﻿42 
  Dr. 
  R. 
  D. 
  Kleeman 
  on 
  the 
  

  

  And 
  since 
  E 
  = 
  KLT, 
  we 
  have 
  

  

  The 
  latent 
  heat 
  of 
  evaporation 
  Li 
  of 
  a 
  liquid 
  is 
  given 
  by 
  

   the 
  well-known 
  thermodynamical 
  relation 
  

  

  L,=(.,-.,)Tg, 
  

  

  where 
  Vi 
  and 
  V2 
  denote 
  respectively 
  the 
  volumes 
  of 
  a 
  gram 
  

   of 
  the 
  saturated 
  vapour 
  and 
  the 
  liquid, 
  and 
  p 
  denotes 
  the 
  

   pressure. 
  For 
  temperatures 
  well 
  removed 
  from 
  the 
  critical 
  

   temperatuie 
  the 
  vapour 
  behaves 
  approximately 
  as 
  a 
  perfect 
  

   gas, 
  and 
  this 
  equation 
  may 
  then 
  be 
  written 
  

  

  _ 
  UT 
  dp 
  

   ' 
  mp 
  dT 
  ' 
  

  

  where 
  R 
  denotes 
  the 
  gas 
  constant. 
  If 
  the 
  equation 
  be 
  written 
  

   in 
  the 
  form 
  

  

  Li?n 
  RT 
  dp 
  

   T 
  " 
  p 
  dT^ 
  

  

  and 
  the 
  temperatures 
  and 
  pressures 
  at 
  the 
  right-hand 
  side 
  

   be 
  expressed 
  in 
  terms 
  of 
  the 
  critical 
  temperature 
  and 
  pressure, 
  

   we 
  obtain 
  

  

  Lim 
  _ 
  RoTe 
  pedp_^d^ 
  

  

  "T" 
  - 
  /3y?e 
  Tc 
  c/« 
  - 
  y8 
  du' 
  

  

  where 
  aTc 
  = 
  T, 
  and 
  y8/)c=p. 
  Thus 
  the 
  relation 
  between 
  Lj, 
  

   T, 
  and 
  m 
  for 
  different 
  liquids 
  at 
  corresponding 
  states 
  is 
  

  

  given 
  by 
  -—^ 
  = 
  constant. 
  Since 
  the 
  temperatures 
  of 
  liquids 
  

  

  at 
  their 
  boiling-points 
  are 
  approximately 
  corresponding 
  tem- 
  

   peratures, 
  this 
  relation 
  holds 
  approximately 
  for 
  liquids 
  at 
  

   their 
  boiling-points 
  ; 
  it 
  is 
  known 
  in 
  this 
  form 
  as 
  Trouton's 
  

   law. 
  If 
  the 
  temperature 
  of 
  evaporation 
  is 
  so 
  far 
  removed 
  

   from 
  the 
  critical 
  temperature 
  that 
  the 
  vapour 
  behaves 
  

   approximately 
  as 
  a 
  perfect 
  gas, 
  the 
  external 
  work 
  of 
  evapo- 
  

  

  RT 
  

   ration 
  p(vi 
  — 
  V2) 
  is 
  approximately 
  equal 
  to 
  — 
  . 
  Therefore 
  

  

  RT 
  

  

  L 
  + 
  p(vi— 
  ^2) 
  = 
  ^ 
  + 
  ^ 
  =Li 
  

  

  and 
  therefore 
  -^ 
  =■ 
  "m 
  - 
  +1^ 
  = 
  constant, 
  or 
  -^ 
  is 
  also 
  

  

  