656  Mr.  Inglis  on  the  Isothermal  Distillation  of 
The  association  varies  with  the  concentration,  so  that  calcu- 
lations from  the  partial-pressure  curve  are  not  very  accurate. 
If  the  log  o£  concentration  be  plotted  against  the  log  o£  the 
partial  pressure  as  before,  the  slope  of  the  curve  at  each 
point  may  be  taken  as  a  measure  of  the  association  for  that 
concentration.  The  result  obtained  is  an  association  factor 
1'50,  when  the  concentration  of  acetic  acid  is  3*51  gr.  per 
100  c.c.  According  to  the  molecular  weight  the  factor 
should  be  1*35.  It  will  be  seen,  therefore,  that  again  the 
agreement  is  not  close  ;  but  since  the  method  of  calculation 
does  not  take  account  of  the  separate  solubilities  of  the  single 
and  double  molecules,  as  is  really  necessary,  a  better  agree- 
ment can  hardly  be  looked  for. 
Zawidzki's  experiments,  therefore,  to  a  certain  extent 
support  the  view  that  the  relation  between  the  partial 
pressure  and  the  concentration  can  be  obtained  by  means  of 
Henry's  law,  so  that  it  may  be  concluded  that  oxygen  is 
associated  when  dissolved  in  nitrogen  and  also  in  the  pure 
state. 
Distillation  of  Argon  and  Oxygen. 
In  the  separation  of  two  gases  from  one  another  by  means 
of  fractionation  at  low  temperatures,  it  often  happens  that 
at  the  temperature  used  the  one  substance  is  below  its 
melting-point.  In  this  case  the  relations  which  hold  during 
distillation  are  modified  by  the  fact  that  the  total  pressure 
of  the  saturated  solution  of  the  one  substance  in  the  other 
may  be  greater  than  the  vapour-pressure  of  either  pure 
substance.  This  happens  in  the  case  of  argon  and  oxygen. 
The  melting-point  of  argon  is  only  a  little  below  its  boiling- 
point;  and  at  the  temperature  of  fairly  fresh  liquid  air,  argon 
is  a  solid  with  a  vapour-pressure  of  over  400  mm.  In  order 
to  see  how  a  mixture  of  argon  and  oxygen  behaved  when 
distilled  isothermally,  a  few  experiments  were  made  at 
82*09  Abs.  with  the  apparatus  already  described.  As,  how- 
ever, the  quantity  of  argon  available  was  only  550  c.c,  the 
greatest  percentage  of  argon  that  could  be  used  was  13*6  per 
cent,  by  volume.  The  results  of  these  experiments  are  given 
in  Table  XL  The  vapour-pressure  of  pure  oxygen  at 
82*09  Abs.  is  300*0  mm.,  and  this  value,  together  with  the 
experiments  2,  3,  4,  5,  gives  five  points  on  the  vapour- 
pressure  diagram.  Experiment  6  was  carried  out  in  quite  a 
different  way.  A  quantity  of  pure  argon  was  prepared  and 
its  vapour-pressure  at  82°*09  Abs.  was  found  to  be  411*0  mm. 
On  adding  a  small  quantity  of  oxygen  the  vapour-pressure 
rose  to  420  mm.,  and  it  remained   equal   to   this  in  spite  of 
