﻿150 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  charged. 
  An 
  a 
  particle 
  is 
  also 
  positively 
  charged, 
  with 
  a 
  known 
  

   charge. 
  Professor 
  Eutherford 
  suggested 
  that 
  a 
  collision 
  between 
  

   them, 
  indicated 
  by 
  a 
  sharp 
  bend 
  in 
  the 
  path 
  of 
  the 
  a 
  particle 
  as 
  it 
  

   passes 
  through 
  the 
  air, 
  may 
  be 
  due 
  simply 
  to 
  the 
  effect 
  of 
  the 
  repul- 
  

   sive 
  force 
  between 
  these 
  two 
  charges 
  when 
  they 
  come 
  very 
  near 
  to- 
  

   gether. 
  Darwin 
  calculated, 
  on 
  this 
  hypothesis, 
  the 
  fraction 
  of 
  all 
  

   the 
  observed 
  deflections 
  of 
  a 
  particles, 
  shooting 
  through 
  air 
  or 
  any 
  

   other 
  substance, 
  which 
  should 
  be 
  within 
  any 
  specified 
  angular 
  limits. 
  

   When 
  this 
  calculation 
  was 
  compared 
  with 
  the 
  experimental 
  measure- 
  

   ments 
  of 
  deflections 
  through 
  various 
  angles, 
  it 
  was 
  found 
  that 
  there 
  

   was 
  exact 
  agreement 
  only 
  provided 
  the 
  force 
  between 
  the 
  a 
  particle 
  

   and 
  the 
  nucleus 
  is 
  taken 
  to 
  vary 
  inversely 
  as 
  the 
  square 
  of 
  the 
  dis- 
  

   tance 
  between 
  them, 
  and 
  provided 
  the 
  charge 
  of 
  the 
  nucleus 
  of 
  the 
  

   atom 
  is 
  taken 
  equal 
  (in 
  electronic 
  units) 
  to 
  its 
  atomic 
  number. 
  The 
  

   atomic 
  number 
  of 
  an 
  element 
  is 
  its 
  order 
  in 
  the 
  periodic 
  table, 
  i. 
  e., 
  

   1 
  for 
  hydrogen, 
  2 
  for 
  helium, 
  3 
  for 
  lithium, 
  etc., 
  

  

  This 
  conclusion 
  was 
  verified 
  by 
  an 
  entirely 
  independent 
  method. 
  

   When 
  a 
  beam 
  of 
  X 
  rays 
  passes 
  through 
  substances, 
  some 
  of 
  its 
  

   energy 
  is 
  abstracted 
  and 
  sent 
  out 
  in 
  all 
  directions. 
  The 
  amount, 
  

   character, 
  and 
  distribution 
  of 
  this 
  scattered 
  radiation 
  have 
  been 
  ex- 
  

   actly 
  accounted 
  for 
  by 
  ascribing 
  the 
  scattering 
  to 
  the 
  action 
  of 
  the 
  

   electrons 
  outside 
  the 
  nuclei 
  of 
  the 
  atoms. 
  These 
  electrons 
  are 
  ac- 
  

   celerated 
  by 
  the 
  electric 
  forces 
  in 
  the 
  X-ray 
  beam, 
  and, 
  as 
  a 
  result 
  

   of 
  their 
  acceleration, 
  give 
  rise 
  to 
  the 
  scattered 
  radiation. 
  Sir 
  J. 
  J. 
  

   Thomson 
  calculated 
  the 
  proportion 
  of 
  the 
  energy 
  of 
  an 
  X-ray 
  beam 
  

   scattered 
  by 
  each 
  negative 
  electron 
  in 
  its 
  path. 
  Dividing 
  the 
  ob- 
  

   served 
  amount 
  of 
  scattering 
  by 
  this 
  gives 
  the 
  number 
  of 
  negative 
  

   electrons 
  taking 
  part 
  in 
  the 
  scattering. 
  Dividing 
  this 
  by 
  the 
  number 
  

   of 
  atoms 
  gives 
  the 
  number 
  of 
  scattering 
  electrons 
  per 
  atom, 
  which 
  

   is 
  found 
  equal 
  to 
  its 
  atomic 
  number. 
  But 
  the 
  number 
  of 
  scattering 
  

   electrons 
  (electrons 
  outside 
  the 
  nucleus) 
  must 
  obviously 
  equal 
  the 
  

   positive 
  charge 
  of 
  the 
  nucleus, 
  in 
  electronic 
  units, 
  thus 
  verifying 
  the 
  

   previous 
  conclusions 
  regarding 
  the 
  nuclear 
  charges 
  of 
  atoms. 
  

  

  Finally, 
  a 
  relation 
  between 
  the 
  atomic 
  number 
  of 
  an 
  element 
  and 
  

   the 
  vibration 
  frequency 
  of 
  the 
  radiation 
  constituting 
  its 
  X-ray 
  spec- 
  

   trum 
  was 
  discovered 
  by 
  Moseley. 
  It 
  can 
  be 
  expressed 
  rather 
  accu- 
  

   rately 
  by 
  saying 
  that 
  the 
  square 
  root 
  of 
  the 
  frequency 
  of 
  any 
  par- 
  

   ticular 
  type 
  of 
  X-radiation 
  is 
  directly 
  proportional 
  to 
  the 
  atomic 
  

   number 
  of 
  the 
  radiating 
  element. 
  This 
  has 
  been 
  satisfactorily 
  ac- 
  

   counted 
  for 
  only 
  by 
  supposing 
  that 
  the 
  atomic 
  number 
  of 
  an 
  element 
  

   is 
  equal 
  to 
  the 
  electronic 
  charge 
  on 
  its 
  nucleus, 
  i. 
  e., 
  to 
  the 
  excess 
  of 
  

   positive 
  over 
  negative 
  electrons 
  in 
  its 
  nucleus. 
  

  

  Atomic 
  constituents. 
  — 
  The 
  foregoing 
  evidence, 
  and 
  much 
  addi- 
  

   tional 
  evidence, 
  leads 
  to 
  the 
  conclusion 
  that 
  the 
  various 
  chemical 
  

   elements 
  have 
  atoms 
  constituted 
  as 
  shown 
  in 
  the 
  following 
  table, 
  

  

  