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  XXII. 
  On 
  the 
  Relative 
  Activity 
  of 
  the 
  Emanation 
  and 
  the 
  

   Active 
  Deposit 
  from 
  Thorium 
  and 
  from 
  Actinium. 
  B;j 
  

   Howard 
  L. 
  Bronson, 
  Ph.D.* 
  

  

  MANY 
  important 
  calculations 
  in 
  radioactivity 
  are 
  based 
  

   on 
  the 
  assumption 
  that 
  each 
  atom 
  of 
  the 
  various 
  

   radioactive 
  bodies, 
  at 
  each 
  transformation, 
  gives 
  off 
  either 
  

   one 
  or 
  no 
  a 
  particle. 
  This 
  is 
  the 
  most 
  natural 
  assumption, 
  

   and, 
  in 
  the 
  case 
  of 
  radium 
  and 
  its 
  transformation 
  products, 
  

   has 
  the 
  support 
  of 
  many 
  theoretical 
  considerations. 
  It 
  is 
  

   also 
  supported 
  by 
  the 
  fact 
  that 
  radium, 
  radium 
  emanation, 
  

   radium 
  A, 
  and 
  radium 
  C, 
  each 
  produces 
  about 
  the 
  same 
  

   amount 
  of 
  ionization, 
  when 
  the 
  four 
  substances 
  are 
  in 
  equi- 
  

   librium 
  (Rutherford, 
  ' 
  Radioactivity,' 
  p. 
  396). 
  This 
  latter 
  

   point 
  was 
  verified 
  by 
  the 
  writer 
  in 
  the 
  case 
  of 
  radium 
  

   emanation, 
  radium 
  A, 
  and 
  radium 
  C. 
  

  

  Dr. 
  Boltwood 
  suggested 
  that 
  the 
  writer 
  should 
  investigate 
  

   this 
  question 
  in 
  the 
  case 
  of 
  thorium 
  and 
  its 
  active 
  deposit, 
  as 
  

   he 
  had 
  evidence 
  that 
  the 
  results 
  obtained 
  in 
  the 
  case 
  of 
  radium 
  

   did 
  not 
  hold 
  true 
  in 
  the 
  case 
  of 
  thorium. 
  The 
  results 
  ob- 
  

   tained 
  in 
  this 
  investigation 
  entirely 
  confirm 
  Dr. 
  Boltwood's 
  

   view, 
  and 
  led 
  the 
  writer 
  to 
  investigate 
  the 
  same 
  question 
  in 
  

   the 
  case 
  of 
  actinium. 
  

  

  Method 
  and 
  Apparatus. 
  

  

  The 
  method 
  adopted 
  was 
  to 
  compare 
  directly 
  the 
  ionization 
  

   produced 
  by 
  the 
  emanation 
  and 
  the 
  active 
  deposit 
  when 
  the 
  

   two 
  substances 
  were 
  in 
  equilibrium. 
  This 
  could 
  be 
  easily 
  

   done 
  in 
  the 
  case 
  of 
  both 
  thorium 
  and 
  actinium, 
  because 
  the 
  

   transformation 
  periods 
  of 
  their 
  emanations 
  are 
  very 
  short 
  

   compared 
  with 
  those 
  of 
  their 
  active 
  deposits. 
  The 
  thorium, 
  

   or 
  actinium, 
  was 
  placed 
  between 
  two 
  layers 
  of 
  filter-paper 
  

   in 
  a 
  small 
  vessel, 
  and 
  a 
  current 
  of 
  air 
  was 
  drawn 
  through 
  

   this 
  filter-paper 
  and 
  into 
  the 
  testing 
  vessel 
  by 
  means 
  of 
  a 
  

   filter-pump. 
  The 
  air 
  current 
  was 
  kept 
  fairly 
  steady 
  by 
  

   placing 
  between 
  the 
  pump 
  and 
  the 
  testing 
  vessel 
  a 
  large 
  air- 
  

   chamber, 
  which 
  was 
  connected 
  to 
  the 
  outside 
  air 
  by 
  a 
  

   mercury 
  trap. 
  After 
  the 
  emanation 
  had 
  been 
  drawn 
  through 
  

   the 
  testing 
  vessel 
  for 
  a 
  known 
  length 
  of 
  time, 
  or 
  until 
  the 
  

   active 
  deposit 
  had 
  reached 
  a 
  maximum 
  value, 
  the 
  ionization 
  

   current 
  due 
  to 
  the 
  sum 
  of 
  the 
  emanation 
  and 
  active 
  deposit 
  

   was 
  measured. 
  The 
  air 
  current 
  was 
  then 
  cut 
  off 
  and 
  the 
  

   emanation 
  allowed 
  to 
  decay 
  to 
  a 
  negligible 
  amount, 
  and 
  the 
  

   ionization 
  current 
  due 
  to 
  the 
  active 
  deposit 
  alone 
  was 
  

   measured. 
  The 
  difference 
  between 
  these 
  two 
  currents, 
  except 
  

  

  * 
  Comniimicated 
  by 
  Professor 
  E. 
  Rutherford, 
  F.R.S. 
  

  

  