﻿and 
  Secondary 
  Gamma 
  Rays. 
  

  

  281 
  

  

  radiations. 
  The 
  readings 
  given 
  in 
  Table 
  II. 
  were 
  obtained 
  

   with 
  a 
  vertical 
  cylindrical 
  zinc 
  electroscope 
  21 
  cm. 
  high, 
  

   16 
  cm. 
  diameter, 
  having 
  a 
  thin 
  aluminium 
  base 
  with 
  the 
  

   radium 
  placed 
  below 
  it 
  at 
  15 
  cm. 
  distance. 
  The 
  plates 
  A 
  

   and 
  B 
  were 
  clamped 
  close 
  to 
  the 
  aluminium. 
  

  

  Using 
  the 
  electroscope 
  previously 
  described 
  (fig. 
  1) 
  the 
  

   mean 
  results 
  obtained, 
  for 
  different 
  screens 
  as 
  stated, 
  and 
  in 
  

   the 
  manner 
  just 
  described, 
  are 
  given 
  in 
  Table 
  III. 
  

  

  Table 
  III. 
  

  

  Screens: 
  — 
  

   Plates. 
  

  

  Lead 
  Cylinder, 
  

   0-6 
  cm. 
  

  

  Lead, 
  

   2-2 
  cm. 
  

  

  Iron 
  Cylinder, 
  

   21 
  cm. 
  

  

  Pb 
  

  

  100 
  

   90 
  

   90 
  

   90 
  

   91 
  

   98 
  

  

  104 
  

  

  100 
  

  

  94-4 
  

  

  96-5 
  

  

  990 
  

  

  107-6 
  

  

  119-4 
  

  

  100 
  

  

  88 
  

  

  87-6 
  

  

  87-0 
  

  

  87-0 
  

  

  900 
  

  

  95-2 
  

  

  Aff 
  

  

  

  Cu 
  

  

  Fe 
  

  

  Al 
  

  

  C 
  

  

  

  The 
  corresponding 
  three 
  curves 
  are 
  shown 
  in 
  fig. 
  3 
  (p. 
  282). 
  

   The 
  changes 
  in 
  the 
  relative 
  positions 
  of 
  carbon 
  and 
  lead 
  

   according 
  to 
  the 
  nature 
  and 
  thickness 
  of 
  the 
  screens 
  employed 
  

   are 
  very 
  striking, 
  and 
  at 
  first 
  sight 
  suggest 
  selective 
  ab- 
  

   sorption 
  ; 
  and 
  a 
  number 
  of 
  experiments 
  were 
  made 
  to 
  examine 
  

   this 
  peculiarity. 
  

  

  Lead, 
  aluminium, 
  and 
  carbon 
  plates 
  were 
  compared 
  with 
  

   screens 
  of 
  lead 
  and 
  iron. 
  

  

  Screens 
  :— 
  

  

  

  Lead. 
  

  

  

  Iron. 
  

  

  0-6 
  cm. 
  

  

  0-72. 
  

  

  1-5. 
  

  

  2-6. 
  

  

  2-1. 
  

  

  5-7, 
  

  

  Pb 
  

  

  100 
  

  

  98 
  

  

  104 
  

  

  100 
  

  

  99 
  

  

  100 
  

   102-5 
  

  

  100 
  

   106 
  

   117 
  

  

  100 
  

   91 
  

  

  98 
  

  

  100 
  

  

  90-5 
  

   99-0 
  

  

  Al 
  

  

  C 
  

  

  In 
  this 
  case 
  the 
  radium 
  was 
  in 
  vessel 
  H 
  (fig. 
  1), 
  and 
  screens 
  

   were 
  interposed 
  near 
  H. 
  Zinc, 
  copper, 
  carbon, 
  and 
  paraffin 
  

  

  