﻿Equilibrium 
  of 
  Heterogeneous 
  Substances. 
  823 
  

  

  chemistry 
  to 
  know 
  how 
  these 
  unstable 
  phases 
  are 
  related 
  to 
  

   those 
  which 
  are 
  relatively 
  or 
  absolutely 
  stable. 
  

  

  The 
  absolutely 
  stable 
  phases 
  are 
  divided 
  from 
  the 
  relatively 
  

   stable 
  phases 
  by 
  a 
  series 
  of 
  pairs 
  of 
  coexistent 
  phases, 
  for 
  

   which 
  the 
  intensities 
  p, 
  t, 
  jjl, 
  &c. 
  are 
  equal 
  and 
  K 
  is 
  zero. 
  

   Thus 
  water 
  and 
  steam 
  at 
  the 
  same 
  temperature 
  and 
  pressure 
  

   are 
  coexistent 
  phases. 
  

  

  As 
  one 
  of 
  the 
  two 
  coexistent 
  phases 
  is 
  made 
  to 
  vary 
  in 
  a 
  

   continuous 
  manner, 
  the 
  other 
  may 
  approach 
  it 
  and 
  ultimately 
  

   coincide 
  with 
  it. 
  The 
  phase 
  in 
  which 
  this 
  coincidence 
  takes 
  

   place 
  is 
  called 
  the 
  Critical 
  Phase. 
  

  

  The 
  region 
  of 
  absolutely 
  unstable 
  phases 
  is 
  in 
  contact 
  

   with 
  that 
  of 
  absolutely 
  stable 
  phases 
  at 
  the 
  critical 
  point. 
  

   Hence, 
  though 
  it 
  may 
  be 
  possible 
  by 
  preventing 
  the 
  body 
  

   from 
  coining 
  in 
  contact 
  with 
  certain 
  substances 
  to 
  bring 
  it 
  

   into 
  a 
  phase 
  far 
  beyond 
  the 
  limits 
  of 
  absolute 
  stability, 
  this 
  

   process 
  cannot 
  be 
  indefinitely 
  continued, 
  for 
  before 
  the 
  

   substance 
  can 
  enter 
  a 
  new 
  region 
  of 
  stability 
  it 
  must 
  pass 
  

   out 
  of 
  the 
  region 
  of 
  relative 
  stability 
  into 
  one 
  of 
  absolute 
  

   instability, 
  when 
  it 
  will 
  at 
  once 
  break 
  up 
  into 
  a 
  system 
  of 
  

   stable 
  phases. 
  

  

  Thus 
  in 
  water 
  for 
  any 
  given 
  pressure 
  there 
  is 
  a 
  corre- 
  

   sponding 
  temperature 
  at 
  which 
  it 
  is 
  in 
  equilibrium 
  with 
  its 
  

   vapour, 
  and 
  beyond 
  which 
  it 
  cannot 
  be 
  raised 
  when 
  in 
  

   contact 
  with 
  any 
  gas. 
  But 
  if, 
  as 
  in 
  the 
  experiment 
  of 
  

   Dufour, 
  a 
  drop 
  of 
  water 
  is 
  carefully 
  freed 
  from 
  air 
  and 
  

   entirely 
  surrounded 
  by 
  liquid 
  which 
  has 
  a 
  high 
  boiling-point, 
  

   it 
  may 
  remain 
  in 
  the 
  liquid 
  state 
  at 
  a 
  temperature 
  far 
  above 
  

   the 
  boiling-point 
  corresponding 
  to 
  the 
  pressure, 
  though 
  if 
  it 
  

   comes 
  in 
  contact 
  with 
  the 
  smallest 
  portion 
  of 
  any 
  gas 
  it 
  

   instantly 
  explodes. 
  

  

  But 
  it 
  is 
  certain 
  that 
  if 
  the 
  temperature 
  were 
  raised 
  high 
  

   enough 
  the 
  water 
  would 
  enter 
  a 
  phase 
  of 
  absolutely 
  unstable 
  

   equilibrium, 
  and 
  that 
  it 
  would 
  then 
  explode 
  without 
  requiring 
  

   the 
  contact 
  of 
  any 
  other 
  substance. 
  

  

  Water 
  may 
  also 
  be 
  cooled 
  below 
  the 
  temperature 
  at 
  which 
  

   it 
  generally 
  freezes, 
  and 
  if 
  the 
  water 
  is 
  surrounded 
  by 
  another 
  

   liquid 
  of 
  the 
  same 
  density 
  the 
  pressure 
  may 
  also 
  be 
  reduced 
  

   below 
  that 
  of 
  the 
  vapour 
  of 
  water 
  at 
  that 
  temperature. 
  If 
  

   the 
  water 
  when 
  in 
  this 
  phase 
  is 
  brought 
  in 
  contact 
  with 
  

   ice 
  it 
  will 
  freeze, 
  but 
  if 
  brought 
  in 
  contact 
  with 
  a 
  gas 
  it 
  will 
  

   evaporate. 
  

  

  Professor 
  Guthrie 
  has 
  recently 
  discovered 
  a 
  very 
  remark- 
  

   able 
  case 
  of 
  equilibrium 
  of 
  a 
  liquid 
  which 
  may 
  be 
  solidified 
  

   in 
  three 
  different 
  ways 
  by 
  contact 
  with 
  three 
  different 
  sub- 
  

   stances. 
  This 
  is 
  a 
  solution 
  of 
  chloride 
  of 
  calcium 
  in 
  water 
  

  

  