PREPARATION  OF  LIQUOR  P0TASSJ5.  243 
latter  to  break,  but  a  small  calico  filter  will  be  found  fully  to 
answer  the  required  purpose.  When  the  solution  is  found  to  be 
perfectly  caustic  it  may  either  be  filtered  at  once  through  calico 
or  linen,  or  the  precipitate  may  be  allowed  to  subside  and  the 
clear  liquor  decanted  off".  In  the  latter  case,  some  days  should 
be  allowed  for  the  subsidence  of  the  precipitate,  the  particles  of 
which  will  become  more  and  more  aggregated,  thus  occupying 
a  smaller  space  and  leaving  a  larger  amount  of  supernatent 
liquid.  The  aggregation  of  the  precipitate  will  thus  take  place 
in  time  to  nearly  the  same  extent  as  occurs  more  quickly  when 
the  ingredients  are  boiled  together.  If  the  solution  be  filtered, 
this  may,  and  indeed  should,  be  done  before  the  precipitate  has 
aggregated  into  a  hard  mass  at  the  bottom  of  the  bottle.  It 
does  this  after  standing  for  some  time,  and  is  then  removed 
with  difficulty.  A  calico  or  linen  filter  should  be  used,  as  neither 
of  these  are  sensibly  acted  upon  by  the  caustic  solution,  especi- 
ally after  they  have  been  once  used.  When  new,  a  little  color 
is  taken  from  them,  and  more  particularly  from  the  linen,  but 
this  is  removed  by  the  first  portion  filtered,  or  the  effect  may  be 
prevented  by  previously  washing  the  filter  with  some  caustic 
alkali,  and  the  same  filter  may  be  afterwards  employed  repeated- 
ly without  any  further  contamination  of  the  liquid.  The  filtra- 
tion will  take  place  most  rapidly  through  the  conical  filter-bag, 
commonly  called  Hippocrates'  sleeve,  which,  when  used  for  this 
purpose,  instead  of  having  a  hoop  to  keep  the  mouth  open, 
should  be  merely  provided  with  a  wide  hem  having  three  equi- 
distant openings  to  admit  three  pieces  of  straight  glass  rod 
passed  through  the  hem  from  one  opening  to  another,  and  these 
must  be  long  enough  to  project  a  little  way  out  of  the  openings 
in  each  direction.  Three  cords  with  loops  placed  over  these 
projecting  ends  will  afford  the  means  of  suspending  the  bag 
when  in  use,  and  on  afterwards  withdrawing  the  rods  the  bag 
may  be  easily  washed  and  put  away  until  again  required.  If  it 
should  be  thought  that  this  method  of  filtration  would  expose 
the  solution  too  much  to  the  air,  a  piece  of  calico  or  linen  may 
be  cut  to  the  form  of  an  ordinary  filter,  and  placed  in  a  funnel 
covered  with  a  common  plate,  the  neck  of  the  funnel  being  in- 
serted into  the  mouth  of  a  green  glass  bottle  intended  for  the  re- 
ception of  the  liquid.    In  the  filtration  of  Liquor  Potassae,  it  is 
4 
