﻿632 
  Messrs. 
  F. 
  Soddy 
  and 
  A. 
  S. 
  Russell 
  on 
  the 
  

  

  6. 
  Absorption 
  of 
  Uranium 
  y-Rays 
  hy 
  Matter. 
  {First 
  Series.) 
  

  

  After 
  preliminary 
  trials 
  with 
  the 
  uranium 
  X 
  of 
  the 
  first 
  

   separation, 
  a 
  careful 
  series 
  of 
  measurements 
  was 
  carried 
  out 
  

   with 
  the 
  preparations 
  of 
  the 
  second 
  separation 
  on 
  the 
  absorp- 
  

   tion 
  of 
  the 
  y-rays 
  by 
  various 
  substances. 
  The 
  selection 
  of 
  

   the 
  particular 
  experimental 
  disposition 
  employed 
  involves 
  

   necessarily 
  a 
  certain 
  amount 
  of 
  arbitrariness, 
  for 
  as 
  is 
  well- 
  

   known 
  the 
  ionization 
  produced 
  by 
  7-rays 
  results 
  largely 
  

   from 
  the 
  secondary 
  radiations 
  it 
  sets 
  up 
  in 
  the 
  walls 
  of 
  the 
  

   measuring 
  instrument. 
  Again, 
  one 
  may, 
  while 
  keeping 
  the 
  

   distance 
  between 
  the 
  active 
  material 
  and 
  the 
  electroscope 
  

   constant, 
  place 
  the 
  absorbing 
  screens 
  either 
  directly 
  over 
  the 
  

   active 
  material, 
  or 
  directly 
  under 
  the 
  electroscope, 
  or 
  at 
  any 
  

   intermediate 
  position. 
  Or, 
  one 
  may 
  have 
  the 
  active 
  material 
  

   near 
  to 
  the 
  electroscope 
  and 
  include 
  a 
  large 
  cone 
  of 
  rays, 
  or 
  

   far 
  away, 
  and 
  work 
  with 
  practically 
  a 
  parallel 
  beam. 
  It 
  is 
  

   curious 
  that 
  in 
  all 
  the 
  published 
  measurements 
  of 
  the 
  absorp- 
  

   tion 
  of 
  7-rays, 
  only 
  Wigger 
  {Jalir. 
  Radioakt. 
  1905, 
  ii. 
  p. 
  430) 
  

   has 
  given 
  full 
  and 
  precise 
  information 
  on 
  these 
  important 
  

   points. 
  

  

  The 
  electroscope 
  employed 
  was 
  a 
  cylindrical 
  brass 
  one 
  of 
  

   internal 
  height 
  12'8 
  cm., 
  internal 
  diameter 
  10*52 
  cm., 
  and 
  

   of 
  wall 
  thickness 
  0'145 
  cm. 
  It 
  was 
  supported 
  by 
  four 
  brass 
  

   pillars 
  15*8 
  cm. 
  above 
  a 
  permanent 
  base-board. 
  Most 
  of 
  the 
  

   absorbing 
  plates 
  used 
  were 
  11 
  cm. 
  square, 
  but 
  lead 
  and 
  brass 
  

   were 
  in 
  the 
  form 
  of 
  circular 
  disks, 
  11 
  and 
  12*7 
  cm. 
  diameter 
  

   respectively. 
  They 
  were 
  clamped 
  up 
  tightly 
  to 
  the 
  bottom 
  

   of 
  the 
  electroscope 
  forming 
  its 
  base. 
  When 
  the 
  absorbers 
  

   were 
  insulators 
  the 
  upper 
  surface 
  was 
  covered 
  with 
  aluminium 
  

   foil, 
  0*0035 
  mm. 
  thick. 
  For 
  light 
  substances, 
  with 
  a 
  density 
  

   below 
  2, 
  a 
  plate 
  of 
  lead 
  1*26 
  cm. 
  thick 
  formed 
  the 
  base 
  of 
  

   the 
  electroscope. 
  The 
  uranium 
  X 
  mounted 
  on 
  one 
  of 
  two 
  

   wooden 
  stands, 
  of 
  heights 
  1*2 
  and 
  7*55 
  cm. 
  respectively, 
  

   according 
  to 
  the 
  nature 
  and 
  amount 
  of 
  the 
  absorbing 
  material 
  

   under 
  investigation, 
  was 
  placed 
  underneath 
  the 
  electroscope. 
  

   The 
  following 
  table 
  gives 
  the 
  results. 
  The 
  first 
  column 
  

   gives 
  the 
  number 
  of 
  the 
  experiment, 
  the 
  second 
  the 
  distance 
  

   of 
  the 
  preparation 
  from 
  the 
  base 
  of 
  the 
  electroscope, 
  the 
  third 
  

   the 
  absorbing 
  material, 
  the 
  fourth 
  its 
  density 
  d, 
  the 
  fifth 
  

   the 
  range 
  over 
  which 
  the 
  absorption 
  is 
  exponential, 
  the 
  sixth 
  

   the 
  value 
  of 
  the 
  absorption 
  coefficient 
  \ 
  (cm.)~^, 
  and 
  the 
  

   seventh 
  the 
  mean 
  value 
  of 
  the 
  ratio 
  Xjd 
  x 
  100. 
  Some 
  of 
  

   these 
  results 
  are 
  plotted 
  as 
  curves 
  in 
  fig. 
  1. 
  The 
  ordinates 
  

   represent 
  logarithms 
  of 
  the 
  ionization 
  leak, 
  in 
  divisions 
  per 
  

   minute 
  corrected 
  for 
  natural 
  leak, 
  and 
  the 
  abscissae 
  are 
  

   thicknesses 
  in 
  cm. 
  To 
  separate 
  the 
  curves 
  in 
  the 
  figure 
  from 
  

  

  