﻿640 
  Messrs. 
  F. 
  Soddy 
  and 
  A. 
  S. 
  Russell 
  on 
  the 
  

  

  smallest 
  distance 
  the 
  curve 
  is 
  exponential 
  (X 
  = 
  *4:65) 
  ; 
  but 
  

   as 
  the 
  radium 
  is 
  removed 
  further 
  and 
  further 
  away 
  the 
  

   curves 
  become 
  more 
  and 
  more 
  convex 
  to 
  the 
  origin. 
  From 
  

   16"4 
  cm. 
  to 
  114*6 
  cm. 
  the 
  preparation 
  is 
  below 
  the 
  level 
  of 
  

   the 
  slate 
  bench. 
  Although 
  at 
  the 
  time 
  it 
  was 
  deemed 
  impos- 
  

   sible 
  for 
  any 
  )8-radiation, 
  secondary 
  or 
  primary, 
  to 
  penetrate 
  

   the 
  walls 
  or 
  windows 
  of 
  the 
  brass 
  electroscope, 
  we 
  very 
  pro- 
  

   bably, 
  owing 
  to 
  our 
  experience 
  having 
  been 
  mainly 
  with 
  the 
  

   /5-rays 
  of 
  uranium, 
  underestimated 
  the 
  penetrating 
  power 
  of 
  

   the 
  secondary 
  y8-rays 
  of 
  radium. 
  From 
  a 
  mature 
  consideration 
  

   of 
  the 
  whole 
  of 
  the 
  effects 
  obtained 
  during 
  the 
  work 
  we 
  thought 
  

   the 
  effect 
  may 
  have 
  been 
  due 
  to 
  secondary 
  /3-radiation, 
  as 
  in 
  

   no 
  other 
  experiments 
  had 
  we 
  been 
  able 
  to 
  obtain 
  any 
  definite 
  

   evidence 
  of 
  secondary 
  7-radiation, 
  generated 
  by 
  fy-rays, 
  more 
  

   penetrating 
  than 
  the 
  /8-rays 
  (compare 
  Sections 
  8 
  A 
  and 
  9). 
  

   When 
  the 
  electroscope, 
  on 
  the 
  other 
  hand, 
  has 
  a 
  base 
  thin 
  

   enough 
  to 
  allow 
  secondary 
  /3-radiations 
  to 
  penetrate 
  it, 
  and 
  the 
  

   radium 
  is 
  placed 
  a 
  few 
  centimetres 
  beneath, 
  it 
  was 
  noticed 
  

   that 
  the 
  leak 
  was 
  always 
  greater 
  with 
  the 
  absorbing 
  screens 
  

   directly 
  over 
  the 
  radium 
  than 
  when 
  they 
  were 
  clamped 
  up 
  to 
  

   the 
  electroscope. 
  This 
  is 
  to 
  be 
  ascribed 
  to 
  lateral 
  7-radiation 
  

   generating 
  secondary 
  yS-radiation 
  at 
  the 
  point 
  when 
  it 
  can 
  

   enter 
  through 
  the 
  electroscope 
  base. 
  

  

  8 
  a. 
  

   [Since 
  the 
  paper 
  was 
  printed 
  we 
  have 
  examined 
  more 
  

   closely 
  the 
  question 
  as 
  to 
  whether 
  secondary 
  /3-radiation 
  

   caused 
  the 
  effect 
  discussed 
  in 
  Section 
  8. 
  The 
  wall 
  of 
  the 
  

   brass 
  electroscope 
  was 
  thickened 
  in 
  places 
  by 
  external 
  sheets 
  

   of 
  various 
  metals. 
  It 
  was 
  found 
  that 
  whereas 
  the 
  addition 
  

   of 
  brass 
  up 
  to 
  a 
  thickness 
  of 
  3 
  mm. 
  did 
  not 
  affect 
  the 
  actual 
  

   value 
  of 
  the 
  leak 
  for 
  a 
  fixed 
  disposition, 
  even 
  thin 
  lead-foil 
  

   cut 
  it 
  down 
  appreciably. 
  Beyond 
  2 
  '2 
  mm. 
  of 
  lead 
  no 
  further 
  

   reduction 
  was 
  noticed. 
  At 
  this 
  stage 
  for 
  the 
  particular 
  dis- 
  

   position 
  used 
  the 
  difference 
  amounted 
  to 
  a 
  third 
  of 
  the 
  total 
  

   effect. 
  The 
  leaks 
  at 
  the 
  end 
  of 
  the 
  lowest 
  curve 
  in 
  fig. 
  2 
  

   were 
  only 
  one-half 
  as 
  great 
  as 
  in 
  this 
  case. 
  These 
  results 
  

   taken 
  in 
  conjunction 
  with 
  those 
  recorded 
  in 
  Section 
  10 
  show 
  

   that 
  the 
  departure 
  of 
  the 
  absorption-curves 
  (tig. 
  2) 
  from 
  the 
  

   straight 
  line 
  is 
  to 
  be 
  ascribed 
  to 
  external 
  secondary 
  radiation 
  

   more 
  penetrating 
  than 
  the 
  /8-rays, 
  so 
  far 
  as 
  brass 
  is 
  con- 
  

   cerned, 
  but 
  readily 
  absorbed 
  by 
  lead. 
  The 
  choice 
  of 
  a 
  thick 
  

   lead 
  electroscope 
  in 
  the 
  final 
  series 
  of 
  measurements 
  described 
  

   in 
  section 
  10 
  was 
  made 
  from 
  quite 
  other 
  considerations, 
  but 
  

   it 
  now 
  appears 
  that 
  secondary 
  radiation 
  effects 
  were 
  thus 
  

   probably 
  avoided, 
  which 
  might 
  not 
  have 
  been 
  the 
  case 
  if 
  an 
  

  

  