﻿280 
  Dr. 
  A. 
  S. 
  Eve 
  on 
  Primary 
  

  

  their 
  thickness 
  the 
  ionization 
  current 
  in 
  the 
  electroscope 
  rises 
  

   quickly 
  to 
  a 
  maximum, 
  owing 
  to 
  the 
  increase 
  of 
  the 
  emergent 
  

   corpuscular 
  secondary 
  radiation 
  from 
  the 
  plate. 
  Further 
  

   increase 
  o£ 
  thickness 
  causes 
  a 
  very 
  gradual 
  decrease 
  in 
  ioni- 
  

   zation 
  owing 
  to 
  the 
  slow 
  absorption 
  of 
  the 
  7 
  rays. 
  In 
  fact 
  

   the 
  curve 
  has 
  an 
  equation 
  involving 
  approximately 
  the 
  

   diiference 
  between 
  two 
  simple 
  exponentials. 
  Provided, 
  

   therefore, 
  that 
  the 
  plates 
  at 
  A 
  and 
  B 
  are 
  more 
  than 
  one 
  or 
  

   two 
  millimetres 
  thick, 
  it 
  makes 
  but 
  a 
  minute 
  difference, 
  in 
  

   the 
  reversal 
  method, 
  if 
  they 
  are 
  several 
  millimetres 
  thick, 
  so 
  

   g'radual 
  is 
  the 
  absorption 
  of 
  the 
  7 
  rays. 
  The 
  changes 
  pro- 
  

   duced 
  in 
  the 
  electroscope 
  on 
  reversing 
  A 
  and 
  B 
  are, 
  then, 
  

   mainly 
  dependent 
  not 
  on 
  the 
  thickness, 
  but 
  on 
  the 
  atomic 
  

   weight 
  of 
  the 
  plate 
  in 
  position 
  B. 
  This 
  conclusion 
  was 
  con- 
  

   firmed 
  experimentally. 
  An 
  exception 
  must 
  be 
  made 
  in 
  

   the 
  case 
  of 
  lead 
  ; 
  for 
  an 
  increase 
  of 
  thickness 
  of 
  the 
  lead 
  

   plates 
  hardens 
  the 
  7 
  rays 
  rapidly 
  unless 
  they 
  have 
  previously 
  

   passed 
  through 
  screens 
  of 
  lead. 
  

  

  The 
  method 
  of 
  procedure 
  adopted 
  was 
  as 
  follows. 
  Tin 
  and 
  

   lead 
  plates 
  were 
  placed 
  at 
  A 
  and 
  B 
  and 
  a 
  measurement 
  taken; 
  

   the 
  plates 
  were 
  reversed, 
  and 
  again 
  a 
  reading 
  made. 
  The 
  

   readings 
  in 
  the 
  electroscopes 
  were 
  in 
  the 
  ratio 
  of 
  100 
  to 
  92. 
  

   Lead 
  was 
  similarly 
  compared 
  with 
  all 
  the 
  other 
  substances, 
  

   as 
  shown 
  in 
  the 
  first 
  column 
  of 
  Tabls 
  II. 
  Then 
  tin, 
  assumed 
  

   92, 
  was 
  compared 
  with 
  the 
  remaining 
  substances, 
  as 
  given 
  in 
  

   the 
  second 
  column, 
  and 
  so 
  forth. 
  

  

  

  

  Table 
  II. 
  

  

  

  RaBrg 
  14 
  mg. 
  

  

  in 
  lead 
  0*6 
  cm, 
  

  

  , 
  thick. 
  

  

  Distance 
  15 
  cm. 
  

  

  Pb 
  100 
  

  

  

  

  

  Mean. 
  

   100 
  

  

  Sn 
  92 
  

  

  92 
  

  

  

  

  920 
  

  

  Zn 
  92-4 
  

  

  93 
  

  

  92-4 
  

  

  

  92-6 
  

  

  Cu 
  92-4 
  

  

  92 
  

  

  94-0 
  

  

  92-4 
  

  

  92-6 
  

  

  Fe 
  94-1 
  

  

  94-3 
  

  

  95-5 
  

  

  93-5 
  

  

  94-1 
  94-3 
  

  

  Al 
  102-3 
  

  

  102-2 
  

  

  101-8 
  

  

  102-0 
  

  

  102-8 
  102-3 
  102-2 
  

  

  C 
  109-2 
  

  

  107-5 
  

  

  110-8 
  

  

  108-2 
  

  

  108-5 
  109-1 
  108-9 
  

  

  The 
  corresponding 
  curve 
  II. 
  (fig. 
  2) 
  shows 
  the 
  effect 
  of 
  the 
  

   atomic 
  weight 
  of 
  plate 
  B 
  on 
  the 
  electroscope 
  reading. 
  Similar 
  

   curves 
  were 
  obtained 
  by 
  Bragg 
  *, 
  who 
  used 
  11 
  elements. 
  It 
  

   appears 
  difficult 
  or 
  impossible 
  to 
  estimate 
  the 
  proper 
  deduction 
  

   to 
  be 
  made 
  from 
  the 
  mean 
  values 
  in 
  the 
  right-hand 
  column 
  

   so 
  as 
  to 
  arrive 
  at 
  the 
  absolute 
  values 
  of 
  the 
  emergent 
  secondary 
  

  

  * 
  Trans. 
  Roy. 
  Soc. 
  S. 
  Aust. 
  7. 
  xxxii. 
  (1908). 
  

  

  