﻿Recoil 
  Atoms 
  from 
  Thorium 
  C 
  and 
  Actinium 
  C. 
  587 
  

  

  For 
  the 
  determination 
  of 
  the 
  range 
  of 
  recoil 
  the 
  arrange- 
  

   ment 
  shown 
  in 
  fig. 
  1 
  was 
  employed. 
  The 
  source 
  of 
  active 
  

  

  deposit 
  (a) 
  was 
  adjusted 
  at 
  the 
  centre 
  of 
  an 
  inverted 
  brass 
  

   hemispherical 
  bowl 
  b, 
  the 
  whole 
  being 
  placed 
  in 
  a 
  bell-jar 
  B. 
  

   The 
  active 
  source, 
  introduced 
  through 
  a 
  ground-glass 
  joint 
  Gr, 
  

   was 
  of 
  highly 
  polished 
  platinum 
  and 
  was 
  connected 
  to 
  the 
  

   negative 
  pole 
  of 
  a 
  battery. 
  When 
  the 
  active 
  deposit 
  dis- 
  

   integrates 
  the 
  recoiling 
  atoms 
  are 
  projected 
  towards 
  the 
  

   hemisphere 
  b. 
  Since 
  they 
  carry 
  a 
  positive 
  charge 
  and 
  the 
  

   hemisphere 
  is 
  also 
  positively 
  charged, 
  those 
  recoil 
  atoms 
  

   which 
  have 
  insufficient 
  momentum 
  to 
  reach 
  b 
  will 
  be 
  pulled 
  

   back 
  to 
  the 
  active 
  plate 
  a, 
  which 
  is 
  charged 
  to 
  a 
  negative 
  

   potential. 
  In 
  the 
  experiments 
  described 
  here 
  the 
  electric 
  

   field 
  between 
  the 
  plate 
  a 
  and 
  the 
  hemisphere 
  /) 
  was 
  insufficient 
  

   to 
  decrease 
  appreciably 
  the 
  range 
  of 
  the 
  recoil 
  atoms. 
  

  

  The 
  product 
  fired 
  on 
  the 
  hemisphere 
  was 
  received 
  on 
  a 
  

   piece 
  of 
  tinfoil 
  which 
  fitted 
  closely 
  over 
  the 
  inner 
  surface 
  

   of 
  b. 
  The 
  activity 
  was 
  measured 
  by 
  a 
  sensitive 
  /3-ray 
  

   electroscope, 
  a 
  new 
  tinfoil 
  being 
  used 
  for 
  each 
  exposure. 
  If 
  

   the 
  recoil 
  atoms 
  have 
  a 
  definite 
  range, 
  it 
  was 
  expected 
  that 
  

   the 
  amount 
  of 
  activity 
  received 
  by 
  the 
  hemisphere 
  b 
  would 
  

   remain 
  sensibly 
  constant 
  below 
  a 
  certain 
  " 
  critical" 
  pressure, 
  

   but 
  that 
  when 
  this 
  pressure 
  was 
  reached 
  this 
  activity 
  w 
  r 
  ould 
  

   suddenly 
  fall 
  to 
  zero. 
  

  

  (a) 
  Recoil 
  from 
  Thorium 
  Active 
  Deposit. 
  

  

  In 
  the 
  earlier 
  experiments 
  the 
  results 
  obtained 
  were 
  ex- 
  

   tremely 
  inconsistent 
  — 
  some 
  experiments 
  would 
  appear 
  to 
  

   support 
  the 
  view 
  that 
  there 
  was 
  only 
  one 
  " 
  critical 
  pressure," 
  

  

  