﻿698 
  On 
  the 
  Kinetic 
  Theory 
  of 
  Matter. 
  

  

  forces 
  in 
  the 
  surface 
  layer 
  which 
  determine 
  the 
  latent 
  heat 
  

   of 
  evaporation, 
  and 
  is 
  the 
  same 
  for 
  the 
  molecules 
  leaving 
  the 
  

   liquid 
  as 
  for 
  any 
  other 
  surface 
  drawn 
  in 
  the 
  vapour 
  at 
  the 
  

   temperature 
  of 
  the 
  liquid. 
  

  

  The 
  distribution 
  of 
  the 
  components 
  of 
  velocity 
  parallel 
  to 
  

   the 
  surface 
  will 
  be 
  unaltered 
  by 
  the 
  passage 
  of 
  the 
  particles 
  

   through 
  it. 
  This 
  is 
  clear 
  even 
  if, 
  as 
  is 
  presumably 
  the 
  case, 
  

   the 
  component 
  O^ 
  of 
  the 
  work 
  function 
  parallel 
  to 
  the 
  surface 
  

   deviates, 
  in 
  individual 
  instances, 
  from 
  its 
  average 
  value 
  zero. 
  

   For, 
  for 
  each 
  class 
  of 
  paths 
  for 
  which 
  <J>s 
  has 
  a 
  given 
  positive 
  

   value, 
  there 
  will 
  be 
  an 
  equal 
  number 
  for 
  which 
  it 
  has 
  a 
  

   negative 
  value, 
  so 
  that 
  the 
  change 
  introduced 
  by 
  those 
  which 
  

   are 
  accelerated 
  parallel 
  to 
  the 
  surface 
  will 
  just 
  be 
  com- 
  

   pensated 
  for 
  by 
  others 
  which 
  are 
  retarded. 
  

  

  The 
  distribution 
  of 
  velocity 
  among 
  the 
  particles 
  leaving 
  any 
  

   surface 
  ivill 
  therefore 
  be 
  completely 
  independent 
  of 
  the 
  work 
  

   done 
  in 
  passing 
  through 
  the 
  surface. 
  

  

  As 
  a 
  particular 
  case 
  it 
  follows 
  that 
  the 
  distribution 
  of 
  

   velocity 
  among 
  the 
  molecules 
  escaping 
  from 
  a 
  liquid 
  by 
  

   evaporation 
  will 
  be 
  just 
  what 
  is 
  proper 
  to 
  a 
  gas 
  of 
  the 
  same 
  

   molecular 
  w^eight 
  at 
  the 
  temperature 
  of 
  the 
  liquid 
  (or 
  its 
  

   vapour). 
  The 
  escaping 
  molecules 
  will 
  be 
  in 
  complete 
  

   statistical 
  equilibrium 
  with 
  those 
  in 
  the 
  surrounding 
  vapour. 
  

  

  It 
  is 
  necessary 
  to 
  lay 
  stress 
  on 
  this 
  point 
  as 
  there 
  is 
  a 
  wide- 
  

   spread 
  heresy 
  to 
  the 
  effect 
  that, 
  since 
  the 
  molecules 
  have 
  to 
  

   do 
  work 
  in 
  escaping 
  from 
  the 
  liquid, 
  their 
  average 
  kinetic 
  

   energy 
  in 
  the 
  vapour 
  will 
  be 
  less 
  than 
  in 
  the 
  liquid 
  ; 
  so 
  that 
  

   the 
  surface 
  layer, 
  with 
  its 
  force 
  directed 
  towards 
  the 
  interior^ 
  

   will 
  act 
  in 
  some 
  sort 
  like 
  Maxwell's 
  Demon, 
  letting 
  only 
  the 
  

   quicker 
  molecules 
  through. 
  When 
  this 
  argument 
  is 
  kept 
  

   to 
  the 
  qualitative 
  plane 
  it 
  may 
  be 
  used 
  to 
  demonstrate 
  a 
  

   violation 
  of 
  the 
  second 
  law 
  of 
  thermodynamics. 
  The 
  

   preceding 
  considerations 
  show 
  that 
  there 
  is 
  nothing 
  in 
  this. 
  

  

  It 
  may 
  be 
  permissible 
  to 
  add 
  that 
  both 
  the 
  case 
  of 
  a 
  liquid 
  

   and 
  the 
  surrounding 
  vapour 
  and 
  that 
  of 
  a 
  conductor 
  and 
  the 
  

   escaped 
  thermions 
  which 
  are 
  in 
  equilibrium 
  with 
  it 
  are 
  

   particular 
  examples 
  of 
  a 
  general 
  theorem 
  in 
  the 
  kinetic 
  theory 
  

   of 
  matter 
  which 
  states 
  that 
  the 
  mode 
  of 
  distribution 
  of 
  

   kinetic 
  energy 
  among 
  the 
  ultimate 
  particles 
  of 
  a 
  system 
  is 
  

   independent 
  of 
  the 
  potential 
  energy 
  of 
  the 
  part 
  of 
  the 
  

   system 
  in 
  which 
  they 
  occur 
  (Jeans' 
  Dynamical 
  Theory 
  of 
  

   Gases, 
  p. 
  78). 
  The 
  potential 
  energy 
  only 
  affects 
  the 
  con- 
  

   centration 
  of 
  the 
  ultimate 
  particles 
  in 
  the 
  different 
  parts 
  of 
  

   the 
  system, 
  the 
  average 
  value 
  and 
  mode 
  of 
  distribution 
  of 
  

   the 
  kinetic 
  energy 
  being 
  unaffected. 
  

  

  