﻿y-Rays 
  of 
  Uranium 
  and 
  Radium. 
  637 
  

  

  a 
  window 
  of 
  the 
  electroscope 
  and 
  the 
  air 
  within 
  thoroughly 
  

   ionized 
  for 
  about 
  40 
  seconds. 
  The 
  radium 
  is 
  then 
  removed 
  

   and 
  the 
  leaf 
  charged 
  up 
  but 
  slightly 
  more 
  than 
  is 
  necessary 
  ta 
  

   get 
  it 
  onto 
  the 
  scale. 
  

  

  The 
  leak 
  so 
  obtained 
  is 
  the 
  minimum 
  leak, 
  and 
  seems 
  ta 
  

   be, 
  if 
  not 
  the 
  true, 
  at 
  least 
  a 
  consistent 
  measure 
  of 
  the 
  ioni- 
  

   zation 
  of 
  the 
  7-rays 
  from 
  the 
  source 
  used. 
  The 
  leak 
  with 
  

   the 
  new" 
  electroscope 
  of 
  lead 
  described 
  later 
  is 
  quite 
  constant 
  

   for 
  a 
  particular 
  disposition 
  over 
  any 
  one 
  day, 
  leaks 
  of 
  

   12 
  minutes 
  duration 
  rarely 
  varying 
  by 
  more 
  than 
  three 
  or 
  

   four 
  seconds. 
  Also 
  the 
  values 
  of 
  X 
  obtained 
  by 
  this 
  method 
  

   for 
  any 
  one 
  substance 
  on 
  different 
  days 
  do 
  not 
  usually 
  differ 
  

   by 
  more 
  than 
  one 
  or 
  two 
  per 
  cent. 
  After 
  this 
  method 
  of 
  

   working 
  was 
  found, 
  the 
  measurement 
  of 
  ionization 
  leaks 
  due 
  

   to 
  7-rays 
  became 
  almost 
  as 
  simple 
  as 
  in 
  the 
  case 
  of 
  the 
  other 
  

   radiations, 
  whereas 
  before 
  they 
  were 
  very 
  uncertain 
  and 
  

   variable. 
  Without 
  some 
  such 
  precautions 
  being 
  employed 
  

   7-ray 
  measurements 
  can 
  hardly 
  be 
  depended 
  upon 
  to 
  10 
  

   per 
  cent. 
  

  

  8. 
  Absorption 
  ofy-Rays 
  of 
  Radium 
  under 
  Various 
  Conditions. 
  

  

  The 
  published 
  values 
  of 
  A, 
  for 
  the 
  7-rays 
  of 
  radium 
  show 
  

   considerable 
  differences 
  among 
  themselves, 
  and 
  we 
  have 
  

   carried 
  out 
  a 
  large 
  number 
  of 
  experiments 
  to 
  determine 
  

   whether 
  this 
  value 
  depends 
  on 
  the 
  conditions. 
  ]\[cClelland 
  

   (Phil. 
  Mag. 
  1904, 
  viii. 
  p. 
  70) 
  and 
  Eve 
  {Pliys. 
  Zeii, 
  1907, 
  viii, 
  

   p. 
  183) 
  agree 
  fairly 
  closely. 
  The 
  latter 
  gives 
  the 
  value 
  of 
  \ 
  as 
  

   varying 
  from 
  0'56 
  to 
  0'46 
  over 
  a 
  range 
  of 
  from 
  "64 
  to 
  3 
  cm. 
  

   of 
  lead. 
  Wigger, 
  who 
  worked 
  over 
  the 
  range 
  from 
  2'% 
  to 
  

   5 
  cm. 
  of 
  lead 
  with 
  a 
  peculiar 
  apparatus, 
  found 
  the 
  value 
  0'241. 
  

   Kleeman 
  (Phil. 
  Mag. 
  1907, 
  xiv, 
  p. 
  643) 
  came 
  to 
  the 
  con- 
  

   clusion, 
  from 
  experiments 
  on 
  the 
  secondary 
  cathode 
  rays 
  

   generated 
  by 
  7-rays, 
  that 
  the 
  latter 
  must 
  consist 
  of 
  three 
  

   groups 
  of 
  rays. 
  Madsen 
  recently 
  (Phil. 
  Mag. 
  1909, 
  xvii. 
  

   p. 
  447) 
  concluded 
  that 
  the 
  7-rays 
  consisted 
  of 
  two 
  homo- 
  

   geneous 
  radiations 
  possessing 
  values 
  for 
  \\d^ 
  independent 
  

   of 
  the 
  nature 
  of 
  the 
  absorbing 
  substance, 
  0*028 
  and 
  0*12 
  

   respectively. 
  Thus, 
  for 
  lead 
  \ 
  has 
  the 
  values 
  0*32 
  and 
  

   1'36. 
  Since 
  the 
  present 
  work 
  was 
  done 
  Y. 
  Tuomikoski 
  has 
  

   published 
  {Pliys. 
  Zeit. 
  1909, 
  x. 
  p. 
  372) 
  a 
  determination 
  of 
  X 
  

   for 
  7-rays 
  of 
  radium 
  over 
  a 
  range 
  up 
  to 
  18 
  cm. 
  of 
  lead. 
  He 
  

  

  gives 
  the 
  rollowmg 
  val 
  

  

  ues 
  : 
  — 
  

  

  

  

  

  

  Thickness 
  / 
  from 
  

   (cm.) 
  y 
  to 
  

  

  •4 
  

   1-0 
  

  

  10 
  

   2"2 
  

  

  2-2 
  

   5-4 
  

  

  5^4 
  

   120 
  

  

  120 
  

   15-8 
  

  

  15-8 
  ! 
  

   18^0 
  

  

  X(cm.)-i 
  ' 
  '70 
  

  

  •58 
  : 
  

  

  •52 
  

  

  •50 
  

  

  •39 
  

  

  •25 
  

  

  