﻿886 
  Messrs. 
  T. 
  H. 
  Laby 
  and 
  G. 
  W. 
  C. 
  Kaye 
  on 
  

  

  probably 
  proportional 
  to 
  the 
  number 
  of 
  molecules 
  present, 
  

   that 
  is 
  to 
  the 
  pressure. 
  

  

  (2) 
  The 
  ionization 
  due 
  to 
  the 
  secondary 
  j3 
  radiation 
  from 
  

   the 
  gas 
  is 
  small 
  * 
  compared 
  to 
  the 
  ionization 
  arising 
  from 
  

   secondary 
  /3 
  radiation 
  from 
  the 
  walls, 
  the 
  latter 
  need 
  only 
  be 
  

   taken 
  into 
  consideration 
  when 
  the 
  gas 
  between 
  the 
  electrodes 
  

   is 
  dense. 
  

  

  The 
  ionization 
  excited 
  in 
  the 
  gas 
  by 
  (say) 
  unit 
  intensity 
  

   of 
  the 
  secondary 
  radiation 
  will 
  vary 
  as 
  the 
  gas 
  pressure, 
  

   but 
  the 
  intensity 
  of 
  this 
  radiation 
  itself 
  varies 
  as 
  the 
  pressure: 
  

   thus 
  this 
  ionization 
  is 
  proportional 
  to 
  the 
  product 
  of 
  the 
  

   intensity 
  of 
  the 
  secondary 
  ft 
  radiation 
  into 
  the 
  pressure, 
  that 
  

   is. 
  to 
  the 
  square 
  of 
  the 
  pressure. 
  

  

  The 
  intensity 
  of 
  the 
  secondary 
  /3 
  radial 
  ion 
  excited 
  in 
  the 
  

   walls 
  Is 
  constant 
  in 
  amount. 
  >o 
  thai 
  the 
  ionization 
  produced 
  

   by 
  it 
  varies 
  as 
  the 
  pressure. 
  

  

  \\ 
  e 
  have 
  seen 
  that 
  the 
  ionization 
  arising 
  from 
  the 
  gas 
  

  

  radiation 
  will 
  lie 
  less 
  than 
  that 
  arising 
  £rom 
  the 
  wall 
  radiation. 
  

  

  Consequently 
  the 
  ionization 
  which 
  varies 
  as 
  the 
  square 
  of 
  the 
  

  

  - 
  ire 
  i- 
  less 
  in 
  amount 
  than 
  that 
  varying 
  directly 
  as 
  the 
  

  

  pressure. 
  

  

  * 
  JJragg 
  and 
  Madsen 
  (Trans. 
  Roy. 
  Soc. 
  S. 
  Australia, 
  xxxii. 
  1908) 
  

   have 
  shown 
  that 
  the 
  secondary 
  /3 
  radiation 
  excited 
  by 
  hard 
  y 
  rajs 
  in 
  a 
  

   plate 
  of 
  aluminium 
  have 
  a 
  A, 
  when 
  absorhed 
  by 
  aluminium, 
  of 
  11 
  cm. 
  -1 
  ; 
  

   further, 
  that 
  the 
  secondary 
  /3 
  radiation 
  proceeds 
  originally 
  (before 
  any 
  

   scattering 
  occursj 
  in 
  the 
  same 
  direction 
  as 
  the 
  primary 
  ft, 
  and 
  that 
  the 
  

   intensity 
  of 
  the 
  emergence 
  radiation 
  considerably 
  exceeds 
  that 
  of 
  the 
  

   incidence. 
  These 
  results 
  enable 
  us 
  to 
  calculate 
  the 
  amount 
  of 
  secondary 
  

   ft 
  radiation 
  from 
  the 
  aluminium 
  walls 
  in 
  our 
  vessel, 
  and 
  show 
  that 
  

   the 
  emergence 
  radiation 
  from 
  the 
  two 
  surfaces 
  A 
  and 
  B 
  (fig. 
  7, 
  p. 
  885) 
  

   remote 
  from 
  the 
  radium 
  need 
  only 
  be 
  considered. 
  The 
  amount 
  of 
  radiation 
  

   in 
  question 
  from 
  either 
  A 
  or 
  B 
  which 
  escapes 
  from 
  the 
  aluminium 
  is 
  

  

  i 
  = 
  i 
  —L_ 
  

  

  where 
  I 
  is 
  the 
  total 
  emergence 
  radiation 
  from 
  an 
  elementary 
  layer 
  of 
  

   thickness 
  ex 
  : 
  putting 
  X 
  = 
  11 
  cm. 
  -1 
  , 
  then 
  I 
  = 
  '1 
  or 
  the 
  secondary 
  

   ft 
  radiation 
  from 
  the 
  two 
  surfaces 
  A 
  and 
  B 
  is 
  equivalent 
  to 
  the 
  total 
  

   emergence 
  radiation 
  from 
  a 
  layer 
  of 
  aluminium 
  2X'l 
  cm. 
  thick, 
  and 
  

   having 
  a 
  mass 
  of 
  '54 
  grams 
  per 
  sq. 
  cm. 
  The 
  radiation 
  from 
  such 
  a 
  layer 
  

   should 
  considerably 
  exceed 
  the 
  radiation 
  from 
  the 
  layer 
  of 
  gas 
  in 
  any 
  of 
  

   our 
  experiments. 
  Of 
  the 
  gases 
  used, 
  carbon 
  dioxide, 
  when 
  most 
  dense 
  (at 
  

   17 
  atmospheres) 
  would 
  give 
  the 
  largest 
  amount 
  of 
  secondary 
  radiation 
  ; 
  

   the 
  layer 
  of 
  it 
  1 
  cm. 
  thick, 
  between 
  A 
  and 
  C 
  (fig. 
  7), 
  has 
  a 
  mass 
  of 
  

   •03 
  gram 
  per 
  sq. 
  cm., 
  which 
  is 
  only 
  1/17 
  of 
  the 
  mass 
  of 
  the 
  aluminium 
  

   walls 
  which 
  emits 
  secondary 
  radiation. 
  In 
  the 
  case 
  of 
  air 
  this 
  radiation 
  

   from 
  the 
  walls 
  would 
  exceed 
  that 
  from 
  the 
  gas 
  to 
  an 
  even 
  greater 
  extent 
  : 
  

   for 
  hydrogen 
  the 
  secondary 
  ft 
  radiation 
  from 
  the 
  gas 
  may 
  certainly 
  be 
  

   neglected. 
  

  

  