788  The  Theory  oj  Percolation.         { Xvimier!**\92": 
moistening  and  packing  separately;  packing  the  finest  drug  first  and 
the  coarsest  last  so  that  the  fresh  menstruum  should  come  into  con- 
tact with  the  most  difficult  drug  to  extract.  This  is  an  early  ex- 
pression of  the  countercurrent  principle  now  so  universally  used  and 
the  idea  is  valuable,  not  only  for  the  above  reason,  but  because  a 
mixture  of  powders  of  all  degrees  of  fineness  from  a  No.  12  up- 
wards is  very  difficult  to  exhaust  satisfactorily. 
THE  ACTUAL  PERCOLATION. 
The  actual  percolation  may  be  considered  as  beginning  with  the 
addition  of  the  menstruum  to  the  packed  drug  in  quantity  sufficient 
Chart  A. 
to  fill  the  interstices  of  the  drug  and  leave  a  stratum  above  it.  To 
facilitate  the  escape  of  air  the  stopcock  is  usually  left  open  until  a 
few  drops  of  percolate  issue.  The  common  practise  at  this  point  is 
to  shut  the  stopcock  and  to  leave  the  whole  apparatus  at  rest  for  forty 
eight  hours  or  more.  This  procedure,  termed  maceration,  has  been 
shown  to  be  very  desirable.  Procter^  approved  of  long  maceration 
and  Campbell'^  adopted  it  as  one  of  the  features  of  his  method.  Sav- 
age^ showed  by  experiments  with  a  long  list  of  drugs  that  long  macera- 
^  This  Journal,  Vol.  36,  i,  (1864).    Pharm.  Jour.  19,  139,  (1859). 
2  This  Journal,  Vol.  41,  384,  (1869). 
2  Pharm.  Jour.  24,  254,  (1864). 
