﻿222 
  JOURNAL 
  OF 
  Tne 
  WASHINGTON 
  ACADEMY 
  OF 
  SCIENCES 
  VOL. 
  12, 
  NO, 
  9 
  

  

  ratio 
  of 
  the 
  differential 
  heat 
  of 
  vaporization 
  to 
  the 
  differential 
  heat 
  of 
  

   solution, 
  and 
  inasmuch 
  as 
  the 
  former 
  is 
  always 
  several 
  times 
  greater 
  

   than 
  the 
  latter, 
  the 
  solution 
  at 
  this 
  point 
  will 
  always 
  contain 
  but 
  

   a 
  small 
  percentage 
  of 
  water; 
  this 
  is 
  especially 
  noticeable 
  when 
  com- 
  

   position 
  is 
  expressed 
  as 
  weight 
  percentage, 
  because 
  of 
  the 
  low 
  molecu- 
  

   lar 
  weight 
  of 
  water. 
  It 
  follows 
  from 
  this 
  fact 
  that 
  the 
  pressure- 
  tem- 
  

   perature 
  curve, 
  BBni 
  in 
  figure 
  1, 
  B, 
  will 
  fall 
  steeply 
  from 
  the 
  point 
  of 
  

   maximum 
  pressure 
  to 
  the 
  melting 
  point 
  of 
  the 
  non-volatile 
  component, 
  

   in 
  this 
  case 
  KNO3. 
  

  

  In 
  a 
  binary 
  system 
  there 
  are 
  three 
  variables 
  to 
  be 
  considered, 
  pres- 
  

   sure, 
  temperature, 
  and 
  composition 
  of 
  the 
  liquid 
  phase 
  in 
  equilibrium 
  

   with 
  crystals 
  and 
  vapor. 
  In 
  figure 
  1, 
  C 
  and 
  B, 
  are 
  shown 
  the 
  relations 
  

   between 
  temperature 
  and 
  composition 
  and 
  between 
  temperature 
  and 
  

   pressure; 
  the 
  relation 
  between 
  pressure 
  and 
  composition 
  is 
  shown 
  in 
  

   figure 
  1, 
  A. 
  The 
  curve 
  BB^ 
  gives 
  the 
  vapor 
  pressure 
  of 
  solutions 
  of 
  

   different 
  composition 
  in 
  equilibrium 
  with 
  KNO3 
  and 
  vapor; 
  the 
  tem- 
  

   perature 
  to 
  which 
  a 
  mixture 
  of 
  any 
  composition 
  must 
  be 
  heated 
  to 
  

   melt 
  all 
  but 
  an 
  infinitesimal 
  portion 
  of 
  the 
  crystals 
  can 
  be 
  obtained 
  

   from 
  either 
  B 
  or 
  C, 
  figure 
  1. 
  This 
  cur\^e 
  shows 
  in 
  a 
  striking 
  manner 
  

   the 
  rapid 
  increase 
  in 
  pressure 
  consequent 
  on 
  but 
  a 
  small 
  increase 
  in 
  

   the 
  water 
  content 
  of 
  the 
  KNO3 
  rich 
  solutions. 
  

  

  If 
  a 
  mixture 
  of 
  composition 
  y 
  (figure 
  1, 
  C) 
  be 
  considered 
  at 
  a 
  tem- 
  

   perature 
  above 
  that 
  of 
  the 
  corresponding 
  point 
  on 
  the 
  saturation 
  

   curve 
  EBnj, 
  its 
  pressure 
  will 
  be 
  represented 
  by 
  a 
  point 
  on 
  the 
  curve 
  

   ay 
  (figure 
  1, 
  B). 
  This 
  curve 
  ay 
  gives 
  the 
  change 
  in 
  vapor 
  pressure 
  

   with 
  temperature 
  of 
  the 
  unsaturated 
  solution 
  of 
  the 
  composition 
  y, 
  

   and 
  its 
  slope 
  will 
  be 
  large 
  and 
  positive, 
  as 
  shown 
  in 
  the 
  figure. 
  As 
  

   the 
  temperature 
  is 
  lowered 
  the 
  presssure 
  will 
  fall 
  along 
  the 
  curve 
  ay 
  

   until 
  the 
  latter 
  curve 
  intersects 
  the 
  curve 
  of 
  saturated 
  solutions 
  EBm. 
  

   At 
  this 
  point 
  crystallization 
  will 
  begin, 
  and 
  with 
  further 
  decrease 
  of 
  

   temperature 
  the 
  pressure 
  will 
  increase 
  along 
  curve 
  BmB 
  in 
  the 
  di- 
  

   rection 
  of 
  y". 
  The 
  rapidity 
  of 
  increase 
  in 
  pressure, 
  and 
  the 
  magni- 
  

   tude 
  of 
  the 
  pressure 
  ultimately 
  developed, 
  will 
  in 
  general 
  depend 
  on 
  

   the 
  solubility 
  of 
  water 
  in 
  the 
  liquid. 
  This 
  effect 
  may 
  be 
  made 
  clearer 
  

   by 
  considering 
  the 
  matter 
  from 
  another 
  point 
  of 
  view. 
  

  

  If 
  liquid 
  KNO3 
  be 
  cooled 
  in 
  an 
  apparatus 
  such 
  that 
  the 
  liquid 
  is 
  

   kept 
  in 
  contact 
  with 
  steam 
  at 
  a 
  pressure 
  of 
  one 
  atmosphere, 
  some 
  H2O 
  

   will 
  be 
  dissolved, 
  and 
  this 
  dissolved 
  water 
  will 
  lower 
  the 
  freezing 
  

   point 
  of 
  the 
  KNO3. 
  If 
  the 
  pressure 
  is 
  kept 
  at 
  one 
  atmosphere 
  about 
  

   1 
  per 
  cent 
  of 
  water 
  will 
  be 
  dissolved, 
  and 
  it 
  will 
  lower 
  the 
  freezing 
  point 
  

  

  