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  Dr. 
  C. 
  G. 
  Barkla 
  and 
  Mr. 
  0. 
  A. 
  Sadler 
  on 
  

  

  Cr-Zn 
  Group. 
  

  

  The 
  radiation 
  proceeding 
  from 
  elements 
  of 
  atomic 
  weight 
  

   between 
  those 
  of 
  chromium 
  and 
  zinc, 
  when 
  subject 
  to 
  X-rays 
  

   of 
  ordinary 
  penetrating 
  power, 
  is 
  of 
  a 
  very 
  different 
  type 
  

   from 
  that 
  discussed, 
  for 
  from 
  no 
  two 
  elements 
  in 
  this 
  group 
  

   is 
  the 
  penetrating 
  power 
  the 
  same. 
  The 
  absorption 
  by 
  a 
  thin 
  

   sheet 
  of 
  aluminium 
  '010-4 
  cm. 
  thick 
  is, 
  between 
  these 
  limits 
  

   of 
  atomic 
  weight, 
  a 
  decreasing 
  function 
  of 
  the 
  atomic 
  weight, 
  

   varying 
  from 
  94 
  per 
  cent, 
  for 
  the 
  radiation 
  from 
  chromium 
  

   to 
  64 
  per 
  cent, 
  for 
  the 
  radiation 
  from 
  zinc. 
  [One 
  of 
  the 
  

   primary 
  beams 
  used 
  was 
  absorbed 
  to 
  the 
  extent 
  of 
  about 
  

   34 
  per 
  cent.] 
  

  

  Homogeneity. 
  — 
  One 
  of 
  the 
  most 
  remarkable 
  features 
  about 
  

   the 
  radiation 
  from 
  any 
  one 
  of 
  these 
  elements 
  is 
  that 
  though 
  

   the 
  primary 
  rays 
  incident 
  upon 
  the 
  substance 
  are 
  very 
  

   heterogeneous, 
  that 
  is 
  consist 
  of 
  rays 
  varying 
  considerably 
  

   in 
  penetrating 
  power, 
  the 
  secondary 
  rays 
  are 
  homogeneous. 
  

   This 
  point 
  has 
  been 
  briefly 
  referred 
  to 
  in 
  a 
  previous 
  

   paper 
  (/). 
  

  

  To 
  give 
  a 
  particular 
  example 
  : 
  — 
  The 
  ionization 
  produced 
  

   in 
  a 
  given 
  electroscope 
  by 
  a 
  primary 
  X-ray 
  beam 
  was 
  

   diminished 
  by 
  51 
  per 
  cent, 
  by 
  placing 
  a 
  sheet 
  of 
  aluminium 
  

   '0208 
  cm. 
  in 
  thickness 
  in 
  its 
  path 
  ; 
  after 
  77 
  per 
  cent, 
  had 
  

   been 
  absorbed 
  by 
  aluminium, 
  a 
  similar 
  plate 
  produced 
  a 
  

   further 
  diminution 
  of 
  this 
  ionization 
  by 
  27 
  per 
  cent. 
  ; 
  after 
  

   91 
  per 
  cent, 
  had 
  been 
  absorbed 
  the 
  same 
  plate 
  cut 
  off 
  only 
  

   18 
  per 
  cent., 
  showing 
  that 
  the 
  rays 
  after 
  each 
  transmission 
  

   became 
  on 
  the 
  average 
  more 
  and 
  more 
  penetrating. 
  This 
  

   effect 
  has 
  been 
  explained 
  as 
  due 
  to 
  the 
  more 
  absorbable 
  

   constituents 
  being 
  sifted 
  out. 
  

  

  Although 
  such 
  a 
  primary 
  beam 
  produced 
  in 
  one 
  of 
  this 
  

   class 
  of 
  substances 
  the 
  secondary 
  radiation 
  experimented 
  

   upon, 
  it 
  was 
  found 
  that 
  the 
  secondary 
  radiation 
  was 
  of 
  an 
  

   entirely 
  different 
  type, 
  being 
  equally 
  absorbed 
  after 
  trans- 
  

   mission 
  through 
  sheet 
  after 
  sheet 
  of 
  absorbing 
  substance. 
  

  

  The 
  radiation 
  from 
  thick 
  copper 
  was 
  found 
  not 
  to 
  differ 
  

   appreciably 
  from 
  that 
  from 
  a 
  very 
  thin 
  sheet 
  which 
  was 
  only 
  

   thick 
  enough 
  to 
  absorb 
  14 
  per 
  cent, 
  of 
  the 
  primary 
  rays. 
  

   Thus 
  the 
  radiations 
  from 
  the 
  deeper 
  layers 
  after 
  transmission 
  

  

  theory 
  shows 
  that 
  these 
  results 
  can 
  at 
  best 
  he 
  only 
  approximately 
  true. 
  

   It 
  is 
  only 
  necessary 
  to 
  consider 
  the 
  action 
  of 
  the 
  magnetic 
  field 
  in 
  the 
  

   primary 
  pulses 
  on 
  the 
  electrons 
  as 
  they 
  begin 
  to 
  move 
  under 
  the 
  action 
  of 
  

   purely 
  electric 
  forces, 
  to 
  see 
  that 
  dissymmetry 
  must 
  exist. 
  A 
  complete 
  

   theory 
  must 
  also 
  take 
  into 
  consideration 
  the 
  distribution 
  of 
  the 
  tubes 
  of 
  

   electric 
  force 
  round 
  each 
  radiating- 
  electron. 
  

  

  