854 
The  Theory  of  Percolation. 
(  Am.  Jour.  Pharm. 
\      December,  1920. 
The  composition  of  the  percolate  depends  upon  several  factors; 
the  nature  of  the  drug,  the  condition  of  the  drug,  the  nature  of  the 
menstruum,  the  shape  of  the  percolator,  the  manner  in  which  the  drug 
is  packed  in  the  percolator,  and  the  rate  at  which  the  percolate  is 
allowed  to  flow  from  the  apparatus. 
The  first  five  factors  have  already  been  considered;  the  last  may 
profitably  engage  our  attention  now.  All  pharmacists  who  have  de- 
voted time  to  the  study  of  percolation  have  recommended  that  the 
flow  of  percolate  be  retarded  to  a  very  slow  dripping.  Squibb^ 
says :  "It  is  an  axiom  in  percolation  that  the  slower  it  is  performed  the 
more  perfect  and  sudden  is  the  exhaustion,  and  with  the  smallest 
quantity  of  menstruum."  Again,  for  percolating  one  pound  of  drug 
he  directs  a  rate  of  flow  of  one  drop  every  two  seconds  or  "about 
3  fl.  oz.  per  hour;  and  for  larger  quantities  in  the  same  ratio."-  Bed- 
ford^ agrees  with  this  but  adds  that  the  rate  of  flow  may  be  in- 
creased toward  the  end  of  the  percolation.  This  latter  idea  as 
I  shall  presently  show,  is  erroneous.  Lloyd^  and  Procter^  have  also 
declared  in  favor  of  slow  percolation. 
How  slow  should  we  percolate?  Obviously  the  criterion  is  the 
strength  of  the  solution  which  issues  from  the  apparatus;  we  should 
control  the  rate  of  flow  just  as  we  control  the  time  of  maceration, 
that  is,  in  such  a  way  as  to  obtain  as  concentrated  a  percolate  as 
is  desirable,  not  necessarily  as  concentrated  as  possible.  For  we 
may,  by  disregarding  the  time  consumed,  keep  increasing  the  strength 
of  our  percolate  at  the  expense  of  time,  though  at  a  diminishing  rate, 
apparently  for  months.  Time,  however,  cannot  be  neglected  for  it  is 
one  of  the  most  important  factors  pharmaceutically  and  financially. 
The  time,  therefore,  which  we  can  allow  for  the  obtaining  of  a  cer- 
tain amount  of  percolate  is  limited  and  must  be  gauged,  like  the 
time  given  to  maceration,  by  the  operator's  experience  and  judg- 
ment. 
Squibb  allows  a  faster  rate  of  flow,  "in  the  same  proportion," 
for  large  quantities  of  drug.  This  is  open  to  some  further  consid- 
eration for,  as  I  have  already  pointed  out,  it  is  not  practicable  to 
use  as  fine  a  powder  of  certain  drugs  in  large  quantities,  enough 
^  This  Journal,  Vol.  39,  400,  (1867). 
2  'pjiis  Journal,  Vol.  30,  97,  (1858). 
2  Pharm.  Record,  6,  19,  (1886). 
4  Proc.  A.  Ph.  A.  1879,  682. 
^  Pharm.  Jour.  19,  139,  (1859). 
