﻿928 
  Prof, 
  Bragg 
  and 
  Mr. 
  Madsen 
  : 
  An 
  Experimental 
  

  

  confusion 
  the 
  representative 
  points 
  are 
  nor 
  marked. 
  The 
  effect 
  

   under 
  discussion 
  is 
  clearly 
  seen 
  on 
  comparing, 
  for 
  example, 
  

   the 
  curve 
  for 
  1*61 
  cm. 
  of 
  Pb 
  with 
  that 
  for 
  3 
  cm. 
  of 
  Zn. 
  

   Kleeman 
  was 
  the 
  first 
  to 
  show 
  that 
  the 
  distinction 
  made 
  by 
  

   the 
  7 
  rays 
  between 
  different 
  atoms 
  in 
  regard 
  to 
  the 
  amount 
  

   of 
  secondary 
  radiation 
  produced 
  by 
  them 
  could 
  be 
  modified 
  

   by 
  screening 
  (Phil. 
  Mag. 
  Nov. 
  1907). 
  Our 
  results 
  agree 
  

   with 
  his 
  to 
  this 
  extent, 
  but 
  they 
  do 
  not 
  show 
  any 
  true 
  

   selective 
  absorption, 
  such 
  as 
  he 
  supposes. 
  

  

  We 
  may 
  add 
  that 
  when 
  using 
  a 
  different 
  specimen 
  of 
  

   radium, 
  with 
  which 
  we 
  were 
  able 
  to 
  allow 
  the 
  7 
  rays 
  to 
  act 
  

   with 
  a 
  minimum 
  of 
  previous 
  screening, 
  the 
  emergence 
  radia- 
  

   tion 
  of 
  Pb 
  was 
  greater 
  than 
  that 
  of 
  0. 
  It 
  would 
  appear 
  

   probable 
  that 
  with 
  \ovy 
  soft 
  rays 
  other 
  atoms 
  lighter 
  than 
  Pb 
  

   would 
  surpass 
  (' 
  in 
  the 
  same 
  way. 
  and 
  that 
  it 
  is 
  quite 
  con- 
  

   ceivable 
  that 
  the 
  emergence 
  radiations 
  should 
  increase 
  with 
  

   the 
  atomic 
  weight 
  throughout 
  the 
  whole 
  range. 
  in 
  a 
  letter 
  

   to 
  Nature 
  (April 
  2, 
  L908, 
  p. 
  509) 
  Cooksey 
  shows 
  that 
  in 
  the 
  

   case 
  <»f 
  X-rays 
  the 
  emergence 
  radiations 
  are 
  greater 
  than 
  the 
  

   incident, 
  thus 
  proving 
  the 
  parallel 
  to 
  our 
  own 
  results 
  in 
  the 
  

   case 
  of 
  7 
  ray-. 
  lie 
  also 
  finds 
  that 
  the 
  emergence 
  radiation 
  

   increases 
  with 
  the 
  atomic 
  weight. 
  This 
  is 
  the 
  opposite 
  to 
  

   what 
  we 
  have 
  found 
  true 
  of 
  the 
  7 
  rays 
  in 
  most 
  cases 
  ; 
  but 
  it 
  

   Is 
  clear 
  thai 
  it 
  can 
  be 
  <piite 
  in 
  accord 
  with 
  our 
  theory. 
  

  

  When 
  the 
  screen 
  through 
  which 
  the 
  7 
  rays 
  pass 
  on 
  their 
  

   way 
  into 
  the 
  chamber 
  is 
  gradually 
  increased 
  in 
  thickness, 
  the 
  

   emergence 
  radiation 
  rises 
  rapidly 
  to 
  a 
  maximum, 
  and 
  then 
  

   Blowly 
  decreases. 
  The 
  rise 
  is 
  due 
  to 
  increasing 
  ft 
  radiation 
  

   from 
  the 
  thickening 
  screen 
  ; 
  the 
  decrease 
  to 
  the 
  absorption 
  

   of 
  the 
  7 
  rays 
  by 
  the 
  screen. 
  There 
  is 
  a 
  maximum 
  when 
  the 
  

   two 
  effects 
  balance. 
  This 
  was 
  clearly 
  shown 
  by 
  Wigger 
  

   (loc. 
  cit. 
  j). 
  429). 
  If 
  we 
  determine 
  the 
  thickness 
  which 
  gives 
  

   half 
  the 
  maximum 
  value, 
  we 
  obtain 
  an 
  easy 
  and 
  fairly 
  

   accurate 
  measure 
  1 
  of 
  the 
  penetration 
  of 
  the 
  secondary 
  ft 
  rays. 
  

   It 
  is 
  easily 
  shown 
  that 
  this 
  particular 
  thickness 
  is 
  also 
  that 
  

   which 
  would 
  absorb 
  half 
  the 
  equally 
  penetrating 
  radiation 
  

   from 
  a 
  radioactive 
  layer, 
  assuming 
  an 
  exponential 
  law, 
  which 
  

   it 
  is 
  permissible 
  to 
  do 
  with 
  sufficient 
  accuracy. 
  

  

  The 
  results 
  are 
  shown 
  graphically 
  in 
  figs. 
  4 
  to 
  8. 
  In 
  each 
  

   case 
  the 
  curve 
  A 
  represents 
  the 
  result 
  of 
  an 
  experiment 
  in 
  

   which 
  the 
  lead 
  plug, 
  1*61 
  cm. 
  in 
  thickness, 
  was 
  used 
  to 
  

   screen 
  the 
  7 
  rays. 
  The 
  curve 
  B 
  shows 
  the 
  result 
  with 
  the 
  

   plug 
  removed. 
  From 
  each 
  curve 
  can 
  be 
  determined 
  the 
  

   thickness 
  of 
  screen, 
  w 
  T 
  hich 
  gives 
  half 
  the 
  full 
  value 
  of 
  the 
  

   emergence 
  radiation 
  ; 
  the 
  points 
  are 
  marked 
  on 
  the 
  diagrams. 
  

  

  