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  XLYIII. 
  The 
  Range 
  of 
  the 
  Recoil 
  Atoms 
  from 
  Thorium 
  C 
  

   and 
  Actinium 
  C. 
  By 
  A. 
  B. 
  Wood, 
  lf.5c* 
  

  

  IT 
  is 
  well 
  known 
  that 
  when 
  an 
  a-particle 
  is 
  expelled 
  from 
  

   a 
  radioactive 
  atom 
  the 
  remainder 
  of 
  the 
  atom 
  recoils 
  

   with 
  the 
  same 
  momentum 
  as 
  the 
  ejected 
  a- 
  particle. 
  The 
  

   penetrating 
  power 
  of 
  the 
  recoil 
  radiation 
  in 
  the 
  case 
  of 
  the 
  

   products 
  of 
  radium 
  has 
  been 
  studied 
  in 
  detail 
  by 
  Makower 
  

   and 
  Russ 
  f 
  and 
  by 
  Wertenstein. 
  In 
  the 
  present 
  paper 
  the 
  

   ranges 
  of 
  the 
  recoil 
  atoms 
  from 
  actinium 
  C 
  and 
  thorium 
  C 
  

   have 
  been 
  investigated 
  with 
  the 
  object 
  of 
  throwing 
  light 
  on 
  

   the 
  arrangement 
  of 
  the 
  products 
  in 
  these 
  series. 
  

  

  Wertenstein 
  % 
  determined 
  the 
  approximate 
  range 
  of 
  recoil 
  

   by 
  reducing 
  the 
  pressure 
  in 
  the 
  recoil-chamber 
  and 
  thus 
  

   increasing 
  the 
  path 
  of 
  the 
  recoiling 
  atoms. 
  The 
  recoiling 
  

   particles 
  were 
  collected 
  on 
  a 
  metal 
  plate 
  which 
  was 
  kept 
  at 
  a 
  

   fixed 
  distance 
  from 
  the 
  radioactive 
  source, 
  and 
  the 
  activity 
  

   of 
  the 
  deposit 
  on 
  the 
  plate, 
  for 
  various 
  pressures 
  within 
  the 
  

   chamber, 
  was 
  measured. 
  In 
  later 
  experiments 
  § 
  he 
  shows 
  

   that 
  ; 
  on 
  account 
  of 
  its 
  large 
  mass 
  and 
  high 
  velocity, 
  a 
  recoil 
  

   atom 
  in 
  its 
  path 
  through 
  a 
  gas 
  produces 
  an 
  extremely 
  intense 
  

   ionization 
  compared 
  with 
  the 
  ionization 
  produced 
  by 
  an 
  

   ^-particle 
  over 
  the 
  same 
  length 
  of 
  path. 
  He 
  describes 
  

   methods 
  of 
  determining 
  the 
  range 
  of 
  the 
  recoiling 
  atom 
  by 
  

   making 
  use 
  of 
  this 
  intense 
  ionization 
  which 
  it 
  produces 
  oyer 
  

   its 
  range. 
  

  

  The 
  methods 
  finally 
  adopted 
  in 
  the 
  present 
  determination 
  

   of 
  the 
  ranges 
  of 
  recoil 
  from 
  thorium 
  C 
  and 
  . 
  actinium 
  C 
  are 
  

   similar 
  to 
  these 
  in 
  principle, 
  but 
  the 
  apparatus 
  was 
  designed 
  

   to 
  give, 
  as 
  far 
  as 
  possible, 
  a 
  homogeneous 
  beam 
  of 
  recoiling 
  

   atoms. 
  

  

  I. 
  First 
  Method. 
  

  

  Experiments 
  by 
  Makower 
  & 
  Russ 
  || 
  and 
  Makower 
  & 
  

   EvansIT 
  have 
  shown 
  that 
  the 
  recoil 
  atoms 
  are 
  positively 
  

   charged 
  and 
  consequently 
  are 
  attracted 
  to 
  the 
  cathode 
  in 
  an 
  

   electric 
  field. 
  In 
  the 
  preliminary 
  experiments 
  of 
  the 
  present 
  

   research, 
  experiments 
  were 
  made 
  to 
  determine 
  the 
  strength 
  

   of 
  the 
  electric 
  field 
  and 
  the 
  best 
  time 
  of 
  exposure 
  to 
  obtain 
  a 
  

   maximum 
  of 
  recoil 
  product. 
  

  

  * 
  Communicated 
  by 
  Prof. 
  E. 
  Rutherford, 
  F.R.S. 
  

  

  f 
  Phil. 
  Mag. 
  xix. 
  p. 
  100 
  (Jan. 
  1910). 
  

  

  X 
  Le 
  Radium, 
  p. 
  288 
  (Oct. 
  1910), 
  and 
  Comptes 
  Rendus, 
  cl. 
  p. 
  932 
  

  

  § 
  Le 
  Radium, 
  p. 
  6 
  (Jan. 
  1912). 
  

  

  II 
  Phil. 
  Mag. 
  xx. 
  p. 
  875 
  (1910). 
  If 
  Phil. 
  Mag. 
  xx. 
  p. 
  882 
  (1910). 
  

  

  