Am.  Jour.  Pharm.  ) 
November,  1920.  ) 
The  Theory  of  Percolation. 
787 
acid  less,  and  benzol,  alcohol,  ether,  chloroform,  and  petroleum  ether 
are  still  less  absorbed.  When  an  hydro-alcoholic  menstruum  is  used 
it  is  not  uncommon  for  the  drug  to  absorb  more  water  than  alcohol 
from  the  menstruum  so  that  the  composition  of  the  liquid  portion  of 
the  drug  mass  will  be  different  from  that  of  the  unabsorbed  men- 
struum, a  phenomenon  which  may  have  an  important  effect  upon 
percolation  and  which,  like  many  others,  needs  investigation.' 
When  the  moistened  drug  has  swollen  sufficiently  it  may  be  passed 
through  a  coarse  sieve  to  break  up  lumps,  though  this  process  is 
usually  omitted,  and  then  packed  in  the  percolator. 
The  outlet  of  the  percolator  is  plugged  with  something  to  prevent 
drug  from  being  washed  out  with  the  percolate;  cotton  is  used  in 
small  operations,  excelsior  for  larger  batches.  The  amount  used  is  so 
small  that  the  effect  on  the  process  due  to  it,  while  definite,  is  negli- 
gible. Covering  the  diaphragm  with  muslin,  cheese  cloth,  or  filter 
paper  is  a  method  quite  commonly  employed. 
The  packing  of  the  drug  in  the  percolator  demands  a  certain 
amount  of  skill  and  the  personal  equation  enters  largely  into  it. 
Individual  operators  differ  widely  in  the  manner  in  which  they  pack 
the  drug  and  it  is  difficult  to  devise  directions  which  will  lead  to  the 
same  result  in  the  hands  of  different  men.  It  is  usually  stated  that 
the  drug  should  be  packed  evenly  and  firmly  so  as  to  lead  to  an  even 
and  not  too  rapid  descent  of  menstruum  through  it  without  the  for- 
mation of  channels  or  regions  of  varying  density  in  the  same  hori- 
zontal plane.  The  degree  of  firmness  to  be  attained  depends  upon 
the  character  of  the  drug  and  the  menstruum.  In  general,  the  looser 
the  drug  fibre  and  the  richer  the  menstruum  is  in  alcohol,  the  firmer 
the  packing  should  be.  All  drugs,  however,  should  be  packed  very 
tightly  particularly  if  they  have  been  allowed  to  swell. 
Rosenwasser^  suggested  that  greater  force  should  be  used  in  packing 
the  upper  layers  of  drug  than  was  used  on  the  lower  layers  since  the 
force  employed  on  the  upper  served  also  to  compress  the  lower  por- 
tions and,  therefore,  if  a  uniform  force  were  to  be  used  throughout 
the  packing,  the  lowermost  drug  would  be  compressed  more  than  the 
top  layers.  This  is  a  valuable  practical  idea  and  does  not  appear  to 
have  attracted  the  attention  which  it  merits. 
Another  neglected  suggestion  is  that  of  J.  W.  Mill'^  who  advised 
the  separation  of  the  drug  into  powders  of  various  degrees  of  fineness, 
'  Proc.  A.  Ph.  A.  1882,  519. 
2  This  Journal,  Vol.  43,   17,  (1871). 
