﻿j-Bai/s 
  of 
  Uranium 
  and 
  liadium. 
  643 
  

  

  12"8 
  cm., 
  inside 
  diameter 
  9*0 
  cm., 
  wall 
  thickness 
  0*65 
  cm., 
  

   thickness 
  o£ 
  top 
  1*18 
  cm., 
  thickness 
  o£ 
  base 
  2'^^ 
  cm. 
  

  

  The 
  electroscope 
  was 
  completely 
  surrounded 
  by 
  two 
  circular 
  

   screens 
  of 
  lead 
  o£ 
  height 
  12'6 
  cm. 
  and 
  of 
  thickness 
  0*244 
  cm., 
  

   provided 
  with 
  small 
  windows 
  cut 
  in 
  them. 
  The 
  whole 
  was 
  

   fitted 
  with 
  a 
  wooden 
  stand 
  and 
  mounted 
  on 
  four 
  brass 
  pillars 
  

   above 
  the 
  wooden 
  table. 
  With 
  this 
  new 
  instrument 
  the 
  effect 
  

   of 
  varying 
  the 
  distance 
  of 
  the 
  radium 
  was 
  again 
  investigated 
  

   in 
  a 
  manner 
  precisely 
  similar 
  to 
  that 
  already 
  described 
  for 
  the 
  

   brass 
  electroscope. 
  In 
  every 
  case 
  the 
  exponential 
  law 
  of 
  

   absorption 
  now 
  held 
  even 
  up 
  to 
  distances 
  of 
  over 
  a 
  metre. 
  In 
  

   contrast 
  to 
  the 
  similar 
  experiments 
  with 
  Wigger^s 
  apparatus, 
  

   the 
  value 
  of 
  X 
  also 
  only 
  varied 
  slightly, 
  if 
  at 
  all, 
  wath 
  the 
  

   distance. 
  The 
  value 
  of 
  A-(cm.)~^ 
  from 
  2*8 
  to 
  8'9 
  cm. 
  of 
  lead 
  

   in 
  this 
  series 
  was 
  0*483 
  with 
  the 
  radium 
  13*1 
  cm. 
  and 
  0*480 
  

   with 
  the 
  radium 
  113 
  cm. 
  below 
  the 
  base 
  of 
  the 
  electroscope. 
  

   At 
  intermediate 
  distances 
  \ 
  was 
  intermediate. 
  

  

  The 
  base 
  was 
  too 
  thick 
  for 
  work 
  with 
  the 
  feeble 
  y-rays 
  of 
  

   uranium 
  X, 
  and 
  it 
  was 
  replaced 
  by 
  a 
  permanent 
  base 
  consisting 
  

   of 
  a 
  plate 
  of 
  lead 
  0*9 
  75 
  cm. 
  thick. 
  With 
  this 
  alteration 
  the 
  

   instrument 
  formed 
  the 
  standard 
  one 
  in 
  all 
  subsequent 
  mea- 
  

   surements. 
  All 
  the 
  active 
  preparations 
  w^ere 
  placed 
  at 
  the 
  

   uniform 
  distance 
  of 
  13 
  cm. 
  from 
  the 
  under 
  surface 
  of 
  the 
  base, 
  

   the 
  absorbing 
  plates 
  were 
  placed 
  directly 
  over 
  the 
  active 
  

   material, 
  and 
  the 
  air 
  in 
  the 
  electroscope 
  was 
  thoroughly 
  

   ionized 
  between 
  each 
  observation. 
  

  

  Before 
  gi^'ing 
  the 
  final 
  results, 
  it 
  is 
  of 
  interest 
  to 
  refer 
  to 
  

   some 
  observations 
  which 
  show 
  that 
  even 
  in 
  this 
  method 
  of 
  

   work 
  a 
  certain 
  amount 
  of 
  arbitrariness 
  is 
  introduced 
  into 
  the 
  

   results 
  by 
  the 
  particular 
  disposition 
  adopted. 
  The 
  eff'ect 
  on 
  

   the 
  absolute 
  value 
  of 
  the 
  leak 
  in 
  the 
  electroscope 
  of 
  alternately 
  

   placing 
  the 
  same 
  plates 
  over 
  the 
  active 
  material 
  and 
  against 
  

   the 
  under 
  side 
  of 
  the 
  base 
  of 
  the 
  instrument 
  was 
  examined. 
  

   For 
  the 
  radium 
  7-rays 
  the 
  leak 
  was 
  slightly 
  less 
  both 
  with 
  

   copper 
  and 
  lead 
  when 
  the 
  plates 
  were 
  clamped 
  up 
  to 
  the 
  base. 
  

   For 
  uranium 
  X 
  the 
  same 
  held 
  true 
  except 
  for 
  great 
  thicknesses 
  

   of 
  lead 
  when 
  the 
  converse 
  held. 
  Thus 
  for 
  uranium 
  X 
  the 
  

   absorption 
  curves 
  for. 
  lead 
  with 
  the 
  two 
  dispositions 
  actually 
  

   cut 
  one 
  another. 
  The 
  efPects 
  so 
  introduced, 
  however, 
  are 
  not 
  

   large, 
  and 
  the 
  advantages 
  in 
  ease 
  and 
  quickness 
  of 
  working 
  

   in 
  placing 
  the 
  plates 
  directly 
  over 
  the 
  active 
  material 
  are 
  

   very 
  great 
  as 
  compared 
  with 
  the 
  first 
  method 
  employed. 
  At 
  

   the 
  same 
  time 
  we 
  do 
  not 
  think 
  that 
  there 
  is 
  much 
  to 
  choose 
  

   between 
  the 
  two 
  methods 
  from 
  a 
  theoretical 
  point 
  of 
  view. 
  

   Each 
  is 
  open 
  to 
  some 
  objection. 
  

  

  