﻿MECHANICAL 
  ACTION 
  OF 
  HEAT. 
  

  

  

  171 
  

  

  C»M\N 
  t 
  T 
  2 
  / 
  

  

  (30.) 
  

  

  (16.) 
  For 
  the 
  vapours 
  of 
  which 
  the 
  properties 
  are 
  known, 
  the 
  negative 
  terms 
  

   of 
  this 
  expression 
  exceed 
  the 
  positive 
  at 
  all 
  ordinary 
  temperatures, 
  so 
  that 
  the 
  

   kind 
  of 
  apparent 
  specific 
  heat 
  now 
  under 
  consideration 
  is 
  a 
  negative 
  quantity 
  : 
  — 
  

   that 
  is 
  to 
  say, 
  that 
  if 
  a 
  given 
  weight 
  of 
  vapour 
  at 
  saturation 
  is 
  increased 
  in 
  tem- 
  

   perature, 
  and 
  at 
  the 
  same 
  time 
  maintained 
  by 
  compression 
  at 
  the 
  maximum 
  elas- 
  

   ticity, 
  the 
  heat 
  generated 
  by 
  the 
  compression 
  is 
  greater 
  than 
  that 
  which 
  is 
  required 
  

   to 
  produce 
  the 
  elevation 
  of 
  temperature, 
  and 
  a 
  surplus 
  of 
  heat 
  is 
  given 
  out 
  ; 
  and 
  

   on 
  the 
  other 
  hand, 
  if 
  vapour 
  at 
  saturation 
  is 
  allowed 
  to 
  expand, 
  and 
  at 
  the 
  same 
  

   time 
  maintained 
  at 
  the 
  temperature 
  of 
  saturation, 
  the 
  heat 
  which 
  disappears 
  in 
  

   producing 
  the 
  expansion 
  is 
  greater 
  than 
  that 
  set 
  free 
  by 
  the 
  fall 
  of 
  temperature, 
  

   and 
  the 
  deficiency 
  of 
  heat 
  must 
  be 
  supplied 
  from 
  without, 
  otherwise 
  a 
  portion 
  of 
  

   the 
  vapour 
  will 
  be 
  liquefied, 
  in 
  order 
  to 
  supply 
  the 
  heat 
  necessary 
  for 
  the 
  expansion 
  

   of 
  the 
  rest. 
  

  

  This 
  circumstance 
  is 
  obviously 
  of 
  great 
  importance 
  in 
  meteorology, 
  and 
  in 
  the 
  

   theory 
  of 
  the 
  steam-engine. 
  There 
  is 
  as 
  yet 
  no 
  experimental 
  proof 
  of 
  it. 
  It 
  is 
  

   true, 
  that, 
  in 
  the 
  working 
  of 
  non-condensing 
  engines, 
  it 
  has 
  been 
  found 
  that 
  the 
  

   steam 
  which 
  escapes 
  is 
  always 
  at 
  the 
  temperature 
  of 
  saturation 
  corresponding 
  to 
  

   its 
  pressure, 
  and 
  carries 
  along 
  with 
  it 
  a 
  portion 
  of 
  water 
  in 
  the 
  liquid 
  state 
  ; 
  but 
  

   it 
  is 
  impossible 
  to 
  distinguish 
  between 
  the 
  water 
  which 
  has 
  been 
  liquefied 
  by 
  the 
  

   expansion 
  of 
  the 
  steam, 
  and 
  that 
  which 
  has 
  been 
  carried 
  over 
  mechanically 
  from 
  

   the 
  boiler. 
  

  

  The 
  calculation 
  of 
  the 
  proportion 
  of 
  vapour 
  liquefied 
  by 
  a 
  given 
  expansion, 
  

   requires 
  the 
  knowledge 
  of 
  the 
  latent 
  heat 
  of 
  evaporation, 
  which 
  forms 
  the 
  subject 
  

   of 
  the 
  next 
  section. 
  

  

  Section 
  III. 
  — 
  Of 
  the 
  Latent 
  and 
  Total 
  Heat 
  of 
  Evaporation, 
  especially 
  

  

  for 
  Water. 
  

  

  (17.) 
  The 
  latent 
  heat 
  of 
  evaporation 
  of 
  a 
  given 
  substance 
  at 
  a 
  given 
  tempe- 
  

   rature, 
  is 
  the 
  amount 
  of 
  heat 
  which 
  disappears 
  in 
  transforming 
  unity 
  of 
  weight 
  

   of 
  the 
  substance 
  from 
  the 
  liquid 
  state, 
  to 
  that 
  of 
  vapour 
  of 
  the 
  maximum 
  density 
  

   for 
  the 
  given 
  temperature, 
  being 
  consumed 
  in 
  producing 
  an 
  increase 
  of 
  volume, 
  

   and 
  an 
  unknown 
  change 
  of 
  molecular 
  arrangement. 
  

  

  It 
  is 
  obvious, 
  that 
  if 
  the 
  vapour 
  thus 
  produced 
  is 
  reconverted 
  into 
  the 
  liquid 
  

   state 
  at 
  the 
  same 
  temperature, 
  the 
  heat 
  given 
  out 
  during 
  the 
  liquefaction 
  must 
  be 
  

  

  vol. 
  xx. 
  part 
  i. 
  2 
  z 
  

  

  