72  Pharmaceutical  Use  of  Filter  Press.    { friary, 
that  it  may  exert  force  upon  the  liquid  which  is  filtering.  Such  ar- 
rangements are  intricate,  involve  an  undue  amount  of  room,  and 
may  present  difficulties  in  cleansing. 
The  Sperry  laboratory  or  10"  filter  press  equipped  with  plates  and  frames 
which  are  convertible  into  the  various  arrangements  used  in  commercial  ap- 
paratus. It  can  be  used  as  a  press  equipped  for  "  Thorough  Washing "  or 
"  Simple  Washing."  Arrangements  are  provided  so  that  either  of  the  above 
two  arrangements  can  be  used  with  separate  discharge  for  washing  or  not,  as 
desired.  Filtrate  can  be  made  to  discharge  from  separate  cocks,  from  each 
plate  or  from  one  large  cock  at  the  head  of  the  press.  In  practice  the  former 
arrangement'  is  called  "  Open  Discharge,"  while  the  latter  is  known  as  "  Closed 
Discharge."  It  is  a  commercial  machine  in  every  respect.  Other  sizes  are 
18",  24",  30",  36",  and  42".  Plates  and  frames  can  be  made  of  iron,  as  shown 
in  the  cut,  or  of  lead  or  wood. 
Added  pressure  secured  by  means  of  the  suction  pump  or  aspi- 
rator is  very  infrequently  of  use  in  pharmacy.  The  most  serious 
objection  to  this  method  arises  from  the  fact  that  the  filtrate  is  neces- 
sarily exposed  to  low  atmospheric  pressure  and  under  such  condi- 
tions the  volatilization  of  alcoholic  menstrua  is  so  rapid  that  it  con- 
demns the  process. 
Centrifugal  filters  are  rapid  and  efficient.  They  are,  however, 
better  adapted  to  cases  where  the  solid  matter  is  the  desired  product, 
as  in  the  separation  of  mother  liquor  from  crystals,  than  for  strictly 
pharmaceutical  purposes  where  the  insoluble  substances  are  com- 
