350 
Facts  Concerning  Filtration. 
f  Am.  Jour.  Pharm. 
1      July,  1883. 
cloth  is  still  better.  For  larger  sizes,  say  of  from  4  to  8  pints,  it  is 
advantageous  to  dispense  with  the  funnel  altogether,  and  to  use  an 
inverted  cone  formed  of  linen  or  stout  calico ;  the  edges  being  fastened 
to  a  wooden  hoop,  which,  resting  on  a  deep  earthenware  pan,  forms  an 
efficient  support  for  the  paper,  the  liquid  passing  through  with  equal 
facility  over  the  entire  surface,  a  suitable  cover  placed  over  it  exclud- 
ing the  air,  and  the  process  goes  on  under  comparatively  satisfactory 
conditions.  A  self-feeding  arrangement  can  be  fitted  to  this,  if  it  be 
so  desired,  in  a  very  simple  manner. 
When,  by  exhausting  the  receiver,  atmospheric  pressure  is  brought 
to  bear  on  the  liquid  in  a  funnel,  then  the  latter  should  be  of  the  ortho- 
dox shape,  as  with  it  air  is  less  likely  to  pass;  but  this  requirement 
militates  against  the  advantages  that  such  a  method  would  otherwise 
possess.  The  point  of  the  filter  should  be  supported  by  a  cone  of 
platinum  or  zinc,  or  by  a  packing  of  tow  or  prepared  wool. 
English  paper  makers  do  not  appear  to  have  devoted  much  attention 
to  the  production  of  filters  in  any  variety,  and  for  this  reason  we 
derive  our  supplies  chiefly  from  the  continent.  It  is  a  well-known 
fact  that  holding  almost  any  of  the  common  filters  up  before  a  strong 
light  they  are  seen  to  be  perforated  more  or  less  with  minute  pinholes, 
so  that  when  in  use  it  is  only  after  these  have  become  filled  up  that 
the  whole  of  the  solid  matter  is  separated,  and  the  liquid  passes  Ihrough 
bright.  Each  time  a  fresh  portion  of  liquid  is  added,  the  disturbance 
caused  thereby  is  liable  to  remove  some  of  the  particles  which  are  act- 
ing as  a  filling,  and  if  this  occurs  filtration  again  becomes  imperfect. 
These  filters,  although  very  cheap,  do  not  pay  to  use  if  time  and  con- 
venience are  taken  into  consideration.  There  is,  however,  considerable 
diffi^rence  in  the  efficiency  of  the  various  kinds  of  filtering  papers,  even 
when  free  from  this  defect.  The  presence  of  animal  matter,  as  in  the 
gray  filter,  increases  the  strength,  but  diminishes  its  working  capabili- 
ties, and  the  existence  of  mineral  matters  therein  does  the  latter,  but 
not  the  former.  The  papers  specially  prepared  by  Messrs.  Schleicher 
and  Schiill  are  practically  free  from  all  extraneous  matters,  the  pulp 
having  been  treated  with  hydrochloric  and  hydrofluoric  acids,  etc. 
They  are  an  example  of  what  can  be  accomplished  in  this  respect,  but 
at  the  same  time  they  are  too  expensive  for  general  pharmaceutical 
purposes,  and,  indeed,  are  only  made  in  comparatively  small  sizes 
suitable  for  analytical  work.  For  operations  requiring  filters  of  7 
inches  diameter  (before  folding),  the  Rhenish  papers,  No.  595,  are,  in 
