﻿210 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1927 
  

  

  are 
  our 
  only 
  guides 
  to 
  a 
  study 
  of 
  atomic 
  nuclei. 
  These 
  simple 
  means 
  

   in 
  the 
  hands 
  of 
  Sir 
  Ernest 
  Rutherford 
  have 
  already 
  sufficed 
  to 
  give 
  

   much 
  information 
  about 
  these 
  minute, 
  but 
  important 
  structures. 
  

  

  Most 
  of 
  us 
  have, 
  at 
  some 
  time 
  or 
  other, 
  discovered 
  the 
  presence 
  

   of 
  a 
  chair 
  in 
  a 
  pitch 
  dark 
  room 
  by 
  the 
  simple 
  process 
  of 
  colliding 
  

   with 
  it. 
  This 
  is 
  essentially 
  the 
  method 
  developed 
  by 
  Sir 
  Ernest 
  

   Rutherford 
  for 
  his 
  investigations 
  of 
  the 
  nucleus. 
  High-speed 
  pro- 
  

   jectiles 
  of 
  suitably 
  small 
  dimensions 
  are 
  fired 
  through 
  matter 
  and 
  

   from 
  the 
  number 
  and 
  magnitude 
  of 
  the 
  deflections 
  which 
  they 
  under- 
  

   go, 
  inferences 
  are 
  drawn 
  as 
  to 
  the 
  number 
  and 
  nature 
  of 
  the 
  obstacles 
  

   Avhich 
  they 
  have 
  met. 
  Suitable 
  projectiles 
  for 
  the 
  purpose 
  are 
  found 
  

   in 
  the 
  a 
  particles 
  from 
  radioactive 
  substances. 
  The 
  a 
  particle 
  is 
  

   an 
  atom 
  of 
  helium 
  which 
  has 
  lost 
  two 
  electrons, 
  and 
  thus 
  carries 
  

   a 
  double 
  positive 
  charge. 
  But 
  it 
  is 
  known 
  from 
  the 
  work 
  of 
  Mosely 
  

   on 
  X-ray 
  spectra 
  that 
  the 
  number 
  of 
  planetary 
  electrons 
  contained 
  

   in 
  the 
  atom 
  of 
  a 
  given 
  element 
  is 
  equal 
  to 
  the 
  atomic 
  or 
  ordinal 
  

   number 
  of 
  the 
  element. 
  Since 
  helium 
  is 
  No. 
  2 
  in 
  the 
  order 
  of 
  the 
  

   elements 
  it 
  has, 
  normally, 
  two 
  planetary 
  electrons 
  only. 
  Hence 
  

   the 
  a. 
  particle 
  is 
  the 
  nucleus 
  of 
  the 
  helium 
  atom, 
  stripped 
  of 
  its 
  plane- 
  

   tary 
  electrons. 
  

  

  If 
  a 
  parallel 
  beam 
  of 
  such 
  particles 
  is 
  projected 
  through 
  a 
  thin 
  

   sheet 
  of 
  matter 
  the 
  majority 
  of 
  the 
  particles 
  emerge 
  without 
  ap- 
  

   preciable 
  deviation. 
  A 
  small 
  fraction 
  are, 
  however, 
  detlected 
  through 
  

   considerable 
  angles, 
  occasionally 
  through 
  more 
  than 
  a 
  right 
  angle. 
  

   It 
  was 
  this 
  observation 
  which 
  originated, 
  and 
  indeed 
  necessitates, 
  

   a 
  nuclear 
  theory 
  of 
  the 
  atom. 
  A 
  particle 
  of 
  the 
  mass 
  and 
  speed 
  

   of 
  an 
  a 
  particle 
  can 
  only 
  be 
  deviated 
  through 
  a 
  sensible 
  angle 
  when 
  it 
  

   collides 
  with 
  an 
  obstacle 
  of 
  mass 
  comparable 
  with 
  or 
  greater 
  than 
  its 
  

   own. 
  The 
  experiments 
  showed 
  that 
  this 
  was 
  a 
  comparatively 
  rare 
  

   occurrence. 
  But 
  in 
  passing 
  through 
  the 
  thinnest 
  sheet 
  of 
  gold 
  leaf 
  

   the 
  a 
  particle 
  must 
  pass 
  through 
  at 
  least 
  a 
  hundred 
  atoms 
  of 
  gold. 
  

   The 
  atom, 
  as 
  such, 
  is, 
  therefore, 
  not 
  a 
  structure 
  which 
  can 
  deflect 
  

   an 
  a 
  particle. 
  On 
  the 
  other 
  hand, 
  particles 
  capable 
  of 
  producing 
  

   large 
  deflections 
  do 
  exist 
  in 
  the 
  material. 
  We 
  are 
  driven 
  to 
  the 
  

   conclusion 
  that 
  the 
  mass 
  of 
  the 
  atom 
  is 
  not 
  distributed 
  uniformly 
  

   through 
  its 
  bulk 
  but 
  is 
  concentrated 
  in 
  a 
  particle 
  whose 
  dimensions 
  

   are 
  much 
  smaller 
  than 
  that 
  of 
  the 
  atom, 
  in 
  other 
  words 
  in 
  a 
  nucleus. 
  

  

  A 
  more 
  detailed 
  study 
  of 
  the 
  deflections 
  undergone 
  by 
  the 
  deflected 
  

   a 
  particles 
  enables 
  us 
  to 
  estimate 
  with 
  fair 
  accuracy 
  the 
  dimensions 
  

   of 
  the 
  nucleus. 
  The 
  majority 
  of 
  the 
  collisions 
  studied 
  are 
  not 
  of 
  the 
  

   nature 
  of 
  the 
  collisions 
  between, 
  say, 
  two 
  billiard 
  balls, 
  where 
  one 
  

   particle 
  actually 
  impinges 
  on 
  the 
  other, 
  though 
  collisions 
  of 
  this 
  

   type 
  have 
  been 
  observed. 
  The 
  a 
  particle 
  and 
  the 
  atomic 
  nucleus 
  are 
  

   each 
  positively 
  charged 
  and 
  thus 
  repel 
  each 
  other 
  with 
  a 
  force 
  vary- 
  

   ing 
  as 
  the 
  inverse 
  square 
  of 
  the 
  distance. 
  The 
  deviation 
  in 
  most 
  

  

  