﻿Pressure, 
  and 
  Latent 
  Heat 
  of 
  Vaporisation. 
  385 
  

  

  remarks* 
  " 
  it 
  is 
  needless 
  to 
  say 
  that 
  the 
  variations 
  to 
  be 
  found 
  

  

  in 
  the 
  values 
  of 
  =-; 
  are 
  enormous." 
  If, 
  however, 
  it 
  be 
  assumed 
  

  

  that 
  T-^= 
  be 
  constant, 
  f 
  it 
  at 
  once 
  follows 
  that 
  — 
  = 
  constant, 
  

  

  which 
  is 
  Trouton's 
  or 
  better 
  Pictet's 
  law. 
  

  

  Le 
  ChatelierJ 
  also 
  has 
  transformed 
  equation 
  (1) 
  into 
  another 
  

   directly 
  comparable 
  with 
  the 
  results 
  of 
  experiment. 
  After 
  

   putting 
  it 
  in 
  the 
  form 
  

  

  ^dT 
  + 
  A(v-v 
  ! 
  )dp 
  = 
  0, 
  (11) 
  

  

  (o 
  in 
  Le 
  Chatelier's 
  calculations 
  is 
  always 
  taken 
  to 
  be 
  the 
  

   molecular 
  heat 
  of 
  vaporization) 
  by 
  multiplying 
  and 
  divid- 
  

   ing 
  the 
  second 
  term 
  by 
  p, 
  he 
  obtained 
  this 
  expression 
  

  

  £dT 
  + 
  Aj>(«-V)— 
  =0 
  (12) 
  

  

  If 
  the 
  volume 
  of 
  the 
  liquid 
  be 
  neglected 
  in 
  comparison 
  with 
  

   that 
  of 
  the 
  vapor, 
  and 
  the 
  gas 
  equation 
  

  

  pv 
  = 
  UT 
  

   be 
  introduced, 
  after 
  division 
  by 
  T, 
  the 
  expression 
  

  

  dT 
  . 
  ^dp 
  iH 
  , 
  

  

  P 
  T 
  2 
  " 
  + 
  AR 
  "7 
  = 
  ° 
  (13) 
  

  

  dT 
  

  

  or 
  p__ 
  + 
  2log 
  j? 
  > 
  = 
  (14) 
  

  

  is 
  obtained. 
  If 
  this 
  equation 
  be 
  integrated 
  between 
  the 
  limits 
  

   T 
  and 
  T 
  , 
  it 
  being 
  admitted 
  that 
  the 
  heat 
  of 
  vaporization 
  is 
  

   constant, 
  the 
  equation 
  

  

  t/T 
  P 
  "^To 
  J- 
  2 
  

  

  results, 
  and, 
  all 
  calculations 
  being 
  made 
  on 
  the 
  assumption 
  that 
  

   p 
  is 
  independent 
  of 
  T, 
  

  

  * 
  Bulletin 
  de 
  l'Academie 
  royale 
  de 
  Belgique, 
  III, 
  ix, 
  p. 
  281, 
  1885. 
  

  

  dip 
  

  

  f 
  The 
  results 
  of 
  Ramsay's 
  and 
  Young's 
  experiments 
  show 
  that 
  T— 
  - 
  is 
  constant 
  

  

  dp 
  ^ 
  T 
  

   for 
  considerable 
  differences 
  of 
  pressure. 
  If 
  it 
  be 
  true 
  that 
  T-~ 
  is 
  constant 
  then 
  

   p 
  dl 
  

  

  — 
  - 
  must 
  be 
  constant 
  also, 
  for 
  

  

  Or 
  

  

  cTp 
  T_ 
  _ 
  p 
  

  

  rfT 
  ' 
  A" 
  ~ 
  drt 
  ' 
  

   Ramsay 
  and 
  Young 
  have 
  also 
  experimentally 
  proven 
  the 
  truth 
  of 
  this 
  relation. 
  

   See 
  Phil. 
  Mag., 
  V, 
  xx, 
  p. 
  515, 
  1885 
  ; 
  ibid., 
  xxi, 
  pp. 
  33 
  and 
  135; 
  and 
  ibid., 
  xxii, 
  

   p. 
  33. 
  1886. 
  

  

  % 
  Recherches 
  expenmentales 
  et 
  theoriques 
  sur 
  les 
  equilibres 
  chimiques, 
  Ann. 
  

   des 
  Mines, 
  Mars-Avril, 
  1888, 
  p. 
  337. 
  

  

  