﻿of 
  the 
  Secondary 
  Ionization 
  due 
  to 
  ft 
  Hays. 
  695 
  

  

  The 
  screens 
  used 
  were 
  as 
  follows 
  : 
  — 
  

  

  Ac 
  — 
  Thin 
  mica 
  + 
  thin 
  tinfoil 
  ; 
  the 
  two 
  equivalent 
  to 
  about 
  

   10 
  cm. 
  of 
  air, 
  and 
  enough 
  to 
  cut 
  off 
  all 
  a 
  rays. 
  

  

  UrX 
  — 
  No 
  screen. 
  

  

  Ra 
  1 
  — 
  Thin 
  mica 
  + 
  tinfoil, 
  the 
  latter 
  *00366 
  cm. 
  thick. 
  

  

  Ra 
  2— 
  Thin 
  mica 
  + 
  tinfoil, 
  the 
  latter 
  '0293 
  cm. 
  thick. 
  

  

  Ra 
  3 
  — 
  Thin 
  mica 
  + 
  tinfoil, 
  the 
  latter 
  '0586 
  cm. 
  thick. 
  

  

  Ra 
  4 
  — 
  Thin 
  mica 
  + 
  lead, 
  the 
  latter 
  equivalent 
  to 
  *11 
  cm. 
  of 
  

   tinfoil. 
  

  

  The 
  figures 
  for 
  Ra 
  1, 
  Ra 
  2, 
  Ra 
  3, 
  are 
  for 
  ft 
  rays 
  only, 
  the 
  

   effects 
  of 
  7 
  rays 
  having 
  been 
  eliminated. 
  The 
  figures 
  for 
  

   Ra 
  4 
  have 
  not 
  been 
  corrected 
  for 
  y 
  rays, 
  and 
  must, 
  indeed, 
  

   refer 
  almost 
  entirely 
  to 
  rays 
  of 
  the 
  latter 
  class. 
  

  

  The 
  velocity 
  of 
  the 
  ft 
  rays 
  of 
  Ac 
  is 
  less 
  than 
  in 
  the 
  case 
  

   of 
  UrX. 
  In 
  the 
  case 
  of 
  radium 
  it 
  varies 
  ; 
  some 
  of 
  the 
  rays 
  

   are 
  as 
  little 
  penetrating 
  as 
  those 
  of 
  Ac, 
  others 
  more 
  than 
  the 
  

   rays 
  of 
  UrX. 
  The 
  figures 
  seem 
  to 
  show 
  that 
  there 
  is 
  a 
  

   certain 
  small 
  dependence 
  on 
  the 
  velocity 
  of 
  the 
  ft 
  rays, 
  but 
  

   it 
  is 
  insufficient 
  to 
  account 
  for 
  the 
  variations 
  in 
  quality 
  which 
  

   have 
  been 
  described 
  above. 
  

  

  "We 
  must 
  therefore 
  fall 
  back 
  upon 
  a 
  second 
  explanation, 
  

   viz., 
  that 
  the 
  scattered 
  rays 
  possess 
  velocities 
  which 
  are 
  less, 
  

   on 
  the 
  average, 
  than 
  those 
  of 
  the 
  primary 
  ; 
  and 
  that 
  the 
  

   difference 
  is 
  greater 
  in 
  the 
  case 
  of 
  the 
  atoms 
  of 
  small 
  atomic 
  

   weight. 
  This 
  is 
  in 
  general 
  accordance 
  with 
  other 
  experi- 
  

   ments 
  and 
  with 
  expectation. 
  It 
  has 
  been 
  shown 
  by 
  Gehrcke 
  

   [Ann. 
  der 
  Phys. 
  viii, 
  p. 
  81, 
  1902) 
  that 
  scattered 
  cathode 
  

   rays 
  possess 
  a 
  variety 
  of 
  velocities, 
  none 
  greater 
  than 
  that 
  

   of 
  the 
  primary, 
  some, 
  however, 
  being 
  equal 
  to 
  it. 
  And 
  it 
  is 
  

   not 
  unreasonable 
  to 
  suppose 
  that 
  ft 
  rays 
  lose 
  more 
  energy 
  in 
  

   being 
  scattered 
  by 
  atoms 
  of 
  less 
  weight 
  or 
  of 
  looser 
  building 
  

   than 
  by 
  those 
  which 
  are 
  heavier 
  or 
  more 
  rigid. 
  If, 
  there- 
  

   fore, 
  a 
  homogeneous 
  beam 
  of 
  ft 
  rays 
  fell 
  upon 
  an 
  aluminium 
  

   plate, 
  there 
  would 
  be 
  found 
  in 
  the 
  scattered 
  secondary 
  rays 
  

   a 
  larger 
  proportion 
  of 
  particles 
  of 
  much 
  reduced 
  speed 
  than 
  

   if 
  the 
  plate 
  had 
  been 
  lead. 
  

  

  When 
  we 
  measure 
  the 
  ionization 
  produced 
  in 
  an 
  ionization- 
  

   chamber, 
  and 
  insert 
  successive 
  thicknesses 
  of 
  absorbing 
  

   material 
  between 
  the 
  radiating 
  substance 
  and 
  the 
  chamber, 
  

   placing 
  them 
  in 
  contact 
  with 
  the 
  wall 
  of 
  the 
  latter, 
  the 
  effect 
  

   is 
  the 
  same 
  as 
  if 
  we 
  gradually 
  removed 
  the 
  chamber 
  to 
  greater 
  

   distances 
  ; 
  if 
  we 
  may 
  assume 
  that 
  the 
  law 
  of 
  absorption 
  in 
  

   air 
  and 
  in 
  the 
  given 
  absorbing 
  material 
  is 
  not 
  greatly 
  affected 
  

   by 
  the 
  alterations 
  in 
  quality 
  which 
  we 
  are 
  considering. 
  We 
  

  

  