﻿644 
  Messrs. 
  F. 
  Soddj 
  and 
  A. 
  S. 
  Russell 
  on 
  the 
  

  

  10. 
  Comparison 
  of 
  the 
  Absorption 
  Coefficients 
  of 
  the 
  j-Rai/s 
  

   of 
  Uranium 
  and 
  Radium. 
  

  

  With 
  the 
  method 
  described 
  in 
  last 
  section 
  a 
  series 
  o£ 
  com- 
  

   parable 
  measurements 
  o£ 
  the 
  absorption 
  coefficients 
  of 
  the 
  

   7-rays 
  of 
  uranium 
  X 
  and 
  radium 
  were 
  finally 
  carried 
  out. 
  

   The 
  only 
  difference 
  between 
  them 
  is 
  that 
  the 
  uranium 
  X 
  was 
  

   in 
  the 
  form 
  of 
  a 
  surface 
  of 
  about 
  30 
  cm.^ 
  area, 
  while 
  the 
  

   radium 
  constituted 
  practically 
  a 
  point 
  source. 
  The 
  uranium 
  X 
  

   preparations 
  employed 
  were 
  those 
  of 
  the 
  third 
  separation. 
  

  

  The 
  results 
  with 
  radium 
  are 
  given 
  in 
  Table 
  II. 
  and 
  fig. 
  3. 
  

   They 
  bear 
  out 
  the 
  values 
  of 
  McClelland 
  and 
  Eve. 
  

  

  Table 
  II. 
  — 
  Radium 
  7-Kays. 
  

  

  Lead 
  Electroscope, 
  base 
  '975 
  cm. 
  thick. 
  Absorbing 
  Plates 
  

   laid 
  directly 
  over 
  Radium, 
  which 
  was 
  13*0 
  cm. 
  below 
  

  

  

  under 
  surface 
  

  

  of 
  base. 
  

  

  0-47 
  mg. 
  of 
  RaBrg 
  used. 
  

  

  No. 
  

   1. 
  

  

  Material, 
  

  

  Density. 
  

  

  Eange 
  (cm,). 
  

  

  X(cm.)-l. 
  

  

  lOOxX/c?. 
  

  

  Mercury. 
  

  

  13-59 
  

  

  •34 
  to 
  3-32 
  

  

  •642 
  

  

  4-72 
  

  

  2. 
  

  

  Lead. 
  

  

  11-40 
  

  

  to 
  7-91 
  

  

  •495 
  

  

  4^34 
  

  

  3. 
  

  

  Copper. 
  

  

  8-81 
  

  

  to 
  7-60 
  

  

  •351 
  

  

  3-98 
  

  

  4. 
  

  

  Brass. 
  

  

  8-35 
  

  

  to 
  5-86 
  

  

  •325 
  

  

  3-89 
  

  

  5. 
  

  

  Iron. 
  

  

  7-62 
  

  

  to 
  7-57 
  

  

  •304 
  

  

  3-99 
  

  

  6. 
  

  

  Tin. 
  

  

  7-245 
  

  

  to 
  5-51 
  

  

  •281 
  

  

  3-88 
  

  

  7. 
  

  

  Zinc. 
  

  

  7-07 
  

  

  to 
  6-00 
  

  

  •278 
  

  

  3-93 
  

  

  8. 
  

  

  Slate. 
  

  

  2-854 
  

  

  to 
  9-44 
  

  

  •118 
  

  

  4-14 
  

  

  9. 
  

  

  Aluminium. 
  

  

  2-77 
  

  

  to 
  11-19 
  

  

  •111 
  

  

  4-01 
  

  

  10. 
  

  

  Glass. 
  

  

  2-52 
  

  

  to 
  11-26 
  

  

  •105 
  

  

  4-16 
  

  

  11. 
  

  

  Magnesia 
  Brick. 
  

  

  1-92 
  

  

  to 
  11-86 
  

  

  •076 
  

  

  3-96 
  

  

  12. 
  

  

  Sulphur. 
  

  

  1-785 
  

  

  to 
  11-59 
  

  

  •0782 
  

  

  4-38 
  

  

  13. 
  

  

  Paraffin-wax. 
  

  

  0-862 
  

  

  to 
  11-39 
  

  

  •040 
  

  

  4-64 
  

  

  Mean 
  value 
  of 
  X/d 
  (Nos. 
  3 
  to 
  11) 
  = 
  -0399. 
  

  

  The 
  results 
  with 
  uranium 
  X 
  are 
  given 
  in 
  Table 
  III. 
  and 
  

   fig. 
  4 
  (pp. 
  646, 
  647). 
  

  

  From 
  the 
  curves 
  (figs. 
  3 
  and 
  4) 
  it 
  can 
  be 
  seen 
  how 
  nearly 
  

   the 
  exponential 
  law 
  is 
  followed 
  in 
  the 
  case 
  of 
  all 
  substances. 
  

   Only 
  the 
  first 
  points 
  on 
  the 
  curve, 
  with 
  zero 
  thickness, 
  show 
  

   slight 
  irregularities. 
  There 
  is 
  no 
  evidence 
  of 
  the 
  existence 
  

   of 
  the 
  second 
  negative 
  exponential 
  term, 
  showing 
  that 
  if 
  

   secondary 
  rays 
  are 
  generated 
  by 
  the 
  7-rays 
  of 
  either 
  uranium 
  

   or 
  radium, 
  they 
  are 
  unable 
  to 
  penetrate 
  1 
  cm. 
  of 
  lead. 
  

   Moreover 
  the 
  value 
  of 
  the 
  absorption 
  coefficient 
  is 
  for 
  all 
  sub- 
  

   stances 
  of 
  average 
  density 
  independent 
  of 
  the 
  nature 
  of 
  the 
  

   substance. 
  

  

  