﻿390 
  

  

  Prof. 
  McLennan 
  and 
  Mr. 
  Kennedy 
  on 
  the 
  

  

  which 
  the 
  value 
  of 
  the 
  currents 
  decreased 
  is 
  indicated 
  byjthe 
  

  

  ing 
  a 
  

  

  Fi<r. 
  7. 
  

  

  curve 
  in 
  no-. 
  7. 
  In 
  arriving 
  at 
  the 
  numbers 
  recorded 
  in 
  

  

  .32 
  Thickness 
  

   in 
  mm. 
  

  

  Table 
  IX. 
  corrections 
  were 
  made 
  from 
  a 
  preliminary 
  set 
  

   of 
  observations 
  for 
  the 
  activity 
  of 
  the 
  tinfoil 
  sheets 
  which 
  

   were 
  found 
  to 
  emit 
  an 
  extremely 
  feeble 
  though 
  measurable 
  

   radiation. 
  From 
  the 
  values 
  given 
  in 
  Table 
  IX. 
  it 
  will 
  be 
  

   seen 
  that 
  it 
  was 
  necessary 
  i 
  o 
  cover 
  the 
  salt 
  with 
  foil 
  repre- 
  

   senting 
  a 
  total 
  thickness 
  of 
  0*6 
  mm. 
  before 
  the 
  radiation 
  

   from 
  the 
  salt 
  was 
  reduced 
  to 
  one-half 
  its 
  original 
  intensity. 
  

  

  The 
  mean 
  values 
  of 
  the 
  absorption 
  constant 
  \ 
  for 
  the 
  

   different 
  thicknesses 
  were 
  deduced 
  from 
  the 
  formula 
  

  

  1 
  = 
  1 
  

  

  e 
  3 
  

  

  where 
  I 
  was 
  the 
  ionization 
  current 
  obtained 
  with 
  the 
  un- 
  

   screened 
  salt, 
  and 
  I 
  the 
  current 
  when 
  d 
  centimetres 
  was 
  the 
  

   thickness 
  of 
  the 
  tinfoil 
  traversed. 
  The 
  deduced 
  values 
  of 
  A 
  

   are 
  given 
  in 
  Table 
  X., 
  and 
  as 
  will 
  be 
  seen 
  these 
  show 
  a 
  

   gradual 
  decrease 
  as 
  the 
  number 
  of 
  sheets 
  of 
  tinfoil 
  used 
  for 
  

   a 
  screen 
  was 
  increased. 
  

  

  