﻿642 
  Messrs. 
  F. 
  Soddy 
  and 
  A. 
  S. 
  Paissell 
  on 
  the 
  

  

  We 
  may 
  merely 
  state 
  here 
  that 
  we 
  have 
  not 
  yet 
  been 
  able 
  

   to 
  obtain 
  any 
  verification 
  apart 
  from 
  the 
  effects 
  recorded 
  in 
  

   Section 
  8 
  A 
  of 
  the 
  existence 
  of 
  secondary 
  or 
  reflected 
  pene- 
  

   trating 
  rays 
  generated 
  by 
  the 
  passage 
  of 
  7-rays 
  through 
  

   matter 
  in 
  any 
  of 
  the 
  dispositions 
  employed. 
  The 
  above 
  

   equations 
  apply, 
  of 
  course, 
  equally 
  to 
  the 
  secondary 
  y8-radia- 
  

   tion 
  which 
  is 
  known 
  to 
  be 
  generated 
  ; 
  but 
  the 
  effect 
  of 
  this 
  

   on 
  the 
  absorption-curve 
  is 
  only 
  apparent 
  over 
  the 
  same 
  range 
  

   as 
  that 
  of 
  the 
  primary 
  /5-rays, 
  which 
  is 
  usually 
  not 
  directly 
  

   investigable. 
  Beyond 
  this 
  range 
  the 
  second 
  term 
  of 
  the 
  

   equation 
  disappears, 
  the 
  absorption 
  is 
  exponential, 
  and 
  merely 
  

   the 
  absolute 
  value 
  of 
  the 
  ionization 
  leak 
  is 
  multiplied 
  by 
  a 
  

   constant 
  factor. 
  This 
  state, 
  by 
  analogy 
  to 
  radioactive 
  equi- 
  

   librium, 
  might 
  be 
  called 
  " 
  radiation 
  equilibrium." 
  

  

  10. 
  1 
  he 
  Form 
  of 
  Aioparatus 
  finally 
  adopted. 
  

   As 
  the 
  7-radiation 
  from 
  uranium 
  X 
  is 
  comparatively 
  feeble, 
  

   it 
  was 
  important 
  to 
  construct 
  the 
  electroscope 
  of 
  metal 
  which 
  

   for 
  a 
  given 
  volume 
  would 
  give 
  the 
  largest 
  effect. 
  Lead 
  was 
  

   found 
  to 
  be 
  easily 
  the 
  best 
  for 
  this 
  purpose. 
  This 
  is 
  curious 
  as 
  

   precisely 
  the 
  opposite 
  effect 
  was 
  expected. 
  For 
  when 
  the 
  

   absorbing 
  plates 
  form 
  the 
  base 
  of 
  the 
  electroscope 
  the 
  leak 
  in 
  

   the 
  instrument 
  for 
  equivalent 
  thicknesses 
  (i. 
  e. 
  thickness 
  X 
  

   density) 
  is 
  least 
  for 
  lead. 
  We 
  have 
  probably 
  here 
  an 
  example 
  

   of 
  the 
  difference 
  discovered 
  by 
  Bragg 
  and 
  Madsen 
  (Phil. 
  Mag. 
  

   1908, 
  XV. 
  p. 
  663) 
  between 
  the 
  incidence 
  and 
  emergence 
  

   secondary 
  radiation. 
  The 
  emergence 
  radiation 
  of 
  lead 
  is 
  not 
  

   greatly 
  different 
  from 
  that 
  of 
  light 
  metals 
  like 
  aluminium, 
  

   whereas 
  the 
  incidence 
  radiation 
  is 
  much 
  greater 
  than 
  that 
  

   from 
  aluminium. 
  Electroscopes 
  of 
  lead 
  and 
  brass 
  with 
  

   circular 
  cross-section 
  and 
  electroscopes 
  of 
  lead, 
  aluminium, 
  

   zinc, 
  and 
  cardboard 
  with 
  square 
  cross-section 
  all 
  of 
  the 
  same 
  

   height 
  were 
  compared 
  under 
  identical 
  conditions 
  of 
  experiment. 
  

   It 
  was 
  found 
  that 
  in 
  the 
  two 
  lead 
  electroscopes 
  the 
  ionization 
  

   produced 
  by 
  the 
  7-rays 
  was 
  proportional 
  to 
  the 
  volume 
  of 
  the 
  

   electroscope. 
  Lead 
  was 
  easily 
  the 
  best, 
  then 
  came 
  zinc 
  and 
  

   brass 
  which 
  gave 
  about 
  equal 
  ionization. 
  Cardboard 
  lined 
  

   with 
  tinfoil 
  came 
  next, 
  and 
  lastly 
  aluminium. 
  The 
  electro- 
  

   scopes 
  were 
  of 
  varying 
  thicknesses, 
  but 
  in 
  no 
  case 
  was 
  the 
  

   ionization 
  increased 
  by 
  increasing 
  the 
  thickness 
  of 
  the 
  walls. 
  

   The 
  relative 
  values 
  of 
  the 
  ionization 
  in 
  the 
  electroscopes 
  were 
  

   approximately: 
  — 
  Lead 
  100, 
  zinc 
  75, 
  brass 
  75, 
  cardboard 
  ^^^ 
  

   aluminium, 
  57. 
  To 
  eliminate 
  the 
  possible 
  effect 
  of 
  secondary 
  

   rays 
  and 
  to 
  secure 
  the 
  maximum 
  ionization 
  by 
  7-rays 
  a 
  cylin- 
  

   drical 
  lead 
  electroscope 
  with 
  small 
  glass 
  windows 
  was 
  con- 
  

   structed 
  with 
  the 
  followino; 
  dimensions 
  : 
  — 
  Inside 
  heio-ht 
  

  

  