﻿560 
  Dr. 
  C. 
  G. 
  Barkla 
  and 
  Mr. 
  C. 
  A. 
  Sadler 
  on 
  

  

  Each 
  o£ 
  these 
  elements 
  therefore 
  emits 
  a 
  characteristic 
  

   radiation 
  which 
  is 
  independent 
  of 
  the 
  penetrating 
  power 
  of 
  

   the 
  primary 
  beam 
  setting 
  up 
  this 
  radiation, 
  and 
  being 
  homo- 
  

   geneous, 
  the 
  absorption 
  -coefficient 
  calculated 
  from 
  the 
  

   relation 
  I 
  = 
  I 
  <? 
  _Ax 
  has 
  a 
  value 
  which 
  is 
  independent 
  of 
  the 
  

   thickness 
  x 
  of 
  absorbing 
  substance 
  * 
  — 
  a 
  property 
  which 
  

   appears 
  to 
  be 
  unknown 
  among 
  X-ray 
  beams 
  hitherto 
  experi- 
  

   mented 
  upon. 
  

  

  The 
  absorption-coefficients 
  for 
  the 
  radiations 
  from 
  Fe, 
  

   Co, 
  iSTi, 
  Cu, 
  and 
  Zn, 
  when 
  absorbed 
  by 
  Al, 
  Fe, 
  Cu, 
  Zn, 
  Ag\ 
  

   !Sn, 
  and 
  Pt 
  have 
  been 
  given 
  in 
  a 
  previous 
  paper 
  (/). 
  

  

  Though 
  it 
  has 
  been 
  shown 
  that 
  the 
  secondary 
  radiation 
  

   from 
  some 
  of 
  the 
  substances 
  in 
  this 
  group 
  (Cr-Zn) 
  is 
  re- 
  

   markably 
  homogeneous 
  in 
  comparison 
  with 
  the 
  primary 
  

   which 
  produces 
  it, 
  the 
  test 
  applied 
  is 
  not 
  one 
  of 
  extreme 
  

   delicacy, 
  and 
  the 
  presence 
  of 
  the 
  scattered 
  radiation 
  similar 
  

   to 
  that 
  of 
  the 
  first 
  group 
  (H-S) 
  and 
  of 
  intensity 
  given 
  by 
  the 
  

   law 
  found 
  for 
  that 
  group 
  would 
  be 
  exceedingly 
  difficult 
  to 
  

   detect, 
  as 
  it 
  would 
  produce 
  only 
  about 
  3 
  J- 
  of 
  the 
  total 
  

   ionization 
  actually 
  produced 
  by 
  the 
  secondary 
  rays 
  from 
  

   a 
  very 
  thin 
  sheet 
  of 
  copper. 
  Now 
  we 
  have 
  seen 
  that 
  a 
  

   radiation 
  after 
  transmission 
  through 
  metals 
  may 
  contain 
  a 
  

   considerable 
  proportion 
  of 
  the 
  radiation 
  which 
  is 
  character- 
  

   istic 
  of 
  the 
  metal 
  traversed 
  and 
  which 
  was 
  not 
  in 
  the 
  incident 
  

   radiation. 
  This 
  acquired 
  radiation 
  may 
  even, 
  if 
  the 
  substance 
  

   traversed 
  has 
  almost 
  completely 
  absorbed 
  the 
  incident 
  radia- 
  

   tion, 
  constitute 
  the 
  bulk 
  of 
  the 
  transmitted 
  radiation. 
  In 
  

   such 
  a 
  case, 
  experiments 
  on 
  the 
  transmitted 
  beam 
  in 
  order 
  

   to 
  analyse 
  its 
  constituents 
  would 
  be 
  misleading, 
  the 
  con- 
  

   stituents 
  not 
  being 
  present 
  in 
  the 
  original 
  beam. 
  

  

  To 
  eliminate 
  the 
  possibility 
  of 
  this 
  error, 
  we 
  have 
  tested 
  

   the 
  radiation 
  from 
  copper 
  after 
  transmission 
  through 
  thin 
  

   sheets 
  of 
  copper 
  by 
  absorbing 
  by 
  further 
  plates 
  of 
  copper. 
  

   Thus 
  radiation 
  of 
  a 
  fresh 
  type 
  was 
  not 
  introduced. 
  

  

  It 
  was 
  found 
  that 
  after 
  absorbing 
  98*3 
  per 
  cent, 
  of 
  the 
  

   copper 
  radiation 
  by 
  copper, 
  the 
  absorption 
  by 
  copper 
  

   •00296 
  cm. 
  thick 
  had 
  dropped 
  from 
  74 
  to 
  70 
  per 
  cent. 
  The 
  

   effect 
  was 
  more 
  pronounced 
  in 
  the 
  case 
  of 
  the 
  radiation 
  from 
  

   iron, 
  the 
  numerical 
  data 
  for 
  which 
  are 
  given 
  below. 
  

  

  * 
  After 
  transmission 
  through 
  sheets 
  of 
  absorbing- 
  substance, 
  secondary 
  

   rays 
  are 
  superposed 
  on 
  the 
  transmitted 
  radiation, 
  and 
  the 
  resultant 
  

   radiation 
  in 
  some 
  cases 
  ceases 
  to 
  be 
  even 
  approximately 
  homogeneous. 
  

   This 
  effect 
  can, 
  however, 
  be 
  readily 
  distinguished 
  from 
  true 
  hetero- 
  

   geneity 
  by 
  using 
  as 
  absorbers 
  only 
  those 
  substances 
  in 
  which 
  a 
  radiation 
  

   of 
  different 
  type 
  is 
  not 
  stimulated. 
  

  

  