78  Pharmaceutical  Use  of  Filter  Press.    {A^/U°aury  F^*r7m' 
ing  the  interstices  of  the  paper  and  clogging  it.  To  line  the  cham- 
bers it  is  necessary  only  to  mix  a  small  amount  of  filtering  agent 
with  five  gallons  of  water  and  pump  the  mixture  through  the  press 
continuing  until  all  the  water  is  drained  out.  In  general,  the  first 
liquid  which  flows  from  the  press  will  be  cloudy  and  a  clear  filtrate 
will  not  be  obtained  until  a  small  layer  has  formed  on  the  papers. 
The  fourth  category  of  liquids  is  represented  by  strongly  alco- 
holic or  ethereal  fluids.  With  these  the  press  has  a  decided  advan- 
tage over  all  other  methods  for  filtration,  for  by  employing  the  chan- 
nel into  which  the  chambers  drain  it  is  possible  to  filter  such  mix- 
tures with  a  minimum  of  loss  of  solvent. 
The  selection  of  the  filtering  agent  may  decide  the  success  or 
failure  of  the  process.  Some  such  agent  is  necessary  in  all  phar- 
maceutical uses  of  the  press  to  assist  in  the  solution  of  oils,  to  dilute 
gummy  or  resinous  precipitates  and  prevent  them  from  clogging 
through  deposition  in  the  interstices  of  the  papers,  and  to  form 
nuclei  on  which  microscopic  particles  of  precipitate  may  agglom- 
erate and  form  grains  of  filterable  size.  Kieselguhr,  talcum,  kaolin, 
magnesium  carbonate,  and  calcium  phosphate  may  be  used  and  their 
practicability  appears  to  be  in  the  stated  order.  Kieselguhr  is  uni- 
versally applicable;  it  is  light  in  texture,  fine  in  size  and  does  not 
pack  firmly  on  the  papers.  It  is  chemically  quite  inert.  Some- 
times an  admixture  with  talcum  may  be  used  and,  for  heavy  work, 
this  is  judicious.  Talcum  itself  is  second  only  to  kieselguhr,  but  is 
likely  to  pack  firmly  on  the  papers,  which  unfits  it  for  use  with 
viscous  liquids. 
Kaolin  cannot  be  used  in  the  presence  of  mucilage,  starch,  di- 
gestive ferments,  or  ox  gall,  with  which  it  forms  a  slimy  mass  that 
clogs  persistently  and  so  decreases  the  rate  of  filtration.  For  simple 
elixir  and  similar  preparations  it  will  serve,  but  presents  no  advan- 
tages over  kieselguhr.  Magnesium  carbonate  and  calcium  phosphate 
are  excluded  from  nearly  all  mixtures  by  their  chemical  activity.  It 
is  probable  that  much  of  the  precipitation  which  has  occurred  in  bot- 
tled preparations  may  be  traced  to  the  use  of  either  one  of  these  salts 
in  filtration.  Neither  of  them  has  any  advantages  not  possessed  by 
kieselguhr.  They  should  never  be  used  in  acid  liquids  and  the  mag- 
nesium compound  will  precipitate  acid  coloring  matters,  i.  e.,  car- 
mine, from  solution. 
The  best  results  cannot  be  obtained  even  if  all  the  above  men- 
tioned points  are  scrupulously  observed  unless  the  apparatus  itself  is 
