻿214 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  phenomena 
  is 
  known 
  as 
  isotopy, 
  and 
  is 
  obviously 
  explained 
  on 
  the 
  

   theory 
  of 
  the 
  nucleus 
  propounded 
  above. 
  The 
  difficulty 
  is, 
  in 
  fact, 
  to 
  

   explain 
  why 
  the 
  number 
  of 
  isotopes 
  is 
  so 
  few. 
  The 
  explanation 
  will 
  

   no 
  doubt 
  be 
  forthcoming 
  when 
  more 
  is 
  known 
  of 
  the 
  laws 
  governing 
  

   the 
  nuclear 
  structure. 
  

  

  It 
  is 
  clear 
  from 
  these 
  facts 
  that 
  the 
  nucleus 
  of 
  an 
  element 
  of 
  hierh 
  

   atomic 
  weight 
  is 
  a 
  complex 
  structure 
  containing 
  many 
  particles. 
  

   Assuming 
  the 
  atomic 
  weight 
  of 
  uranium 
  to 
  be 
  238 
  and 
  its 
  atomic 
  

   number 
  92, 
  its 
  nucleus 
  must 
  contain 
  238 
  protons 
  and 
  146 
  electrons 
  

   condensed 
  in 
  a 
  volume 
  the 
  radius 
  of 
  which 
  is 
  no 
  more 
  than 
  6 
  X 
  10~^^ 
  

   cm. 
  The 
  question 
  naturally 
  arises 
  what 
  is 
  the 
  " 
  cement 
  " 
  which 
  holds 
  

   together 
  this 
  swarm 
  of 
  highly 
  electrified 
  particles 
  in 
  so 
  small 
  a 
  space 
  

   and 
  how 
  are 
  they 
  arranged 
  in 
  it? 
  To 
  these 
  questions 
  we 
  have 
  at 
  

   present 
  only 
  the 
  beginning 
  of 
  an 
  answer. 
  Some 
  quite 
  recent 
  experi- 
  

   ments 
  of 
  Bieler 
  have 
  shown 
  that 
  in 
  very 
  close 
  proximity 
  to 
  the 
  nucleus 
  

   new 
  and 
  unexpected 
  forces 
  are 
  apparently 
  brought 
  into 
  play. 
  We 
  

   have 
  seen 
  that 
  the 
  a 
  particles 
  are 
  able 
  to 
  penetrate 
  much 
  nearer 
  to 
  

   the 
  nuclei 
  of 
  elements 
  of 
  low 
  than 
  of 
  high 
  atomic 
  number. 
  Experi- 
  

   ments 
  on 
  the 
  scattering 
  of 
  a 
  particles, 
  similar 
  to 
  those 
  by 
  which 
  Chad- 
  

   wick 
  was 
  enabled 
  to 
  verify 
  the 
  inverse 
  square 
  law 
  of 
  force, 
  were 
  

   carried 
  out 
  with 
  scattering 
  materials 
  of 
  much 
  lower 
  atomic 
  number. 
  

   It 
  was 
  found 
  that 
  the 
  deflections 
  of 
  the 
  a 
  particles 
  in 
  such 
  substances 
  

   were 
  distinctly 
  less 
  than 
  the 
  theoretical 
  values. 
  With 
  aluminum 
  

   as 
  the 
  scattering 
  substance, 
  the 
  difference 
  amounted 
  to 
  nearly 
  7 
  per 
  

   cent, 
  even 
  with 
  the 
  more 
  slowly 
  moving 
  a 
  particles, 
  and 
  increased 
  

   to 
  as 
  much 
  as 
  29 
  per 
  cent 
  with 
  the 
  fastest 
  rays, 
  which, 
  of 
  course, 
  

   approach 
  the 
  nuclei 
  more 
  nearly. 
  

  

  It 
  seems 
  clear 
  that 
  at 
  the 
  very 
  small 
  distances 
  reached 
  in 
  these 
  

   encounters 
  the 
  inverse 
  square 
  law 
  is 
  no 
  longer 
  sufficient 
  to 
  account 
  

   for 
  the 
  observed 
  phenomena. 
  Either 
  the 
  law 
  of 
  force 
  between 
  two 
  

   electrical 
  charges 
  changes 
  when 
  their 
  distance 
  apart 
  is 
  very 
  minute 
  

   or 
  some 
  new 
  force 
  is 
  called 
  into 
  play 
  which 
  falls 
  off 
  with 
  increasing 
  

   distance 
  so 
  much 
  more 
  rapidly 
  than 
  the 
  ordinary 
  electrical 
  force 
  that 
  

   it 
  becomes 
  inappreciably 
  small 
  at 
  finite 
  distances. 
  Bieler 
  found 
  that 
  

   his 
  results 
  could 
  best 
  be 
  explained 
  by 
  assuming 
  the 
  existence 
  of 
  an 
  

   attractive 
  force 
  between 
  the 
  nucleus 
  and 
  the 
  a 
  particle, 
  varying 
  

   inversely 
  as 
  the 
  fourth 
  power 
  of 
  the 
  distance 
  between 
  them. 
  This 
  

   attractive 
  force 
  would 
  exactly 
  balance 
  the 
  electrostatic 
  repulsion 
  

   between 
  the 
  nuclear 
  charges 
  at 
  a 
  distance, 
  which 
  was 
  calculated 
  to 
  

   be 
  3.4X10"^^ 
  cm. 
  from 
  the 
  center 
  of 
  the 
  aluminum 
  nucleus. 
  Out- 
  

   side 
  this 
  distance 
  the 
  force 
  between 
  the 
  colliding 
  particles 
  was 
  entirely 
  

   repulsive; 
  inside 
  this 
  distance 
  it 
  would 
  be 
  entirely 
  attractive. 
  A 
  

   positive 
  particle 
  which 
  succeeded 
  in 
  penetrating 
  within 
  this 
  charmed 
  

   circle 
  would 
  thus 
  fall 
  into 
  the 
  nucleus. 
  

  

  That 
  the 
  law 
  of 
  force 
  between 
  two 
  complex 
  structures 
  like 
  atomic 
  

   nuclei 
  when 
  brought 
  very 
  close 
  together 
  can 
  be 
  adequately 
  repre- 
  

  

  