﻿572 
  Dr. 
  C. 
  G. 
  Barkla 
  and 
  Mr. 
  C. 
  A. 
  Sadler 
  on 
  

  

  Ag-I 
  Group. 
  

   When 
  subject 
  to 
  primary 
  beams 
  of 
  moderate 
  penetrating 
  

   power 
  such 
  as 
  those 
  used 
  in 
  these 
  experiments, 
  the 
  elements 
  

   of 
  atomic 
  weights 
  from 
  that 
  of 
  silver 
  to 
  that 
  o£ 
  iodine 
  are 
  

   exceedingly 
  susceptible 
  to 
  small 
  changes 
  in 
  the 
  penetrating 
  

   power 
  of 
  that 
  primary 
  radiation, 
  both 
  as 
  regards 
  the 
  character 
  

   and 
  the 
  intensity 
  of 
  the 
  secondary 
  rays 
  that 
  they 
  emit. 
  This 
  

   is 
  indeed 
  the 
  most 
  remarkable 
  feature 
  of 
  this 
  group 
  of 
  

   elements. 
  

  

  Silver 
  and 
  tin 
  are 
  the 
  only 
  elements 
  of 
  this 
  group 
  which 
  

   have 
  been 
  examined 
  in 
  any 
  detail, 
  but 
  they 
  appear 
  from 
  

   superficial 
  observation 
  to 
  be 
  typical 
  of 
  the 
  whole 
  group. 
  

   In 
  previous 
  papers 
  it 
  has 
  been 
  shown 
  that 
  the 
  radiation 
  from 
  

   these 
  two 
  when 
  subject 
  to 
  a 
  primary 
  beam 
  of 
  only 
  moderate 
  

   penetrating 
  power, 
  is 
  not 
  a 
  scattered 
  radiation 
  like 
  that 
  which 
  

   proceeds 
  from 
  elements 
  of 
  the 
  H-S 
  group, 
  for 
  it 
  differs 
  

   much 
  more 
  in 
  penetrating 
  power 
  from 
  the 
  primary 
  producing 
  

   it 
  than 
  the 
  radiation 
  from 
  the 
  elements 
  of 
  that 
  group, 
  though 
  

   not 
  so 
  much 
  as 
  the 
  radiation 
  from 
  Cr, 
  Fe, 
  Cu, 
  &c. 
  The 
  

   radiation 
  does 
  not 
  exhibit 
  the 
  polarity 
  of 
  a 
  primary 
  beam 
  

   where 
  such 
  exists, 
  and 
  it 
  is 
  not 
  distributed 
  in 
  the 
  manner 
  of 
  

   the 
  radiation 
  from 
  substances 
  of 
  low 
  atomic 
  weight, 
  the 
  in- 
  

   tensity 
  in 
  a 
  direction 
  almost 
  opposite 
  to 
  that 
  of 
  primary 
  

   propagation 
  being 
  approximately 
  equal 
  to 
  that 
  in 
  a 
  direction 
  

   at 
  right 
  angles. 
  It 
  was 
  not 
  possible, 
  however, 
  to 
  perform 
  

   this 
  experiment 
  with 
  anything 
  like 
  the 
  degree 
  of 
  accuracy 
  

   with 
  which 
  it 
  was 
  done 
  in 
  the 
  case 
  of 
  the 
  radiation 
  from 
  Fe> 
  

   Cu, 
  &c, 
  because 
  the 
  ionization 
  produced 
  by 
  the 
  secondary 
  

   beams 
  from 
  Ag 
  and 
  Sn 
  was 
  much 
  less 
  intense. 
  

  

  Secondary 
  Scattered 
  Rays. 
  — 
  Experiments 
  with 
  the 
  softest 
  

   rays 
  procurable 
  from 
  an 
  X-ray 
  tube 
  of 
  ordinary 
  type, 
  how- 
  

   ever, 
  considerably 
  simplified 
  the 
  secondary 
  radiation, 
  for 
  it 
  

   was 
  found 
  that 
  the 
  secondary 
  rays 
  from 
  silver 
  then 
  differed 
  

   very 
  little 
  in 
  penetrating 
  power 
  from 
  the 
  soft 
  primary. 
  

   When 
  the 
  intensity 
  of 
  secondary 
  radiation 
  set 
  up 
  by 
  a 
  

   powerfully 
  polarized 
  primary 
  was 
  examined 
  in 
  the 
  two 
  

   principal 
  directions 
  at 
  right 
  angles, 
  it 
  was 
  found 
  that 
  it 
  

   varied 
  by 
  approximately 
  the 
  same 
  amount 
  as 
  that 
  from 
  

   elements 
  of 
  the 
  H-S 
  group, 
  thus 
  exhibiting 
  the 
  same 
  per- 
  

   fection 
  of 
  scattering. 
  When 
  tin 
  was 
  examined 
  in 
  the 
  same 
  

   way, 
  the 
  variation 
  in 
  intensity 
  was 
  found 
  to 
  be 
  about 
  half 
  

   that 
  exhibited 
  by 
  the 
  rays 
  from 
  paper 
  and 
  from 
  silver. 
  Thus 
  

   by 
  using 
  a 
  very 
  soft 
  primary 
  radiation, 
  an 
  almost 
  purely 
  

   scattered 
  secondary 
  radiation 
  was 
  emitted 
  by 
  silver, 
  and 
  a 
  

   radiation 
  consisting 
  of 
  a 
  large 
  proportion 
  of 
  scattered 
  rays 
  

   was 
  emitted 
  from 
  Sn. 
  

  

  