790 
The  Theory  of  Percolation. 
(  Am.  Jour.  Pharm. 
\    November,  1920. 
A  number  of  questions  arise  concerning  it.  Is  it  displaced  by  fresh 
menstruum?  Is  it  constant  or  variable?  Does  it  differ  from  the 
precolate?    What  effect  does  it  have  on  percolation? 
The  earlier  views  of  the  percolation  of  plant  drugs  was  that  fresh 
portions  of  menstruum  continually  forced  those  which  immediately 
preceded  them  through  the  mass  and  finally  out  at  the  bottom 
of  the  apparatus/  this  being  a  true  displacement,  each  layer  of  men- 
struum acting  as  a  piston  upon  those  layers  beneath  it.  The  Boul- 
lays  therefore  termed  it  "the  method  of  displacement"  and  the  term 
percolation  did  not  come  into  general  use  until  later. 
Vauquelin^  proved  the  displacement  of  salt  solutions  from  sand 
by  fresh  water  and  his  experiments  were  repeated  by  Deane  who 
verified  his  conclusions. 
Rossenwasser^  appears  to  have  been  the  first  to  challenge  in 
print  the  idea  of  displacement.  He  says  that  cell  percolation  depends 
upon  osmosis,  not  on  displacement  and  offers  experiments  in  support 
of  his  views."^  Guillermond^  and  Soubieran^  had  long  before  shown 
that  alcohol  cannot  be  displaced  from  a  marc  by  water  without  some 
intermixture  of  the  two  liquids. 
Experiments  based  on  the  displacement  ol  liquids  from  sand  offer 
little  to  parallel  ordinary  percolation.  The  difference  in  conditions 
is  obvious;  with  such  "marcs"  as  sand  there  is  no  absorption  of 
menstruum  or  solution  though  there  may  be,  and  probably  always 
is,  adsorption  to  some  extent.  With  cellular  structures  there  is 
always  absorption  of  the  menstruum,  consequently  there  is  an  at- 
tracting force  between  the  fibre  and  the  liquid  which  resists  the  effort 
of  the  fresh  menstruum  to  wash  out  the  absorbed  liquid.  To  appreci- 
ate the  magnitude  of  this  force  one  has  only  to  recall  the  fact  that 
huge  rocks  may  be  split  by  driving  a  wooden  wedge  into  some  crack 
in  them  and  then  wetting  the  wood.  The  wood  absorbs  the  water  in 
spite  of  the  enormous  pressure  exerted  against  the  act  by  the  rock  and, 
swelling  in  consequence,  shatters  the  stone. 
Displacement  is,  however,  one  of  the  factors  which  govern  the 
^  The  Boullays,  Jour.  de.  Pharm.  21,  i,  (1835).  Soubieran,  This  Journal, 
Vol.  8,  221  ,(1836).  Duhammel,  This  Journal,  Vol.  10,  i,  (1838);  Deane, 
Pharm.  Jour,  i,  61,  (1841).    Graham,  This  Journal,  31,  354,  (1859). 
2  Quoted  by  Deane,  (v.  s.). 
3  Proc.  A.  Ph.  A.  1882,  519. 
^  Proc.  A.  Ph.  A.  1885,  399. 
^  Jour.  de.  Pharm.  21,  349,  (1835). 
6  This  Journal,  Vol.  8,  221,  (1836). 
