﻿614 
  Mr. 
  T. 
  S. 
  Taylor 
  on 
  the 
  Retardation 
  of 
  

  

  Hence 
  the 
  rates, 
  at 
  which 
  the 
  hydrogen- 
  equivalents 
  o£ 
  the 
  

   Gold 
  and 
  Celloidin 
  sheets 
  decrease 
  with 
  the 
  speed 
  o£ 
  the 
  

   alpha-particle 
  entering 
  the 
  sheets^ 
  are 
  proportional 
  to 
  the 
  

   square 
  roots 
  of 
  their 
  respective 
  atomic 
  weights. 
  Moreover 
  

   the 
  slope 
  of 
  the 
  curve 
  " 
  Celloidin 
  in 
  Hydrogen 
  '''' 
  is 
  numeri- 
  

   cally 
  equal 
  (but 
  of 
  opposite 
  sign) 
  to 
  the 
  slope 
  of 
  the 
  curve 
  

   " 
  B 
  Hydrogen 
  '^ 
  in 
  air. 
  The 
  hydrogen-equivalent 
  of 
  the 
  

   celloidin 
  sheet 
  was 
  somewhat 
  larger 
  than 
  the 
  thickness 
  of 
  the 
  

   " 
  B 
  Hydrogen 
  ^^ 
  cell, 
  but 
  it 
  seems 
  entirely 
  proper 
  to 
  conclude 
  

   that 
  the 
  rate, 
  at 
  which 
  the 
  hydrogen-equivalent 
  of 
  the 
  

   celloidin 
  sheet 
  decreases 
  with 
  the 
  speed 
  of 
  the 
  alpha-particle, 
  

   is 
  the 
  same 
  as 
  the 
  rate 
  at 
  which 
  the 
  air-equivalent 
  of 
  the 
  

   " 
  B 
  Hydrogen 
  '^ 
  increases 
  as 
  the 
  speed 
  of 
  the 
  entering 
  alpha- 
  

   particle 
  decreases. 
  

  

  The 
  possibility 
  that 
  the 
  observed 
  variations 
  in 
  the 
  ioniza- 
  

   tion, 
  which 
  have 
  been 
  taken 
  to 
  be 
  the 
  measures 
  of 
  the 
  changes 
  

   in 
  the 
  air-equivalents, 
  may 
  be 
  due 
  to 
  secondary 
  rays 
  is 
  

   precluded 
  by 
  the 
  fact 
  that 
  numerous 
  direct 
  determinations 
  of 
  

   the 
  Bragg 
  ionization-curves, 
  with 
  and 
  without 
  the 
  metal 
  sheets 
  

   near 
  the 
  polonium, 
  and 
  again 
  near 
  the 
  ionization-chamber, 
  

   showed 
  no 
  irregularities 
  in 
  the 
  curves 
  as 
  would 
  be 
  expected 
  

   were 
  secondary 
  rays 
  present 
  in 
  any 
  appreciable 
  amount. 
  The 
  

   behaviour 
  of 
  the 
  air-equivalents 
  of 
  the 
  hydrogen 
  sheets 
  in 
  no 
  

   way 
  conforms 
  to 
  what 
  might 
  be 
  expected 
  to 
  be 
  produced 
  by 
  

   secondary 
  rays. 
  

  

  The 
  increasing 
  of 
  the 
  air-equivalents 
  of 
  the 
  hydrogen 
  

   sheets 
  and 
  the 
  decreasing 
  of 
  the 
  hydrogen-equivalents 
  of 
  

   the 
  celloidin 
  sheets 
  when 
  moved 
  away 
  from 
  the 
  source 
  

   of 
  rays 
  gave 
  occasion 
  for 
  suspecting 
  that 
  some 
  differences 
  

   might 
  be 
  found 
  to 
  exist 
  between 
  the 
  Bragg 
  ionization-curves 
  

   obtained 
  in 
  atmospheres 
  of 
  air 
  and 
  hydrogen 
  respectively. 
  

   To 
  determine 
  these 
  curves, 
  use 
  was 
  made 
  of 
  an 
  apparatus 
  

   constructed 
  for 
  Mr. 
  F. 
  E. 
  Wheelock, 
  of 
  this 
  Laboratory, 
  

   which 
  was 
  similar 
  to 
  the 
  one 
  used 
  thus 
  far 
  in 
  the 
  work, 
  

   except 
  that 
  the 
  vessel 
  enclosing 
  the 
  main 
  part 
  of 
  the 
  

   apparatus 
  could 
  be 
  completely 
  exhausted. 
  To 
  make 
  any 
  

   comparison 
  of 
  the 
  two 
  ionization 
  curves, 
  it 
  was 
  necessary 
  

   to 
  determine 
  them 
  under 
  similar 
  conditions 
  — 
  i. 
  e., 
  the 
  same 
  

   source 
  of 
  rays 
  was 
  used 
  in 
  the 
  two 
  cases, 
  and 
  the 
  pressure 
  

   of 
  the 
  air 
  was 
  so 
  reduced 
  as 
  to 
  make 
  the 
  range 
  of 
  the 
  

   alpha-particles 
  in 
  air 
  equal 
  to 
  their 
  range 
  in 
  hydrogen 
  at 
  

   normal 
  pressure. 
  Polonium 
  was 
  used 
  as 
  the 
  source 
  of 
  

   rays, 
  and 
  several 
  Bragg 
  curves 
  were 
  obtained 
  in 
  hydrogen 
  

   at 
  normal 
  pressure 
  and 
  in 
  air 
  at 
  a 
  reduced 
  pressure 
  of 
  about 
  

   17 
  cm. 
  of 
  mercury. 
  Two 
  of 
  the 
  curves 
  are 
  shown 
  in 
  

   fig. 
  2. 
  The 
  dotted 
  portion 
  of 
  each 
  curve 
  is 
  assumed 
  to 
  be 
  of 
  

  

  