782 
The  Theory  of  Percolation. 
(Am.  Jour.  Pharm. 
November,  1920. 
The  general  manufacturing  practise  at  present  is  to  use  drugs 
ground  to  about  a  No.  12  powder.  This  is  much  coarser  than  the 
pharmacopoeia  recognizes  but  is  justified  by  the  magnitude  of  the 
operations.  There  is  present  in  such  a  drug  powders  of  all  degrees  of 
fineness  up  to  impalpability  and  this  lack  of  uniformity  is  undesir- 
able but  unavoidable. 
There  is,  too,  a  practical  limit  to  the  fineness  of  a  powder  which 
can  be  extracted  with  diluted  alcohol.  Consider  a  mucilaginous  drug 
such  as  senna,  gentian,  uva  ursi,  or  buchu,  and  in  No.  60  powder. 
As  the  precolate  descends  through  the  drug  it  dissolves  the  extractive 
and  also  the  drug  moisture,  an  important  constituent  for  practical 
reasons.  This  means  that  the  alcoholic  strength  of  the  precolate 
is  constantly  diminishing  and,  if  the  column  of  drug  be  long  enough,  a 
point  will  be  reached  where  its  alcoholic  strength  is  so  low  that  the 
mucilage  will  begin  to  swell,  preparing  to  dissolve,  and,  covering 
every  particle  of  drug  with  a  continuous  slimy  film,  will  clog  the  per- 
colator, oppose  the  passage  of  the  precolate,  and  probably  terminate 
the  process.  In  such  a  case  one  of  two  expedients  must  be  resorted 
to  in  avoiding  such  an  exigency ;  either  the  drug  must  be  coarse  enough 
to  permit  the  passage  of  the  precolate  before  it  has  become  so  dilute 
in  alcoholic  strength,  or  these  large  lots  of  very  fine  drugs  must  be 
extracted  with  menstrua  of  high  alcoholic  content.  The  variation 
of  the  menstruum  is,  in  most  cases,  permissible  only  within  very  nar- 
row limits  and  so  the  operator  is  forced  to  use  coarser  drugs  for  large 
batches.  With  pharmacopoeial  quantities  of  1,000  grams  such  fac- 
tors do  not  enter  into  consideration,  and  it  is  possible  that  there  exists 
a  rational  relationship  between  the  quantity  of  drug  to  be  extracted 
and  the  fineness  required.  Perhaps  a  No.  20  powder  is  to  a  No.  60  as 
a  five  litre  product  is  to  a  one  litre. 
Another  factor  which  must  be  considered  is  adsorption.  How 
important  this  phenomenon  is  will  be  discussed  later  but  in  this  con- 
nection it  should  be  noted  that  adsorption  resists  extraction.  It  is 
an  opposing  force,  and,  being  a  surface  phenomenon,  increases  with 
the  enlargement  of  the  drug  surface,  that  is  with  increasing  fineness 
of  drug.  It  is  known  that  in  many  cases  the  amount  of  substance 
adsorbed  is  less  in  alcoholic  liquids  than  in  aqueous  solutions  and  the 
factor  may  disappear  in  strongly  alcoholic  percolates.  Lloyd  has 
established  the  fact  of  adsorption  in  pharmaceutical  processes^  and 
the  literature  on  the  physicochemical  aspects  of  the  phenomenon  is 
1  Proc.  A.  Ph.  A.  1885,  411. 
