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  Prof. 
  A. 
  S. 
  Eve 
  on 
  the 
  Secondary 
  y-JRays 
  

  

  impact 
  of 
  /3 
  rajs. 
  As 
  matters 
  stand 
  at 
  present, 
  the 
  most 
  

   notable 
  difference 
  between 
  X 
  and 
  7 
  rays 
  is 
  that 
  the 
  former 
  

   arise 
  where 
  the 
  cathode 
  rays 
  are 
  stopped 
  or 
  absorbed, 
  and 
  the 
  

   latter 
  where 
  the 
  /3 
  rays 
  originate. 
  

  

  The 
  following 
  experiments 
  show 
  clearly 
  that 
  primary 
  7 
  rays 
  

   give 
  rise 
  to 
  secondary 
  7 
  rays, 
  that 
  the 
  intensity 
  depends 
  

   upon 
  the 
  material 
  surrounding 
  the 
  radium 
  employed, 
  upon 
  

   the 
  nature 
  of 
  the 
  secondary 
  radiator 
  and 
  upon 
  the 
  thickness 
  

   and 
  material 
  of 
  the 
  screens 
  placed 
  in 
  front 
  of 
  the 
  electroscope, 
  

   and 
  that 
  the 
  intensity 
  of 
  the 
  secondary 
  7 
  radiation 
  does 
  not 
  

   follow 
  the 
  order 
  of 
  the 
  atomic 
  weight, 
  or 
  of 
  the 
  density, 
  of 
  

   the 
  secondary 
  radiator. 
  In 
  all 
  these 
  particulars 
  there 
  is 
  a 
  

   similarity 
  between 
  secondary 
  X 
  and 
  secondary 
  7 
  rays, 
  and 
  

   this 
  accounts 
  to 
  some 
  extent 
  for 
  the 
  apparent 
  differences 
  

   noted 
  at 
  the 
  beginning 
  of 
  this 
  paper. 
  

  

  In 
  a 
  previous 
  paper 
  (Phil. 
  Mag. 
  June 
  1908) 
  I 
  have 
  

   noted 
  that 
  books, 
  carbon, 
  cement, 
  or 
  bricks 
  give 
  rise 
  to 
  

   more 
  penetrating 
  rays 
  than 
  lead, 
  when 
  these 
  substances 
  are 
  

   exposed 
  to 
  the 
  ft 
  and 
  7 
  rays 
  of 
  radium, 
  provided 
  the 
  electro- 
  

   scope 
  is 
  screened 
  from 
  the 
  secondarv 
  radiator 
  by 
  a 
  sufficient 
  

   thickness 
  of 
  aluminium, 
  or 
  by 
  a 
  thick 
  book. 
  It 
  was 
  shown, 
  

   too, 
  that 
  some 
  of 
  these 
  secondary 
  rays 
  came 
  from 
  a 
  depth 
  of 
  at 
  

   least 
  4 
  cms. 
  in 
  the 
  case 
  of 
  carbon, 
  6 
  cms. 
  for 
  wood, 
  3'5 
  cms. 
  for 
  

   slate. 
  Moreover, 
  the 
  primary 
  7 
  rays 
  had 
  first 
  to 
  penetrate 
  

   those 
  thicknesses. 
  It 
  is 
  these 
  very 
  penetrating 
  rays 
  that 
  are 
  

   under 
  consideration. 
  The 
  secondary 
  radiators 
  and 
  screens 
  

   were 
  all 
  tested, 
  when 
  the 
  radium 
  was 
  not 
  present, 
  and 
  they 
  were 
  

   found 
  free 
  from 
  any 
  radioactive 
  effects 
  under 
  the 
  conditions 
  

   of 
  the 
  experiments. 
  

  

  Fourteen 
  milligrams 
  of 
  pure 
  radium 
  bromide 
  were 
  placed 
  

   in 
  a 
  test-tube 
  within 
  cylinders 
  of 
  (1) 
  lead, 
  1 
  cm. 
  thick, 
  or 
  

   (2) 
  nickel-steel, 
  2'2 
  cms. 
  thick. 
  The 
  radiators 
  were 
  5 
  ems. 
  

   or 
  more 
  thick, 
  and 
  measured 
  22*5 
  X 
  22*5 
  centimetres. 
  The 
  

   necessarily 
  large 
  size 
  and 
  thickness 
  of 
  the 
  radiators 
  made 
  

   it 
  difficult 
  to 
  procure 
  suitable 
  substances 
  for 
  the 
  experiments, 
  

   and 
  I 
  have 
  worked 
  mainly 
  with 
  lead, 
  iron, 
  brick, 
  and 
  slate. 
  

   The 
  screens 
  placed 
  in 
  front 
  of 
  the 
  electroscope 
  were 
  either 
  of 
  

   (1) 
  aluminium 
  or 
  (2) 
  lead. 
  The 
  electroscope. 
  10 
  x 
  L0xl6cms., 
  

   was 
  made 
  of 
  zinc 
  and 
  mounted 
  on 
  a 
  platform. 
  One 
  face 
  of 
  

   the 
  electroscope 
  was 
  removed 
  and 
  replaced 
  by 
  two 
  very 
  

   thin 
  aluminium 
  sheets, 
  cadi 
  '0003J 
  cm. 
  thick. 
  The 
  electro- 
  

   scope 
  and 
  screens 
  were 
  guarded 
  from 
  the 
  radium 
  and 
  the 
  

   cylinder 
  containing 
  it 
  by 
  blocks 
  of 
  lead 
  L0 
  cm-, 
  thick. 
  The 
  

   ciVect 
  with 
  no 
  radiator 
  present, 
  except 
  air. 
  was 
  deduced 
  from 
  

   the 
  effect 
  with 
  the 
  stated 
  radiator 
  present 
  ; 
  tlii- 
  was 
  done 
  for 
  

   all 
  the 
  screens 
  used. 
  Th 
  • 
  secondary 
  rays 
  from 
  air 
  are 
  easily 
  

  

  