86o 
The  Theory  of  Percolation. 
(  Am.  Jour.  Pharm. 
\     December,  1920. 
large  amount  of  water  concentrated  in  a  small  volume  must  materially 
affect  the  alcoholic  strength  of  the  solvent.  Conversely,  a  fibrous 
drug  will  actually  absorb  water  frOiU  a  menstruum  of  low  alcohol 
content  so  that  the  first  portions  of  the  percolate  will  contain  a  sol- 
vent of  higher  alcoholic  strength  than  the  original  menstruum. 
It  is,  perhaps,  unnecessary  to  comment  upon  the  possibility  that 
conditions  of  this  sort  are  almost  certain  to  lead  to  extensive  precip- 
itation in  liquid  products. 
The  actual  alcoholic  strength  of  the  percolate  increases  in  every 
percolation  as  the  content  of  extract  diminishes  and  the  volume  of 
solvent  becomes  greater.  There  has  not  been,  up  to  the  present 
time,  any  published  determination  which  shows  the  regularity  of 
this  increase  in  alcoholic  percentage  but  the  fact  is  well  known  if 
too  often  disregarded.  Lloyd  has  suggested  that  the  solvent  power 
of  a  partially  saturated  menstruum  is  qualitatively  different  from 
that  of  the  fresh  menstruum;  it  would  be  of  considerable  interest 
in  connection  with  the  problems  of  percolation  if  this  question 
were    fully  investigated. 
THE  MARC. 
With  the  exhaustion  of  the  drug  the  process  of  percolation  enters 
a  new  phase.  The  residue  within  the  percolator,  however,  may  still 
claim  attention  for  it  usually  contains  a  quantity  of  valuable  material, 
especially  alcohol.  The  recovery  of  this  valuable  substance  properly 
belongs  to  the  economic  side  of  the  process  and  is  highly  important. 
A  variety  of  methods  are  used  for  this  purpose:  the  most  common 
procedure  and  the  one  which  is  always  applied  in  large  scale  opera- 
tions is  the  use  of  a  dreg  still  into  which  the  wet  marc  is  packed  and 
then  subjected  to  the  action  of  live  steam  which  drives  off  nearly 
all  the  alcohol  in  vapor  and  this  is  then  condensed  by  suitable  means 
and  collected.  About  two-thirds  of  the  residual  menstruum  may 
be  recovered  by  pressing  out  the  marc  in  a  tincture  press  but  this  is 
neither  convenient  nor  economical  for  large  operations.  Washing 
out  or  "displacing"  the  menstruum  with  water  is  the  process  first 
used  by  pharmacists^  to  recover  the  valuable  alcohol.  Smith^ 
has  described  a  percolator  which  has  an  ingenious  device  whereby 
the  apparatus  may  be  reversed  and  the  alcohol  floated  out  of  the 
1  Jour,  de  Pharm.  2,  165,  468,  (1816).    This  Journal,  Vol.  91,  17,  (19  19). 
-  Pharm.  Jour.  18,  291,  (1858). 
