''Noviaer!^^™:}         The  Theovy  of  PeTcolatiou.  795 
from  digitalis  is  readily  soluble  in  chloroform  but  not  so  in  water, 
yet  owing  to  the  presence  of  tannins  in  the  leaf,  it  may  be  perfectly 
extracted  therefrom  by  water  but  cannot  be  extracted  by  chloro- 
form. The  glucoside  of  oleander  leaves,  oleandrin,^  although  prac- 
tically insoluble  in  water  is  readily  and  completely  extracted  by  cold 
water  owing  to  the  modifying  influence  of  a  phenolic  glucoside 
present  which  is  soluble  in  water.  Jowett^  reports  that  emodin, 
though  only  slightly  soluble  in  water,  is  extracted  from  cascara 
sagrada  by  an  aqueous  menstruum,  and  Squibb^  stated  that,  while 
buchu  could  be  perfectly  extracted  by  dilute  alcohol,  the  extract  ob- 
tained by  evaporating  the  percolate  will  not  redissolve  in  dilute 
alcohol  but  requires  the  strong  solvent.  Such  experiences  are  every- 
day occurrences  in  the  manufacture  of  fluidextracts. 
The  effect  of  temperature  on  the  composition  of  the  percolate  has 
been  investigated  by  Norris^  and  Smith^  who  find  that  the  percolate 
obtained  at  the  higher  temperature  contains  somewhat  more  extract. 
Smith  reported  that  the  rate  of  extraction  of  alkaloids  is  not  greatly 
affected  by  increased  temperature. 
The  great  factor  in  determining  the  composition  of  the  precolate  is 
time.  It  is  certain  that,  as  percolation  is  ordinarily  conducted,  not 
enough  time  is  allowed  for  equilibrium  between  the  marc  and  the 
solution  to  be  reached.  The  net  effect  of  this  is  that  those  constituents 
which  are  quickly  soluble  are  dissolved  out  while  others,  just  as 
soluble  but  more  slowly  dissolved,  are  extracted  in  a  fraction  of  their 
possible  amount  only  and  so  are  stretched  through  a  greater  volume 
of  percolate  than  is  necessary  or  desirable.  The  composition  of  the 
percolate  will  vary  qualitatively  according  to  the  time  it  has  been  in 
contact  with  the  drug.  Several  of  the  older  writers  seem  to  have 
thought  a  free  flow  of  percolate  desirable  because  they  disapprove 
certain  manipulations  which  "interfere  with  percolation"  and  ad- 
vocate measures  designed  to  remove  impediment  to  rapid  descent 
of  the  menstruum.  I  believe  that  such  ideas  are  founded  upon  false 
principles;  that  the  rate  of  flow  of  percolate  is  not  the  essential  factor 
but  that  the  rate  of  extraction  is  and  the  flow  of  percolate  should  be 
adjusted  to  yield  as  concentrated  a  liquid  as  the  time  at  the 
^  vStraub,  Arch.  exp.  Path.  u.  Pharm.  82,  327-343,  (1918). 
^  Proc.  A.  Ph.  A.  1904,  288. 
3  In  discussion,  Proc.  A.  Ph.  A.  1880,  550. 
4  Proc.  A.  Ph.  A.  1898,  684. 
5  Proc.  A.  Ph.  A.  1897,  245 
