Am  jour.  Pharm.)    Pharmaceutical  Use  of  Filter  Press. 
February,  19 17.    }  J 
75 
above  the  amount  which  the  press  is  filtering.  This  affords  a  second 
means  of  controlling  the  pressure  and  is  of  service  in  those  cases 
where  the  pressure  gradually  rises  during  the  filtration  as  the  cake 
of  precipitate  thickens  in  the  chambers.  With  easily  filtered  liquids 
the  safety  tube  may  be  kept  closed  and  it  should  never  be  opened 
until  rising  pressure,  indicated  on  the  gauge,  demands  some  release. 
The  air  chamber  may  be  placed  anywhere  between  the  pump  and 
first  chamber  of  the  press  on  the  connecting  pipe.  If  a  single- 
plunger  type  of  pump  is  employed  the  air  chamber  becomes  a  neces- 
sity to  equalize  the  pressure.  Without  it,  the  pressure  will  vary 
during  each  stroke  of  the  plunger,  sometimes  as  much  as  eighty 
pounds,  rising  and  falling  in  pulsations.  The  effect  of  such  a  con- 
tinual pounding  is  to  pack  the  cake  firmly  in  the  chambers  and  even- 
tually this  concreted  wall  becomes  impenetrable  and  filtration  ceases. 
The  air  chamber  takes  up  the  excess  pressure  and,  even  with  the 
most  refractory  liquids,  diminishes  the  variation  in  pressure  to,  at 
most,  twenty  pounds.  A  pet  cock  should  be  fitted  to  the  lower  ex- 
tremity of  the  air  chamber  to  allow  it  to  be  drained  and  cleaned. 
The  pressure  gauge  is  usually  placed  just  before  the  first  cham- 
ber of  the  press.  Its  functions  are  too  obvious  to  require  comment 
here. 
After  leaving  the  pump  the  liquid  enters  the  press,  is  conducted 
by  a  channel  into  the  several  chambers,  passes  through  the  filter- 
paper  walls,  and  drains  out  at  the  bottom  into  a  trough  which  leads 
it  into  a  covered  receptacle. 
The  press  consists  of  a  number  of  solid  plates  of  iron  separated 
by  hollow  "  distance "  frames  so  arranged  that  the  whole  may  be 
securely  clamped  together,  forming  a  series  of  chambers  or  cells. 
Lateral  openings  connect  them  with  the  channel,  through  which  the 
mixture  enters,  bored  throughout  the  length  of  the  press  and  termi- 
nating in  a  pet  cock  which  is  used  to  permit  the  escape  of  air  when 
the  press  is  started.  The  vertical  walls  of  the  chambers  are  formed 
by  the  solid  plates  which  are  corrugated  in  such  a  way  as  to  lead  the 
filtered  liquid  to  an  outlet  in  one  of  the  lower  corners  through  which 
the  filtrate  is  discharged. 
The  actual  filtration  is  accomplished  in  these  chambers.  Heavy 
sheets  of  canvas  are  suspended  over  the  faces  of  the  solid  plates 
and,  for  technical  use,  these  serve  sufficiently  well ;  for  pharmaceu- 
tical purposes,  however,  better  filtration  is  demanded  and  the  cloth 
is,  therefore,  covered  with  one  or  more  sheets  of  filter  paper.  The 
