﻿406 
  Dr. 
  T. 
  S. 
  Taylor 
  on 
  the 
  Range 
  and 
  

  

  that 
  does 
  exist 
  indicates 
  that 
  the 
  average 
  range 
  of 
  the 
  alpha 
  

   particle 
  is 
  about 
  one 
  millimetre 
  nearer 
  the 
  end 
  of 
  the 
  range 
  

   in 
  air 
  than 
  it 
  is 
  in 
  oxygen. 
  A 
  similar 
  difference 
  is 
  shown 
  

   between 
  hydrogen 
  and 
  helium. 
  If 
  we 
  compare 
  the 
  decrease 
  

   in 
  the 
  number 
  of 
  particles 
  toward 
  the 
  end 
  of 
  the 
  range 
  in 
  

   oxygen 
  or 
  air 
  with 
  that 
  in 
  hydrogen 
  or 
  helium, 
  a 
  more 
  marked 
  

   difference 
  is 
  observed. 
  For 
  example, 
  if 
  the 
  air 
  and 
  hydrogen 
  

   curves 
  in 
  fig. 
  2 
  were 
  plotted 
  on 
  the 
  same 
  scale, 
  the 
  decrease 
  

   in 
  the 
  number 
  of 
  particles 
  would 
  commence 
  about 
  2'2 
  cm. 
  

   before 
  the 
  end 
  of 
  the 
  range 
  in 
  hydrogen, 
  while 
  that 
  for 
  air 
  

   (on 
  hydrogen 
  scale) 
  would 
  begin 
  about 
  4*5 
  cm. 
  before 
  the 
  

   end 
  of 
  the 
  range. 
  From 
  this 
  it 
  is 
  seen 
  that 
  — 
  for 
  the 
  gases 
  of 
  

   small 
  atomic 
  weight 
  — 
  hydrogen 
  and 
  helium 
  — 
  the 
  beam 
  of 
  

   particles 
  remains 
  more 
  nearly 
  uniform 
  towards 
  the 
  end 
  of 
  the 
  

   range 
  than 
  it 
  does 
  for 
  the 
  gases 
  — 
  oxygen 
  and 
  air 
  — 
  of 
  higher 
  

   atomic 
  weight. 
  This 
  is 
  what 
  should 
  be 
  expected 
  from 
  the 
  

   theory 
  of 
  the 
  scattering 
  of 
  the 
  alpha 
  particles 
  by 
  matter 
  as 
  

   developed 
  by 
  Rutherford 
  *. 
  The 
  heavier 
  the 
  atom 
  through 
  

   which 
  the 
  particle 
  passes, 
  the 
  greater 
  the 
  angle 
  through 
  

   which 
  the 
  particle 
  is 
  turned 
  from 
  its 
  course. 
  The 
  decrease 
  

   in 
  the 
  number 
  of 
  particles 
  near 
  the 
  end 
  of 
  the 
  range 
  has 
  been 
  

   investigated 
  carefully 
  by 
  Geiger 
  f 
  . 
  The 
  results 
  given 
  herein 
  

   for 
  air 
  agree 
  with 
  his 
  investigations. 
  

  

  The 
  Ionization 
  Curves 
  for 
  the 
  Alpha 
  Particles 
  from 
  

   Polonium 
  in 
  Helium, 
  Hydrogen, 
  and 
  Air. 
  

  

  The 
  apparatus 
  used 
  in 
  the 
  determination 
  of 
  the 
  ionization 
  

   curves 
  was 
  a 
  modified 
  form 
  of 
  a 
  Bragg 
  ionization 
  vessel. 
  

   The 
  essential 
  parts 
  are 
  shown 
  in 
  fig. 
  3. 
  An 
  electroscope, 
  

   however, 
  was 
  used 
  to 
  measure 
  the 
  ionization 
  current 
  on 
  

   account 
  of 
  its 
  extreme 
  steadiness 
  and 
  the 
  ease 
  with 
  which 
  

   observations 
  could 
  be 
  repeated. 
  By 
  using 
  an 
  electroscope 
  it 
  

   was 
  not 
  convenient 
  to 
  use 
  a 
  shallow 
  ionization 
  chamber 
  on 
  

   account 
  of 
  the 
  length 
  of 
  time 
  required 
  for 
  a 
  single 
  reading. 
  

   Consequently, 
  the 
  ionization 
  chamber 
  I, 
  fig. 
  3, 
  was 
  a 
  brass 
  

   cylinder 
  about 
  2'5 
  centimetres 
  in 
  diameter 
  and 
  2'4 
  centi- 
  

   metres 
  high, 
  and 
  was 
  fastened 
  to 
  the 
  end 
  of 
  a 
  glass 
  tube 
  

   about 
  2*4 
  centimetres 
  in 
  diameter. 
  This 
  cylinder 
  was 
  

   charged 
  to 
  a 
  negative 
  potential 
  of 
  160 
  volts 
  by 
  being 
  always 
  

   connected 
  to 
  a 
  battery. 
  A 
  small 
  central 
  electrode, 
  insulated 
  

   by 
  means 
  of 
  good 
  sealing-wax, 
  was 
  joined 
  through 
  a 
  mercury 
  

   earthing 
  and 
  charging 
  key 
  to 
  an 
  electroscope. 
  The 
  electro- 
  

   scope 
  and 
  electrode 
  were 
  always 
  charged 
  to 
  a 
  positive 
  

  

  * 
  Rutherford, 
  'Radioactive 
  Substances 
  and 
  their 
  Radiations/ 
  p. 
  180. 
  

   t 
  Geiger, 
  Proc. 
  Roy. 
  Soc. 
  A. 
  lxxxiii. 
  p. 
  505 
  (1910). 
  

  

  