﻿932 
  Prof. 
  Bragg 
  and 
  Mr. 
  Madsen 
  : 
  An 
  Experimental 
  

  

  passed 
  through 
  the 
  ping, 
  1*61 
  cm. 
  of 
  lead, 
  is, 
  of 
  course, 
  

   shown 
  by 
  the 
  curve 
  A. 
  It 
  is 
  a 
  little 
  difficult 
  to 
  say 
  precisely 
  

   what 
  it 
  would 
  have 
  been 
  if 
  the 
  rays 
  had 
  not 
  gone 
  through 
  

   the 
  plug, 
  since 
  the 
  absorption 
  coefficient 
  for 
  7 
  rays 
  varies 
  so 
  

   much 
  with 
  the 
  hardness. 
  From 
  '8 
  cm. 
  Pb 
  to 
  1*25 
  cm. 
  Pb, 
  

   the 
  value 
  of 
  X 
  is 
  given 
  by 
  McClelland 
  as 
  # 
  I4 
  cm. 
  -1 
  ; 
  from 
  

   2*8 
  cm. 
  Pb 
  onwards 
  Wigger 
  gives 
  '241 
  cm. 
  -1 
  . 
  We 
  have 
  

   taken 
  a 
  value 
  between 
  these, 
  towards 
  the 
  harder 
  side, 
  and 
  

   drawn 
  the 
  curve 
  A' 
  for 
  each 
  metal, 
  derived 
  from 
  A 
  by 
  

   increasing 
  the 
  ordinate* 
  of 
  A 
  by 
  two-thirds, 
  a 
  factor 
  calcu- 
  

   lated 
  from 
  the 
  absorption 
  coefficient 
  chosen. 
  Subtracting 
  A' 
  

   from 
  B, 
  we 
  obtain 
  1> 
  . 
  a 
  curve 
  which 
  may 
  be 
  taken 
  as 
  repre- 
  

   senting 
  approximately 
  the 
  behaviour 
  of 
  soft 
  7 
  rays 
  alone. 
  

   The 
  form 
  of 
  the 
  curve 
  for 
  both 
  Pb 
  and 
  Sn 
  seems 
  to 
  suggest 
  

   that 
  tlie 
  operation 
  has 
  been 
  a 
  success. 
  The 
  half-values 
  are 
  

   now 
  '024 
  and 
  '029 
  respectively. 
  Thus 
  the 
  j3 
  rays 
  due 
  to 
  

   these 
  Boft 
  7 
  rays 
  have 
  only 
  one-quarter 
  the 
  penetration 
  of 
  

   those 
  derived 
  from 
  the 
  harder 
  / 
  rays. 
  Possibly 
  wo 
  have 
  

   here 
  an 
  association 
  with 
  t 
  lie 
  facts 
  stated 
  by 
  H. 
  W. 
  Schmidt 
  

   (.1////. 
  der 
  I'ln/s. 
  Bd. 
  xxi. 
  p. 
  654), 
  viz., 
  that 
  the 
  ft 
  rays 
  

   emitted 
  by 
  EtaC 
  con 
  si 
  si 
  of 
  two 
  groups, 
  of 
  which 
  one 
  has 
  four 
  

   tim<'< 
  the 
  penetration 
  of 
  the 
  other. 
  The 
  thickness 
  of 
  Al 
  

   screen 
  accessary 
  to 
  reduce 
  the 
  harder 
  set 
  to 
  half-value 
  was 
  

   •53, 
  which 
  agrees 
  with 
  the 
  '50 
  given 
  in 
  the 
  table 
  above 
  ; 
  for 
  

   the 
  softer 
  rays 
  the 
  value 
  was 
  '131, 
  nearly 
  a 
  quarter 
  of 
  the 
  

   other. 
  If 
  there 
  are 
  two 
  main 
  groups 
  of 
  /3 
  rays 
  we 
  should 
  

   certainly 
  expect 
  to 
  find 
  two 
  corresponding 
  groups 
  of 
  7 
  rays. 
  

   Kleeman 
  has 
  also 
  argued 
  in 
  the 
  same 
  direction, 
  though 
  for 
  

   different 
  reasons. 
  

  

  A- 
  will 
  be 
  -ecu 
  from 
  tig-. 
  6 
  to 
  8 
  we 
  have 
  made 
  the 
  same 
  

   attempt 
  to 
  sort 
  the 
  soft 
  from 
  the 
  hard 
  rays 
  in 
  respect 
  to 
  Cu, 
  

   Al, 
  and 
  celluloid 
  as 
  in 
  the 
  case 
  of 
  Pb 
  and 
  Sn. 
  The 
  propor- 
  

   tion 
  of 
  soft 
  7 
  rays 
  converted 
  into 
  /5 
  rays 
  is 
  now 
  much 
  less 
  

   relatively. 
  The 
  results 
  for 
  soft 
  rays 
  are 
  therefore 
  of 
  less 
  

   accuracy, 
  and 
  depend 
  appreciably 
  on 
  the 
  allowance 
  made 
  for 
  

   the 
  hard 
  rays. 
  Nevertheless, 
  they 
  are 
  of 
  considerable 
  interest. 
  

   They 
  show 
  clearly 
  the 
  difference 
  between 
  the 
  effects 
  of 
  hard 
  

   and 
  soft 
  rays, 
  and 
  the 
  varying 
  proportion 
  of 
  soft 
  to 
  hard 
  from 
  

   substance 
  to 
  substance. 
  

  

  It 
  may 
  be 
  gathered 
  from 
  consideration 
  of 
  all 
  these 
  curves 
  

   that 
  secondary 
  7 
  rays 
  play 
  a 
  very 
  small 
  part 
  in 
  ionizing 
  the 
  

   air 
  within 
  the 
  chamber. 
  If 
  there 
  were 
  any 
  appreciable 
  

   amount 
  of 
  it, 
  we 
  should 
  expect 
  a 
  gradual 
  rise 
  long 
  after 
  the 
  

   effect 
  of 
  the 
  secondary 
  /3 
  rays 
  had 
  reached 
  its 
  maximum 
  

   value. 
  

  

  