792 
The  Theory  of  Percolation. 
(  Am.  Jour.  Pharm. 
(    November,  1920. 
ment  of  the  absorbed  menstruum  we  should  find  the  conditions  which 
Procter^  assumed.  Experimental  evidence,  however,  disproves  this 
contention  and  even  in  those  rare  cases  where  the  second  fraction  of 
the  percolate^  contains  more  extractive  than  the  first,  an  explana- 
tion may  usually  be  found  in  the  fact  that  the  first  portion  has  lain 
in  the  bottom  of  the  percolator  quite  out  of  contact  with  the  drug, 
at  least  in  part,  while  the  second  fraction  was  macerating  in  contact 
with  soluble  material. 
To  the  foregoing  considerations  may  be  ascribed  the  fact  that 
Squibbs'  repercolation  process  does  not  possess  as  great  advantage 
over  simple  percolation  as  it  would  if  we  were  dealing  with  displace- 
ment or  "osmosis,"  and  also  the  failures  of  complicated  and  ingenious 
vacuum  and  pressure  apparatus  may  be  due  to  this  same  cause. 
Percolation  does  not  depend  upon  "osmosis"  except  insofar  as  that 
term  was  used  for  "diffusion"  by  the  early  writers.  Osmotic  pres- 
sure, if  it  influences  the  process  at  all,  serves  to  hinder  rather  than 
to  facilitate  the  extraction,  for  it  would  operate  to  draw  the  menstruum 
into  the  cell  instead  of  forcing  solution  out  of  the  cell  and,  consequently, 
we  should  have  to  wait  until  the  cells  burst  open  from  internal 
pressure  before  we  could  have  dissolved  extractive  in  the  precolate. 
There  is  no  evidence  that  this  is  the  case;  rather  is  it  probable  that 
the  cell  wall  is  so  much  altered  in  drying  that  it  can  no  longer  serve 
as  a  semipermeable  membrane. 
THE  PRECOIvATE. 
The  precolate  is  a  heterogeneous  mixture  of  solutions  which  fills 
the  interstices  of  the  packed  drug  from  the  upper  layers  to  the  stop- 
cock. Its  composition  varies  by  infinitesimal  degrees  from  pure  sol- 
vent to  partly  saturated  solution.  Only  rarely,  if  ever,  is  any  por- 
tion of  the  precolate  a  fully  saturated  solution ;  this  is  shown  by  the 
fact  that  the  first  portions  of  percolate,  that  is,  the  most  concentrated 
portions,  seldom  contain  as  much  extract  as  the  fluidextract  made 
from  the  particular  drug  in  question. 
The  character  of  the  changes  in  the  composition  of  the  precolate 
from  the  upper  to  the  lowermost  portions  is  of  considerable  interest 
from  the  practical  as  well  as  the  theoretical  point  of  view.  Lloyd^ 
^  This  Journal,  Vol.  31,  317,  (1859). 
2  Cf.  Lloyd,  This  Journal,  Vol.  50,  438,  (1878). 
3  Proc.  A.  Ph.  A.  1881,  498;  1882,  508;  1884,  410;  1885,  411.    This  Journal, 
Vol.  80,  39,  (1908). 
