﻿and 
  Secondary 
  Gamma 
  Rays. 
  

  

  285 
  

  

  Thickness 
  of 
  

  

  Divisions 
  

  

  Absorbing- 
  

  

  Lead. 
  

  

  a 
  minute. 
  

  

  0-0 
  

  

  mm. 
  

  

  7-13 
  

  

  1-7 
  

  

  

  4-01 
  

  

  3-4 
  

  

  

  2-61 
  

  

  5-1 
  

  

  

  1-77 
  

  

  &^ 
  

  

  

  1-34 
  

  

  8-5 
  

  

  

  •95 
  

  

  10-2 
  

  

  

  •75 
  

  

  3-4 
  

  

  2-5 
  

   2-3 
  

   1-9 
  

   1-6 
  

   1-4 
  

  

  The 
  last 
  three 
  values 
  o£ 
  \ 
  are 
  not 
  exact. 
  

  

  The 
  values 
  o£ 
  X 
  aud 
  of 
  X/density 
  for 
  thicknesses 
  between 
  

   2 
  to 
  3 
  mm. 
  and 
  5 
  to 
  6 
  mm. 
  were 
  also 
  found 
  for 
  various 
  

   substances. 
  

  

  Absorber. 
  

  

  Secondary. 
  

  

  Peijiary. 
  

  

  Lead 
  2-4 
  

  

  Zinc 
  : 
  -86 
  

  

  Copper 
  ■' 
  1-09 
  

  

  Iron 
  I 
  -84 
  

  

  Aluminium 
  ...' 
  '21 
  

  

  X/d. 
  

  

  •21 
  

  

  •121 
  

  

  •122 
  

  

  •118 
  

  

  •08 
  

  

  X. 
  

  

  •64 
  

  

  •28 
  

  

  •10 
  

  

  A/^. 
  

  

  •056 
  

   •039 
  

  

  •039 
  

  

  In 
  the 
  right 
  columns 
  are 
  values 
  for 
  the 
  more 
  easily 
  absorbed 
  

   primary 
  7 
  rays 
  as 
  found 
  by 
  McClelland 
  ; 
  these 
  serve 
  to 
  

   illustrate 
  the 
  relative 
  softness 
  of 
  the 
  secondary. 
  

  

  The 
  secondary 
  7 
  rays 
  were 
  next 
  investigated 
  by 
  the 
  method 
  

   of 
  reversal 
  of 
  pairs 
  of 
  plates 
  at 
  A 
  and 
  B, 
  precisely 
  as 
  described 
  

   earlier 
  in 
  this 
  paper 
  when 
  using 
  primary 
  rays. 
  Taking 
  the 
  

   mean 
  values 
  for 
  brick 
  or 
  iron 
  radiators, 
  on 
  wood, 
  iron, 
  and 
  

   lead 
  platforms, 
  the 
  following 
  results 
  were 
  obtained. 
  

  

  Emergent 
  secondary 
  radiation 
  using 
  secondary 
  7 
  rays. 
  

  

  Pb 
  

  

  ]00 
  

  

  Fe 
  

  

  G4-5 
  

  

  Sn 
  

  

  68-5 
  

  

  Al 
  

  

  70-0 
  

  

  Zn 
  

  

  63-0 
  

  

  

  

  75-0 
  

  

  Cu 
  .... 
  

  

  63-5 
  

  

  

  

  The 
  curve 
  obtained, 
  shown 
  in 
  fig. 
  5, 
  is 
  typical 
  of 
  easily 
  

   absorbed 
  7 
  rays. 
  

  

  