The  Theory  of  Percolation. 
(  Am.  Jour.  Pharm. 
i     December,  1920. 
and  the  drug  macerated  for  four  days  at  the  end  of  which  time 
three  pints  more  of  percolate  were  coUected.  Then  the  drug  was 
macerated  for  six  days  when  a  third  three  pints  were  collected; 
the  next  maceration  was  for  eight  days  at  the  end  of  which  time 
three  pints  of  percolate  were  run  out.  The  last  period  of  macera- 
tion was  ten  days  long  and  then  four  pints  of  percolate  were  collected. 
The  specific  gravities  only  of  the  percolates  were  determined. 
Below  is  a  tabulation  of  the  results: 
Table. 
Fraction.  Period   of   Maceration.  Specific  Gravity. 
1  2  days  0.9853  at  25°  C. 
2  4    '<  0.9848 
3  6    ,<  0.9833 
4  8  0.9826 
5  10  0.9657 
The  specific  gravities  of  the  first  four  percolates  were  almost  identi- 
cal and  this  shows  a  distinct  advantage  in  the  process  especially  when 
it  is  remembered  that  the  first  portions  of  percolate  usually  con- 
tain all  of  the  drug  moisture  and  so  are  of  higher  specific  gravity 
than  equahy  concentrated  solutions,  the  solvent  of  which  is  the  un- 
diluted menstruum.  The  above  process  occupied  30  days  which, 
it  ma}^  reasonably  be  argued,  is  too  long  a  time  to  devote  to  a  per- 
colation of  this  sort.  But  if  one  considers  the  economy  of  men- 
struum, the  elimination  of  many  details  of  the  other  processes,  and 
the  quality  of  the  product,  this  process  must  be  recommended  as 
practicable.  At  present  the  writer  is  using '  it  exclusively  in  ex- 
tensive work  on  the  extraction  of  plants  and  finds  it  satisfactory. 
METHODS  INVOI^VING  THE  USE  OF  PRESSURE. 
From  time  to  time  it  has  been  considered  advisable  for  a  variety 
of  reasons  to  hasten  the  rate  at  which  the  precolate  passes  through 
the  drug  by  applying  extra  pressure  to  the  percolation  apparatus. 
A  large  number  of  ingenious  forms  of  apparatus  have  been  devised 
to  carry  out  this  purpose.  They  may  be  classified  in  three  ways 
according  to  the  manner  of  applying  the  extra  pressure.  The  first 
class  follows  the  method  of  Count  Real^  who  contrived  a  tall  col- 
umn of  liquid  so  that  the  hydrostatic  pressure  would  act  directly 
upon  the  vSurface  of  the  packed  drug  and  some  of  this  class  are 
^  Jour,  de  Pharm.  2,  165,  (1816). 
