IS 
652  Mr.  Inglis  on  the  Isothermal  Distillation  of 
of  that  coefficient  being  the  normal  one,  so  that  ox}Tgen  must 
be  slightly  associated.     The  association  calculated  from  these 
figures    would   be   (     Q  '_  j    =  1*068.       Now   if    oxygen 
associated  so  that  n  molecules  in  the  vapour  become  one 
molecule  in  the  liquid,  we  should  have,  according  to  Henry's 
law,  concentration  =  const,  x  (pressure)". 
But  this  equation  takes  no  account  of  any  relation  between 
the  association  and  the  concentration,  and  therefore  at  its 
best  can  only  approximately  represent  the  facts  when  the 
concentration  is  small.  In  the  equation,  n  and  the  constant 
are  both  unknown,  but  may  be  determined  very  easily  by 
plotting  the  logarithm  of  the  concentration  against  the 
logarithm  of  the  pressure.  If,  now,  this  be  done  for  experi- 
ments 23-20  of  Table  IV.  (for  all  of  which  the  concentration 
of  the  oxygen  is  low),  it  is  found  that  the  four  points 
obtained  lie  close  to  a  straight  line  the  slope  of  which  indi- 
cates that  the  pressure  and  concentration  satisfy  the  relation, 
—  concentration  =  const,  x  (press.)1"09,  thus  indicating  an 
association  factor  =1'09.  This  is  of  the  same  order  as  Baly 
and  Donnan's  factor  1*068,  so  that  the  agreement  is  satis- 
factory. Similarly  the  results  of  Table  V.  point  to  an 
association  factor  1*15,  but  a  slight  error  in  the  composition 
of  the  vapour  in  experiment  11  in  that  table  would  be 
sufficient  to  explain  the  increase  from  l-09  to  1*15.     Values 
„  -  .,  .  concentration  02         -,    concentration  02 
or    the    expressions    — -. rr^ —   and    — -. -j-p — 
L  (pressure) l  uy  (pressure) i  l0 
are  given  in  Tables  IV.  and  V.  (for  low  concentrations  of 
oxygen),  and  the  values  obtained  show  how  the  relation  is 
satisfied.  Since  the  size  of  the  associated  oxygen  molecule 
is  not  known,  one  cannot  introduce  the  corresponding  modi- 
fications in  the  formula  deduced  from  Henry's  law,  so  that 
the  formula  used  cannot  be  expected  to  give  good  results. 
In  addition  to  this,  Richardson  (Phil.  Mag.  [6]  vii.  p.  266) 
has  shown  that  one  must  consider  separately  the  solubilities 
of  the  simple  and  associated  molecules. 
The  results  point,  therefore,  to  nitrogen  obeying  Henry's 
law  and  to  oxygen  only  obeying  it  when  we  allow  for  asso- 
ciation. Now  in  the  results  of  the  many  isothermal  distil- 
lations which  have  been  carried  out  at  ordinary  temperatures 
(see  Zawidzki,  loc.  cit.)  agreement  with  Henry's  law  has 
not  been  looked  for,  as  investigators  have  only  looked  for  a 
linear  relation  between  the  partial  pressure  and  the  molecular 
percentage.  The  same  difficulty  that  we  have  experienced 
in  the  calculation  of  the  densities  of  the  mixtures  arises  here 
also  in  calculating  the  concentrations  ;  but  it  may  be  over- 
