Am.  Jour.  Pharm.  ) 
November,  1920.  j 
The  Theory  of  Percolation. 
791 
percolating  of  the  menstruum ;  with  the  force  of  gravity  it  serves  to 
overcome  the  surface  tension  holding  liquid  between  the  drug  parti- 
cles and  so  assists  the  descent  of  percolate. 
If  the  process  of  percolation  were  wholly  one  of  displacement  the 
absorbed  menstruum  would  present  no  such  problem  as  it  now  does. 
It  would  simply  dissolve  a  certain  fraction  of  the  extractive  and  be 
completely  replaced  by  a  layer  of  fresher  menstruum,  itself  moving  to 
the  next  lower  section  of  drug.  It  is  not  so  displaced,  however,  a;s 
anyone  may  easily  conclude  after  watching  the  descent  of  menstruum 
through  a  moistened  drug  packed  in  a  glass  percolator.  The  new 
menstruum  fills  up  the  air  spaces  between  the  swollen  drug  particles 
and  part  of  it  may  be  absorbed  if  the  drug  was  not  completely  satu- 
rated with  liquid  when  it  was  packed.  The  absorbed  liquid  does  not 
appear  to  be  disturbed. 
The  composition  of  the  absorbed  menstruum  is,  of  course,  variable 
for  it  will  contain  a  large  amount  of  dissolved  extractive  during  the 
early  part  of  the  process  and  at  the  end  is  practically  pure  menstruum. 
However,  it  may  vary  considerably  from  the  composition  of  the  per- 
colate especially  where  the  menstruum  contains  any  large  percentage 
of  water.  The  marc  will  exert  a  selective  absorption  in  such  cases 
with  the  result  that  more  water  than  alcohol  will  be  absorbed  by  the 
drug.  ^  The  solvent  powers  of  this  absorbed  menstruum  willl  be  differ- 
ent from  those  of  the  original  menstruum  and,  consequently,  the  na- 
ture and  quantities  of  the  dissolved  substances  will  not  be  the  same 
in  the  two  solutions. 
I  do  not  regard  it  as  probable  that  the  absorbed  portions  and  the 
precolate  liquid  are  consolute,  that  they  freely  mingle.  The  ab- 
sorbed liquid  is,  for  practical  purposes,  a  solid  body.  It  is  a  part  of 
the  drug  being  acted  upon  by  the  precolate  precisely  as  we  have  been 
in  the  habit  of  thinking  the  drug  was  itself  acted  upon  by  the  men- 
struum. It  is  much  more  likely  that  the  extractive  is  first  dissolved 
in  the  absorbed  liquid  whence  it  diffuses  into  the  precolate  which 
bathes  each  particle  of  drug  while  a  further  quantity  of  soluble  matter 
goes  into  solution  in  the  absorbed  liquid.  There  may  be  some  small 
diffusion  of  molecules  of  absorbed  liquid  into  the  precolate  with  con- 
comitant replacement  by  molecules  from  the  latter  but  this  must  be 
on  so  small  a  scale  as  to  be  negligible  to  all  but  the  most  minute  scru- 
tiny. 
If  there  was  a  complete  diffusion  into  the  precolate  or  a  displace- 
'  Cf.  Patch,  in  discussion.    Proc.  A.  Ph.  A.  1892,  176. 
