D^ce^iber,  ^19^-}  The  ThcoTy  of  PercolaUofi.  855 
to  prepare  ten  or  twenty-five  gallons  of  fiuidextract  for  instance, 
as  one  may  use  in  the  much  smaller  pharmacopoeial  amounts,  and 
it  is  my  opinion  that,  the  coarser  the  drug,  the  slower  should  be  the 
rate  of  flow  of  percolate. 
The  time  factor  will  materially  influence  the  rate  at  which  the 
percolate  changes  from  a  nearly  saturated  solution  to  nearly  pure 
meiistruum  so  that  no  mathematical  rule  which  does  not  take  it 
into  consideration  can  be  applied  to  the  variation  in  the  composi- 
tion of  the  percolate.  We  may,  however,  state  in  general  terms 
the  character  of  this  variation  and  present  a  definite  idea  of  the 
phenomena  involved. 
Specific  Gravity. — The  first  portions  of  percolate  are  laden  with 
extracted  matter  and  are  specifically  heavier  than  any  succeeding 
fraction.  They  may,  indeed,  present  a  greater  specific  gravity 
than  the  fiuidextract  made  from  the  drug  due  to  the  fact  that  they 
may  contain  not  only  more  extract  but  a  larger  proportion  of  water 
which  originates  in  the  natural  moisture  of  the  drug.  If  the  fractions 
of  percolate  are  taken  in  small  volumes  it  may  be  found  that  the 
second  fraction  is  of  greater  specific  gravity  than  the  first  due  to  an 
actually  greater  content  of  extract.  ^  As  above  suggested  this  anom- 
alous condition  is  probably  due  to  the  first  portion  of  percolate 
lying  during  the  larger  part  of  the  maceration  quite  out  of  con- 
tact with  the  drug. 
The  lowest  specific  gravity  possible  is,  of  course,  that  of  the 
menstruum  and  this  is  the  limit  which  the  changing  values  for 
the  fractions  approach.  It  has  been  found^  that  the  character 
of  the  change  in  the  specific  gravity  of  successive  fractions  as  per- 
colation proceeds  is  quite  regular  and  furnishes  a  rough  indication 
of  the  degree  to  which  the  drug  has  been  exhausted. 
Accompanying  the  change  in  specific  gravity  we  find  a  decrease 
in  the  amount  of  extract  contained  in  the  individual  fractions, 
each  being  a  little  less  concentrated  than  its  immediate  predeces- 
sor. If  the  percolation  has  not  been  interrupted  so  that  no  period 
of  maceration  has  occurred  since  the  start  of  fiow  of  percolate, 
the  decrease  of  extract  concentration  proceeds  with  regularity  as 
the  following  table,  taken  from  one  of  Lloyd's  publications,^  shows. 
^  Lloyd,  This  Journal,  Vol.  50,  434,  (1878). 
•vSquibb,  This  Journal,  Vol.  50,  229,  223,  (1878). 
