﻿Scattering 
  of 
  a 
  Particles 
  by 
  Gases. 
  

  

  705 
  

  

  In 
  the 
  same 
  figure, 
  curve 
  B 
  gives 
  the 
  result 
  of 
  a 
  typical 
  

   experiment 
  showing 
  the 
  effect 
  of 
  both 
  scattering 
  and 
  ab- 
  

   sorption 
  together. 
  From 
  the 
  two 
  curves 
  A 
  and 
  B, 
  a 
  curve 
  

   can 
  be 
  obtain 
  ed 
  which 
  gives 
  the 
  effect 
  of 
  scattering 
  alone. 
  

   To 
  obtain 
  this, 
  the 
  ratios 
  of 
  the 
  ordiriates 
  of 
  the 
  curves 
  A 
  

   and 
  B 
  at 
  each 
  pressure 
  must 
  be 
  plotted. 
  It 
  is 
  obvious 
  that 
  

   this 
  ratio 
  gives 
  the 
  fraction 
  of 
  a 
  particles 
  scattered 
  by 
  the 
  

   gas 
  at 
  each 
  particular 
  pressure, 
  and 
  is 
  independent 
  of 
  any 
  

   change 
  of 
  ionization 
  due 
  to 
  loss 
  of 
  range. 
  Curve 
  C 
  obtained 
  

   in 
  this 
  way 
  indicates 
  the 
  amount 
  of 
  unscattered 
  radiation 
  

   passing 
  between 
  the 
  plates 
  at 
  various 
  pressures. 
  It 
  is 
  seen 
  

   that, 
  over 
  a 
  certain 
  range 
  of 
  the 
  u 
  particle 
  at 
  least, 
  the 
  

   amount 
  of 
  unscattered 
  radiation 
  passing 
  between 
  the 
  plates 
  

   varies 
  nearly 
  exponentially 
  with 
  the 
  pressure 
  of 
  gas, 
  i. 
  e. 
  if 
  

   N 
  is 
  the 
  number 
  of 
  a. 
  particles 
  at 
  zero 
  pressure 
  and 
  N 
  p 
  the 
  

   number 
  at 
  pressure 
  p 
  cm., 
  then 
  N 
  p 
  /N 
  = 
  <? 
  -a 
  p, 
  where 
  X 
  may 
  

   be 
  called 
  the 
  "scattering 
  coefficient" 
  for 
  the 
  particular 
  gas 
  

   under 
  the 
  given 
  conditions. 
  It 
  should 
  be 
  noted 
  that 
  this 
  

   formula 
  does 
  not 
  hold 
  accurately 
  when 
  the 
  pressure 
  of 
  the 
  

   gas 
  becomes 
  so 
  large 
  that 
  the 
  velocity 
  of 
  the 
  a 
  particle 
  is 
  

   much 
  reduced. 
  For 
  pressures 
  of 
  the 
  gas, 
  whereby 
  the 
  loss 
  

   of 
  range 
  of 
  the 
  a 
  particle 
  is 
  equivalent 
  to 
  more 
  than 
  2 
  cm. 
  

   of 
  air 
  at 
  atmospheric 
  pressure, 
  the 
  scattering 
  will 
  increase 
  

   more 
  rapidly. 
  Indications 
  of 
  this 
  were 
  obtained 
  in 
  some 
  of 
  

   the 
  experiments. 
  Table 
  I. 
  gives 
  the 
  results 
  of 
  a 
  typical 
  

   experiment, 
  air 
  being 
  used 
  as 
  the 
  scattering 
  gas. 
  

  

  Table 
  I. 
  

  

  i 
  

  

  Pressure 
  of 
  Gas 
  Scattering 
  and 
  

   p. 
  absorption. 
  

  

  Absorption 
  

   alone. 
  

  

  Intensity 
  N 
  due 
  

  

  to 
  scattering 
  

  

  alone. 
  

  

  loglVNp 
  

  

  v. 
  ■ 
  

  

  cm. 
  100 
  

   •92 
  84-2 
  

   1-9 
  726 
  

   3-55 
  580 
  

   5-10 
  45-9 
  

   7-15 
  335 
  

  

  900 
  24-6 
  

  

  i 
  

  

  100 
  

   98-2 
  

   96-5 
  

   930 
  

   88-6 
  

   82-5 
  

   76-5 
  

  

  100 
  

   86-8 
  

   75-2 
  

   62-5 
  

   519 
  

   40-6 
  

   32-0 
  

  

  •66 
  

   •65 
  

   •58 
  

   •56 
  

   •55 
  

   •55 
  

  

  The 
  numbers 
  in 
  the 
  last 
  column 
  show 
  that 
  the 
  radiation 
  

   is 
  scattered 
  approximately 
  according 
  to 
  an 
  exponential 
  law 
  

   over 
  a 
  considerable 
  range 
  of 
  pressure. 
  

  

  