﻿212 
  ANNUAL 
  KEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  which 
  can 
  easily 
  be 
  calculated. 
  Using 
  a 
  particles 
  of 
  comparatively 
  

   small 
  velocity 
  these 
  calculations 
  were 
  actually 
  verified. 
  When, 
  how- 
  

   ever, 
  the 
  experiments 
  were 
  repeated 
  using 
  the 
  fastest 
  a 
  particles 
  avail- 
  

   able, 
  those 
  emitted 
  by 
  Radium 
  C, 
  the 
  agreement 
  between 
  the 
  experi- 
  

   mental 
  observations 
  and 
  the 
  theory 
  completely 
  broke 
  down. 
  The 
  

   hydrogen 
  nuclei 
  instead 
  of 
  being 
  scattered 
  in 
  all 
  directions 
  as 
  

   required 
  by 
  the 
  simple 
  theorj^ 
  were 
  found 
  to 
  be 
  projected 
  mainly 
  in 
  

   the 
  direction 
  of 
  the 
  oncoming 
  a 
  particles, 
  and 
  the 
  number 
  set 
  in 
  

   motion 
  was 
  also 
  greater 
  than 
  was 
  to 
  be 
  expected. 
  At 
  the 
  very 
  close 
  

   distances 
  of 
  approach 
  reached 
  in 
  these 
  experiments 
  (about 
  4X10'" 
  

   cm.) 
  the 
  inverse 
  square 
  law 
  of 
  force 
  completely 
  fails 
  to 
  describe 
  the 
  

   phenomena, 
  and 
  the 
  particles 
  no 
  longer 
  act 
  like 
  point 
  charges; 
  in 
  

   other 
  words 
  they 
  are 
  revealed 
  as 
  structures 
  with 
  an 
  extension 
  in 
  space 
  

   which 
  now 
  has 
  to 
  be 
  taken 
  into 
  account. 
  

  

  The 
  observed 
  direction 
  of 
  projection 
  of 
  the 
  hydrogen 
  nuclei 
  would 
  

   be 
  obtained 
  if 
  the 
  projectiles, 
  instead 
  of 
  being 
  spherical, 
  were 
  flat- 
  

   tened 
  disks. 
  A 
  number 
  of 
  flat 
  disks 
  traveling 
  face 
  forward 
  through 
  

   a 
  cloud 
  of 
  pellets 
  would 
  project 
  all 
  the 
  pellets 
  with 
  which 
  they 
  came 
  

   into 
  contact 
  in 
  a 
  direction 
  identical 
  with 
  that 
  in 
  which 
  they 
  them- 
  

   selves 
  were 
  traveling. 
  The 
  parallelism 
  of 
  the 
  projected 
  hydrogen 
  

   nuclei 
  is 
  not 
  quite 
  so 
  extreme 
  as 
  would 
  be 
  observed 
  if 
  the 
  colliding 
  

   particles 
  were 
  actually 
  flat. 
  Chadwick 
  and 
  Bieler 
  were 
  able 
  to 
  sliow 
  

   that 
  the 
  actual 
  distribution 
  of 
  directions 
  among 
  the 
  hydrogen 
  par- 
  

   ticles 
  was 
  just 
  what 
  Avould 
  be 
  obtained 
  if 
  the 
  a 
  particles 
  behaved 
  like 
  

   perfectly 
  elastic 
  spheroids 
  having 
  major 
  and 
  minor 
  axes 
  of 
  8X 
  

   10~^^ 
  cm. 
  and 
  4X10-^^ 
  cm., 
  respectively. 
  It 
  is, 
  of 
  course, 
  more 
  than 
  

   improbable 
  that 
  this 
  rather 
  crude 
  picture 
  represents 
  the 
  actual 
  a 
  

   particle, 
  or 
  helium 
  nucleus. 
  What 
  we 
  can 
  assert, 
  however, 
  is 
  that 
  

   outside 
  this 
  spheroid 
  the 
  helium 
  nucleus 
  behaves 
  approximately 
  as 
  a 
  

   single 
  point 
  charge, 
  while 
  inside 
  the 
  spheroid 
  the 
  repulsive 
  forces 
  

   increase 
  so 
  rapidly 
  that 
  the 
  hydrogen 
  nuclei 
  are 
  driven 
  from 
  it, 
  as 
  

   if 
  from 
  a 
  highly 
  elastic 
  and 
  rigid 
  surface. 
  At 
  such 
  distances 
  then 
  

   we 
  are 
  approaching 
  the 
  actual 
  structure 
  of 
  the 
  helium 
  nucleus 
  itself. 
  

  

  These 
  calculations 
  were 
  made 
  on 
  the 
  assumption 
  that 
  the 
  dimen- 
  

   sions 
  of 
  the 
  hydrogen 
  nucleus, 
  the 
  other 
  partner 
  to 
  the 
  collision, 
  were 
  

   negligibly 
  small 
  in 
  comparison 
  with 
  those 
  of 
  the 
  a 
  particle. 
  There 
  is 
  

   much 
  fairly 
  string, 
  though 
  indirect 
  evidence, 
  for 
  this 
  assumption. 
  

   Aston's 
  important 
  measurements 
  with 
  the 
  mass 
  spectrograph 
  have 
  

   shown 
  that 
  every 
  atom 
  has 
  a 
  mass 
  which 
  is 
  represented, 
  almost 
  

   exactly, 
  by 
  a 
  whole 
  number 
  if 
  the 
  mass 
  of 
  the 
  oxygen 
  atom 
  is 
  taken 
  

   as 
  16. 
  The 
  most 
  reasonable 
  explanation 
  of 
  this 
  remarkable 
  fact 
  is 
  

   that 
  every 
  nucleus 
  is 
  built 
  up 
  of 
  a 
  whole 
  number 
  of 
  particles 
  each 
  of 
  

   mass 
  equal 
  to 
  unity 
  on 
  this 
  scale, 
  and 
  presumably, 
  therefore, 
  hy- 
  

   drogen 
  nuclei. 
  It 
  is 
  true 
  that 
  the 
  mass 
  of 
  a 
  hydrogen 
  nucleus, 
  on 
  this 
  

   scale, 
  is 
  somewhat 
  greater 
  than 
  unity, 
  1.0077, 
  but 
  modern 
  electrical 
  

  

  