Am  jour,  pharm.j    Pharmaceutical  Use  of  Filter  Press. 
February,  19 17.    '  1 
71 
THE  PHARMACEUTICAL  USE  OF  THE  FILTER  PRESS. 
By  James  F.  Couch  and  James  E.  Kersey. 
Filtration  may  be  denned  as  the  resolution  of  a  disperse  system 
into  liquid  and  solid  phases  by  means  of  a  diaphragm  permeable  by 
the  liquid  only.  In  practise  such  a  perfect  separation  of  the  system 
is  seldom,  probably  never,  realized.  We  submit  mixtures  to  a 
process  of  straining  whereby  all  particles  which  are  larger  than  a 
certain  minimum  size  are  retained  by  the  strainer  and  all  others  pass 
through  into  the  nitrate.  The  filtered  liquid  may  at  first  appear 
crystalline,  yet  after  months  of  undisturbed  standing  on  a  shelf  a 
slight,  fine  precipitate  will  be  found  in  the  bottom  of  the  container 
caused  by  the  agglomeration  and  deposition  of  the  invisible  particles 
which  the  strainer  did  not  retain. 
Accomplishment  of  the  most  complete  separation  of  the  phases 
in  a  minimum  of  time  constitutes  the  problem  of  filtration.  Let  us 
consider  the  various  means  employed  for  its  solution. 
In  general,  filtration  processes  may  be  divided  into  two  classes : 
first,  those  which  depend  upon  gravity,  and  second,  those  in  which 
extra  pressure  is  employed.  Of  the  first  class  the  most  commonly 
used  is  the  funnel  and  filter  paper  method.  This  is  adapted  to  all 
sorts  of  variations  and  is  quite  satisfactory  for  small  volumes  of 
liquids.  Its  great  disadvantages  consist  in  the  continual  diminu- 
tion in  the  rate  of  filtration  and  the  evaporation  of  solvent  from  the 
surface  of  the  mixture  which  causes  a  change  in  the  menstruum,  a 
rearrangement  of  the  solution,  and,  possibly,  precipitation  with  the 
first  portions  of  filtrate.  If  a  filtering  agent  is  used  with  the  paper 
the  rate  of  filtration  is  retarded  still  more  and  may  be  entirely 
stopped  while  the  volume  of  liquid  absorbed,  and  consequently  lost, 
is  very  much  increased. 
Filter  bags  of  felt  are  occasionally  employed.  They  are  useful 
for  hot  filtration  or  for  syrups  but  are  slow  and  permit  visible  parti- 
cles, notably  starch,  to  pass  through,  so  that  filtrates  from  them  are 
frequently  cloudy.  Straining  through  cotton  or  woolen  cloths  is 
resorted  to  when  coarse  precipitates  are  to  be  separated  but  this 
method  has  only  a  limited  application  in  pharmacy. 
There  are  in  use  devices  for  securing  added  pressure  in  filtration 
by  contriving  a  column  of  liquid  several  feet  in  height  in  such  a  way 
