Am.  Jour.  Pharm.  ) 
December,  1920.  ) 
The  Theory  oj  Percolation. 
853 
THE  THEORY  OF  PERCOLATION. 
By  James  F.  Couch, 
washington,  d.  c. 
(Continued  from  November  number,  page  796) 
THE  PKRCOIvATE. 
The  immediate  product  of  percolation  is  the  solution  which  issues 
from  the  percolator,  the  percolate.  It  represents  a  summation  of  the 
various  complexes  of  the  precolate  and,  to  a  certain  extent,  furnishes 
an  index  to  the  conditions  within  the  apparatus.  The  changes  which 
it  undergoes  are  regular  and  have  been  the  object  of  much  careful 
study.  There  are,  how^ever,  many  questions  involved  which  cannot, 
at  present  be  answered  and  the  whole  subject  needs  revision  and  ex- 
tension particularly  since  much  of  the  study  of  percolates  was  made 
before  the  pharmacopoeial  standard  for  fluidextracts  was  changed 
from  a  grain  per  minim  to  a  gram  per  mil. 
The  first  question  which  confronts  us  in  this  consideration  is, 
what  does  the  percolate  represent?  With  our  present  knowledge 
this  is  exceedingly  difficult,  if  not  impossible,  to  answer.  For  such 
drugs  as  peppermint,  ginger,  or  aspidium,  which  we  percolate  with  a 
simple  menstruum  as  alcohol  or  acetone,  thereby  dissolving  but  few 
of  the  constituents  of  the  plant  while  the  larger  number  of  them  is 
excluded,  the  lack  of  complexity  of  the  precolate  enables  us  to  decide 
that  the  percolate  represents  that  portion  of  the  drug  which  is  soluble 
in  the  given  menstruum  and  the  quantitative  composition  of  the  per- 
colate bears  a  direct  and  simple  relation  to  the  degree  of  exhaustion 
and  composition  of  the  partially  exhausted  drug  then  in  the  percolator. 
When,  however,  we  are  dealing  with  drugs  of  more  complex  com- 
position, and  are  extracting  them  with  a  complex  solvent  such  as 
diluted  alcohol,  the  strength  of  which,  and  consequently  its  solvent 
powers,  is  variable  the  answer  to  the  question  is  not  so  easy  to  formu- 
late. As  I  have  fully  outlined  above,  the  composition  of  the  pre- 
colate is  variable  under  such  circumstances,  the  qualitative  composi- 
tion of  the  drug  varies  in  different  parts  of  the  percolator  through 
physical  reactions  and,  as  a  result  the  percolate  represents  a  sum- 
mation of  the  conditions  within  the  percolator  some  of  which  are 
mathematically  negative  and  none  of  which  have  been  satisfactorily 
investigated.  A  percolate  collected  in  such  a  case  will  vary  in  the  corn- 
position  of  its  different  parts  and  this  variation  will  be  not  only  quan- 
titative but  qualitative  and  may  lead  to  ])recipitation  on  mixing.^ 
1  Cf.  Lloyd,  Froc.  A.  Ph.  408. 
