Am.  Jour.  Pharm.  ) 
November,  1920.  j 
The  Theory  of  Percolation. 
793 
has  devoted  much  attention  to  this  question  especially  with  reference 
to  its  bearing  on  precipitation  in  fluidextracts.  The  writings  of 
Squibb/  Diehl,^  and  others  present  data  which  bears  on  this  prob- 
lem. 
As  the  menstruum  descends  through  the  drug  it  dissolves  the  ex- 
tractive which  diffuses  into  it  and  its  concentration  increases.  With 
this  increase  in  concentration  its  solvent  powers  diminish  so  that  the 
rate  at  which  it  extracts  soluble  material  from  the  moist  drug  decreases 
while  the  loss  of  soluble  material  due  to  adsorption  becomes  greater. 
All  solutions  are  subject  to  adsorption,  for  contact  with  a  surface 
leads  to  this  under  the  proper  conditions  of  surface  tension.  In- 
crease of  surface  area  and  of  concentration  in  solution  lead  to  in- 
creased adsorption.  This  is  a  factor  which  opposes  extraction  and 
one  which  intimately  concerns  the  composition  of  the  precolate. 
Out  of  the  great  variety  of  classes  of  extractible  drug  constituents 
from  the  water-soluble  proteins  to  the  alcohol -soluble  terpenes  and 
resins,  there  is  present  in  the  percolate  an  extremely  complex  and 
variable  mixture,  the  components  of  which  differ  in  the  degree  to 
which  they  are  adsorbed  as  well  as  in  their  relative  solubilities  and 
concentrations.  In  such  a  solution  slight  variations  of  temperature 
and  pressure  as  well  as  gain  of  certain  constituents  by  solution  and 
part  loss  of  others  through  adsorption  may  readily  upset  the  nicely 
adjusted  equilibrium  and  lead  to  precipitation  within  the  percolator. 
Such  a  condition  as  this  would  lead  to  striking  differences  in  different 
portions  of  the  percolate. 
Again,  as  the  precolate  descends  the  percolator  and  becomes  more 
and  more  saturated,  two  things  happen ;  its  solvent  power  for  more  of 
the  substances  already  in  solution  diminishes  while  its  alcoholic 
strength  decreases.  The  latter  fact  may  so  alter  the  quality  of  its 
solvent  action  that  there  may  occur  a  solution  in  quantity  of  sub- 
stances which  were  but  slightly  dissolved  out  of  the  upper  layers  of 
the  drug.  In  certain  cases  this  will  result  in  a  readjustment  of  the 
solution  and  precipitation  may  result.  The  net  effect  of  such  an 
occurrence  is  an  actual  transfer  of  soluble  materials  from  the  upper 
to  the  lowers  layers  of  the  percolator,  the  deposited  matter  being 
forced  to  wait  for  menstruum  of  proper  quality  before  it  can  be  re- 
dissolved  and  so  extracted.  The  effect  of  mixing  the  first  and  last 
portions  of  percolate  under  such  conditions  is  obvious. 
1  This  Journal,  Vol.  38,  109.  (1866);  39,  289,  398,  513,  (1867);  40,  i,  (1868). 
Proc.  A.  Ph.  A.  1878,  681;  1880,  424. 
