774 
The  Theory  oj  Percolation. 
\  Am.  Jour.  Pharm. 
\    November,  1920. 
Since  the  Seventies,  a  large  number  of  memoirs  bearing  directly 
on  the  problems  of  percolation  has  been  published  and,  of  these,  the 
larger  proportion  has  resulted  from  the  work  of  American  phar- 
macists. The  behaviors  of  different  drugs  in  percolation  have  been 
compared,  the  effects  of  altered  conditions  studied,  the  factors  which 
govern  the  permanence  of  the  products  investigated,  new  menstrua 
experimented  with,  much  analytical  data  accumulated,  and  a  host 
of  new  apparatus  and  methods  developed  and  proposed.  All  of  this 
work  shall  be  considered  in  detail  in  the  course  of  this  survey  and  with 
this,  we  terminate  the  brief  historical  sketch,  which  to  develop  more 
fully  would  carry  us  into  discussions  which  properly  belong  under 
other  headings. 
DEFINITIONS. 
The  term  percolation,  as  generally  used  in  this  country,  may  be 
defined:  a  continuous  process  for  the  selective  extraction,  by  a  suit- 
able solvent  or  mixture  of  solvents,  of  the  soluble  constituents  of  a 
mixture  of  substances  part  of  which  are  insoluble  in  the  solvent  se- 
lected, conducted  in  such  a  way  that  the  solvent  travels  through  the 
mass  to  be  extracted,  from  one  surface  of  which  the  partially  saturated 
solution  is  removed  while  fresh  solvent  is  admitted  at  the  opposite 
surface.  The  process  is  carried  out  in  an  apparatus  called  a  percolator, 
of  various  designs  and  made  of  various  materials  for  special  purposes, 
usually  of  glass,  shaped  like  a  frustrated  cone  with  the  smaller  end 
sharply  constricted  to  a  small  tubular  opening  which  may  be  fitted  with 
a  stopcock.  In  pharmaceutical  practise  the  material  percolated  is 
usually  called  a  drug  and  the  solvent  used  is  termed  the  menstruum. 
The  solution  as  it  runs  from  the  percolator  is  known  as  the  percolate, 
its  dissolved  matter  is  termed  the  extract,  and  the  exhausted  drug 
which  remains  in  the  percolator  at  the  conclusion  of  the  process  is 
called  the  marc.  The  soluble  constituents  of  the  drug  before  their 
appearance  in  the  percolate  are  included  in  the  term  extractive.  It 
appears  desirable,  for  theoretical  reasons,  to  make  a  distinction  be- 
tween the  undissolved  soluble  material  and  that  actually  in  solution. 
Moreover,  as  there  is  frequently  a  chemical  difference  between  the 
solution  within  the  percolator  and  in  contact  with  the  drug,  and  the 
percolate,  which  is  no  longer  in  contact  with  the  drug,  it  would  seem 
advisable  to  distinguish  between  them  by  a  specific  name  for  the  for- 
mer. In  this  survey  the  term  precolate  will  be  used  to  specify  the 
liquid  found  between  the  initial  and  terminal  surfaces  of  the  drug 
and  in  contact  with  it. 
