﻿552 
  Dr. 
  C. 
  G. 
  Barkla 
  and 
  Mr. 
  0. 
  A. 
  Sadler 
  

  

  on 
  

  

  arbitrary, 
  as 
  elements 
  of 
  intermediate 
  atomic 
  weight 
  emit 
  

   radiations 
  possessing 
  intermediate 
  properties, 
  and 
  the 
  classi- 
  

   fication 
  depends 
  to 
  a 
  certain 
  extent 
  on 
  the 
  character 
  of 
  the 
  

   primary 
  radiation. 
  It 
  is, 
  however, 
  convenient 
  for 
  the 
  

   purpose 
  of 
  description. 
  

  

  H-S 
  Group. 
  

  

  The 
  group 
  of 
  substances 
  of 
  atomic 
  weights 
  from 
  that 
  

   of 
  hydrogen 
  to 
  that 
  of 
  sulphur 
  appears 
  simplest 
  in 
  behaviour 
  

   under 
  X-rays 
  of 
  ordinary 
  penetrating 
  power. 
  

  

  Each 
  element, 
  when 
  subject 
  to 
  such 
  a 
  primary 
  beam, 
  

   emits 
  a 
  secondary 
  radiation 
  which 
  has 
  almost 
  exactly 
  the 
  

   same 
  penetrating 
  power 
  as 
  the 
  primary 
  producing 
  it. 
  The 
  

   secondary 
  beam 
  is 
  complex 
  like 
  the 
  primary, 
  and 
  contains 
  

   the 
  rays 
  of 
  various 
  penetrating 
  powers 
  in 
  approximately 
  the 
  

   same 
  proportion 
  as 
  the 
  primary 
  (a 
  & 
  b). 
  

  

  Though 
  it 
  is 
  very 
  difficult, 
  if 
  not 
  impossible, 
  to 
  detect 
  by 
  

   direct 
  methods 
  a 
  difference 
  in 
  the 
  penetrating 
  powers 
  of 
  

   primary 
  and 
  secondary 
  beams, 
  when 
  the 
  primary 
  is 
  not 
  

   more 
  than 
  moderately 
  penetrating, 
  it 
  appears 
  from 
  indirect 
  

   evidence 
  that 
  the 
  secondary 
  radiation 
  is 
  always 
  slightly 
  

   more 
  absorbable 
  than 
  the 
  primary 
  (6). 
  With 
  more 
  pene- 
  

   trating 
  primary 
  rays 
  the 
  difference 
  is 
  more 
  marked 
  (b 
  & 
  *). 
  

  

  The 
  iutensity 
  of 
  radiation 
  emitted 
  by 
  these 
  elements 
  is 
  

   proportional 
  merely 
  to 
  the 
  quantity 
  of 
  matter 
  passed 
  through 
  

   by 
  a 
  primary 
  beam 
  of 
  definite 
  intensity, 
  if 
  of 
  low 
  to 
  moderate 
  

   penetrating 
  power 
  : 
  in 
  other 
  words, 
  the 
  intensity 
  of 
  radiation 
  

   from 
  an 
  atom 
  is 
  proportional 
  to 
  the 
  atomic 
  weight 
  f 
  (a 
  & 
  b). 
  

   The 
  secondary 
  radiation 
  proceeding 
  from 
  one 
  of 
  these 
  

   substances 
  in 
  a 
  direction 
  perpendicular 
  to 
  that 
  of 
  pro- 
  

   pagation 
  of 
  the 
  primary 
  is 
  fairly 
  completely 
  polarized, 
  

   when 
  the 
  rays 
  are 
  of 
  the 
  absorbable 
  type 
  (d). 
  

  

  The 
  intensity 
  of 
  secondary 
  radiation 
  from 
  each 
  of 
  these 
  

   substances 
  varies 
  in 
  different 
  directions 
  perpendicular 
  to 
  

   that 
  of 
  propagation 
  of 
  a 
  polarized 
  primary 
  beam 
  (<?). 
  

  

  The 
  amount 
  of 
  polarization 
  in 
  a 
  primary 
  beam, 
  as 
  in- 
  

   dicated 
  by 
  the 
  secondary 
  rays, 
  diminishes 
  with 
  an 
  increase 
  

   in 
  the 
  u 
  hardness 
  " 
  of 
  a 
  given 
  X-ray 
  tube 
  emitting 
  the 
  

   primary 
  radiation. 
  

  

  * 
  Beatty, 
  Phil. 
  Mag. 
  Nov. 
  1907, 
  pp. 
  604-614. 
  

  

  t 
  It 
  was 
  considered 
  possible 
  that 
  the 
  discrepancy 
  in 
  the 
  case 
  of 
  

   hydrogen, 
  as 
  found 
  by 
  one 
  of 
  us, 
  might 
  be 
  explained 
  by 
  the 
  mixture 
  of 
  a 
  

   small 
  quantity 
  of 
  air. 
  Crowther, 
  however, 
  from 
  more 
  recent 
  experiments 
  

   has 
  concluded 
  that 
  hydrogen 
  and 
  helium 
  in 
  this 
  group 
  are 
  exceptions 
  to 
  

   this 
  law 
  of 
  intensity. 
  

  

  