﻿on 
  ft 
  and 
  7 
  Rays. 
  543 
  

  

  the 
  electroscope. 
  The 
  readings 
  are 
  uniformly 
  higher, 
  and 
  

   consequently 
  when 
  calculating 
  the 
  absorption 
  coefficient 
  in 
  

   the 
  usual 
  way 
  we 
  get 
  a 
  lower 
  value. 
  This 
  effect 
  may 
  be 
  

   quite 
  large, 
  as 
  very 
  simple 
  experiments 
  show. 
  It 
  is 
  more 
  

   noticeable 
  in 
  the 
  case 
  of 
  ft 
  rays. 
  In 
  a 
  similar 
  experiment 
  

   Kovarik 
  has 
  shown 
  that 
  by 
  placing 
  aluminium 
  above 
  the 
  

   active 
  material 
  (radium 
  (B-f-E 
  + 
  F) 
  in 
  a 
  position 
  corre- 
  

   sponding 
  to 
  A 
  it 
  was 
  actually 
  possible 
  to 
  increase 
  the 
  ioniza- 
  

   tion 
  as 
  much 
  as 
  20 
  per 
  cent. 
  After 
  a 
  certain 
  thickness, 
  

   however, 
  the 
  ionization 
  decreased 
  in 
  the 
  usual 
  way. 
  It 
  is 
  

   only 
  when 
  the 
  rays 
  have 
  passed 
  through 
  sufficient 
  material 
  

   to 
  nullify 
  the 
  two 
  effects 
  of 
  scattering 
  noted 
  above, 
  that 
  we 
  

   can 
  speak 
  of 
  absorption 
  according 
  to 
  an 
  exponential 
  law. 
  In 
  

   considering 
  the 
  first 
  effect, 
  it 
  has 
  been 
  assumed 
  that 
  the 
  

   absorbing 
  plates 
  were 
  placed 
  directly 
  under 
  the 
  ionization 
  

   chamber 
  or 
  electroscope. 
  

  

  In 
  some 
  experiments 
  to 
  be 
  described 
  in 
  the 
  following 
  

   paper, 
  the 
  writer 
  has 
  occasion 
  to 
  compare 
  the 
  intensities 
  of 
  

   7 
  rays 
  of 
  different 
  penetrating 
  powers. 
  It 
  is 
  first 
  necessary 
  

   to 
  define 
  intensity. 
  A 
  tentative 
  definition 
  of 
  the 
  intensity 
  

   of 
  a 
  beam 
  of 
  7 
  rays 
  is 
  made 
  by 
  assuming 
  that 
  the 
  

   number 
  of 
  rays 
  excited 
  by 
  a 
  beam 
  of 
  intensity 
  I 
  in 
  passing 
  

   through 
  a 
  layer 
  of 
  thickness 
  dx 
  is 
  simply 
  proportional 
  

   to 
  jxldx, 
  where 
  fidx 
  is 
  the 
  fraction 
  of 
  the 
  rays 
  absorbed. 
  We 
  

   measure 
  7 
  rays 
  by 
  ionization, 
  and 
  the 
  ionization 
  caused 
  by 
  

   7 
  rays 
  in 
  an 
  ordinary 
  electroscope 
  is 
  due 
  to 
  two 
  causes 
  : 
  — 
  

  

  (1) 
  Ionization 
  due 
  to 
  the 
  absorption 
  of 
  7 
  rays 
  in 
  the 
  air 
  of 
  

   the 
  electroscope 
  ; 
  

  

  (2) 
  Ionization 
  caused 
  by 
  ft 
  rays 
  coming 
  from 
  the 
  walls 
  of 
  

   the 
  electroscope. 
  

  

  From 
  the 
  first 
  cause 
  the 
  ionization 
  will 
  be 
  greater, 
  the 
  

   softer 
  the 
  rays, 
  as 
  a 
  greater 
  percentage 
  of 
  them 
  will 
  be 
  

   absorbed 
  in 
  the 
  gas. 
  The 
  number 
  of 
  ft 
  rays 
  escaping 
  from 
  

   the 
  walls 
  and 
  the 
  ionization 
  from 
  them 
  depend 
  very 
  largely 
  

   on 
  the 
  nature 
  of 
  the 
  walls. 
  The 
  number 
  will 
  depend 
  on 
  (a) 
  

   the 
  absorption 
  coefficient 
  of 
  the 
  7 
  rays, 
  (b) 
  the 
  penetrating 
  

   power 
  of 
  the 
  ft 
  rays 
  excited 
  by 
  the 
  7 
  rays. 
  

  

  In 
  the 
  experiments 
  described 
  later, 
  7 
  rays 
  for 
  the 
  most 
  part 
  

   enter 
  and 
  leave 
  the 
  electroscope 
  by 
  carbon. 
  Now 
  carbon 
  

   absorbs 
  the 
  y 
  rays 
  of 
  radium 
  E 
  about 
  three 
  times 
  as 
  much 
  as 
  

   the 
  7 
  rays 
  of 
  radium, 
  and 
  the 
  X 
  rays 
  characteristic 
  of 
  silver 
  

   about 
  10 
  times 
  as 
  much. 
  The 
  ft 
  rays 
  excited 
  by 
  the 
  7 
  rays 
  

   of 
  radium 
  can 
  penetrate 
  more 
  than 
  a 
  centimetre 
  of 
  carbon, 
  

   those 
  excited 
  by 
  the 
  7 
  rays 
  of 
  radium 
  E 
  0*015 
  cm.*, 
  and 
  those 
  

   excited 
  by 
  X 
  rays 
  characteristic 
  of 
  silver 
  about 
  0*00003 
  cm. 
  

   * 
  A 
  result 
  obtained 
  last 
  year 
  but 
  not 
  yet 
  published. 
  

  

  