﻿558 
  

  

  Dr. 
  C. 
  G. 
  Barkla 
  and 
  Mr. 
  C. 
  A. 
  Sadler 
  

  

  on 
  

  

  passed 
  through 
  various 
  thicknesses 
  of 
  zinc 
  to 
  absorb 
  different 
  

   proportions 
  of 
  it. 
  

  

  The 
  radiation 
  from 
  copper 
  was 
  examined 
  in 
  the 
  same 
  way. 
  

  

  Table 
  II. 
  

   Radiation 
  from 
  Copper 
  (thick 
  sheet) 
  

  

  I. 
  

  

  Percentage 
  Absorption 
  

  

  by 
  Zn 
  

  

  previous 
  to 
  absorption 
  

  

  in 
  column 
  II. 
  

  

  II. 
  

  

  Percentage 
  Absorption 
  

  

  by 
  Zn 
  (-00131 
  cm.) 
  

  

  after 
  absorption 
  

  

  in 
  column 
  I. 
  

  

  

  

  4$ 
  

  

  44 
  

  

  44 
  

  

  

  

  41-4 
  

  

  42-6 
  

  

  42-6 
  

  

  67*5 
  

  

  43-1 
  

  

  81-5 
  

  

  42 
  

  

  96-7 
  

  

  41-8 
  

  

  

  

  42-5 
  

  

  All 
  these 
  experiments 
  show 
  extremely 
  little 
  change 
  in 
  the 
  

   percentage 
  absorption 
  even 
  after 
  almost 
  complete 
  absorption. 
  

   The 
  contrast 
  between 
  primary 
  and 
  secondary 
  beams 
  is 
  

   strikingly 
  shown 
  in 
  fig. 
  1 
  (p. 
  559) 
  5 
  in 
  which 
  the 
  absorptions 
  are 
  

   represented 
  by 
  ordinates 
  and 
  the 
  amount 
  previously 
  absorbed 
  

   by 
  abscissae. 
  The 
  corresponding 
  curve 
  for 
  the 
  secondary 
  

   radiation 
  from 
  paper 
  (in 
  this 
  case 
  subject 
  to 
  penetrating 
  

   primary 
  rays) 
  is 
  given 
  for 
  comparison 
  (fig. 
  1). 
  

  

  Independence 
  of 
  Primary 
  Rays. 
  — 
  To 
  exhibit 
  the 
  inde- 
  

   pendence 
  of 
  the 
  penetrating 
  power 
  of 
  the 
  secondary 
  radiation 
  

   from 
  one 
  of 
  these 
  metals 
  Cu, 
  of 
  that 
  of 
  the 
  primary 
  pro- 
  

   ducing 
  it, 
  we 
  have 
  tabulated 
  below 
  the 
  absorbability 
  of 
  

   various 
  primary 
  rays 
  and 
  that 
  of 
  the 
  secondary 
  rays 
  produced 
  

   by 
  these. 
  Though 
  the 
  absorption 
  of 
  the 
  primary 
  by 
  alu- 
  

   minium 
  '0208 
  cm. 
  thick 
  varied 
  from 
  52 
  to 
  18 
  per 
  cent., 
  the 
  

   absorptions 
  of 
  the 
  corresponding 
  secondary 
  beams 
  from 
  

   copper 
  by 
  aluminium 
  '0104 
  cm. 
  thick 
  were 
  as 
  nearly 
  as 
  

   observable 
  the 
  same, 
  the 
  experimentally 
  determined 
  values 
  

   being 
  58*3 
  and 
  58- 
  1 
  per 
  cent, 
  respectively. 
  This 
  constancy 
  

   in 
  character 
  makes 
  accurate 
  experiments 
  on 
  these 
  radiations 
  

   possible. 
  

  

  