74  Pharmaceutical  Use  of  Filter  Press.    { ^arm. 
tically  free  from  pulsations  but,  if  an  air  chamber  is  attached,  a 
single  plunger  will  answer  pharmaceutical  requirements. 
The  plunger  should  admit  of  adjustment  so  that  the  volume  of 
liquid  delivered  at  each  stroke  may  be  varied  to  compensate  the  vis- 
cosity of  the  mixture  being  filtered.  True,  this  might  be  controlled 
by  varying  the  speed  at  which  the  pump  is  driven  or  by  proper  regu- 
lation of  the  safety  tube  but  either  of  these  methods  involves  other 
difficulties  which  make  it  more  practicable  to  adjust  the  plunger. 
Our  plunger  is  2^4  in-  in  diameter  with  a  full  stroke  of  in.,  so 
that  the  total  volume  which  may  be  delivered  at  each  stroke  is  about 
5  (4-935)  fluidounces.  It  may  be  adjusted  to  any  lower  volume 
and,  for  filtering  light  liquids,  we  regulate  it  to  three  fluidounces  per 
stroke. 
The  speed  at  which  the  pump  should  be  run  is  determined  by  the 
filtering  area  of  the  press  and  the  volume  of  liquid  delivered  by  each 
stroke  of  the  pump.  This  must  be  determined  by  direct  experiment. 
The  ideal  condition  is  the  one  where  the  volume  of  liquid  forced  into 
the  press  by  the  pump  is  just  enough  to  fill  the  chambers  and  keep 
up  a  steady  flow  of  nitrate  without  causing  an  undue  increase  in 
pressure.  We  operate  our  pump  at  a  rate  of  sixty  strokes  per  min- 
ute, which  gives  us  a  possible  capacity  of  140  gallons  per  hour.  This 
is  never  attained  in  practise;  with  most  fluid  mixtures  we  operate 
at  a  rate  of  from  seventy  to  ninety-five  gallons  per  hour. 
The  valves  of  the  pump  should  be  easily  accessible  so  that  they 
may  be  readily  cleaned  during  and  after  the  nitration.  Small  par- 
ticles of  solid  matter  derived  from  sugar  or"  chemicals  frequently 
lodge  in  the  valves,  interfering  with  their  proper  closing  and  so 
causing  leakage.  This  decreases  the  efficiency  of  the  pump  and  may 
become  serious  enough  to  stop  the  nitration.  A  little  practise  will 
enable  the  operator  to  determine  which  valve  is  affected. 
The  packing  around  the  plunger  should  be  of  the  cold-water  type 
firmly  embedded  in  place.  Yet,  with  the  utmost  precautions,  there 
will  be  a  small  amount  of  leakage  through  it  whenever  the  pressure 
rises  above  one  hundred  pounds  per  square  inch.  Below  this  point 
leakage  may  be  prevented. 
The  pump  is  connected  with  the  press  by  an  iron  pipe  of  suitable 
size  in.  is  convenient),  so  arranged  that  it  may  be  taken  apart 
and  cleaned.  A  safety  tube,  closed  by  a  gate  valve,  is  .fitted  into 
this  by  a  T.  The  function  of  the  safety  tube  is  to  return  to  the 
barrel  any  excess  liquid  which  the  pump  is  delivering  to  the  press 
