244  PREPARATION  OE  LIQUOR  POTASS^. 
important  that  the  precipitate  as  well  as  the  solution  should  be 
put  into  the  filter.  In  fact,  it  is  the  precipitate  that  forms  the 
most  efficient  filtering  medium;  without  it  the  liquid  would  not 
pass  through  clear ;  but  if  the  whole  of  the  precipitate  be  turned 
into  the  bag  or  other  filter  with  the  liquid,  the  latter  will  almost 
immediately  run  through  sufficiently  clear  for  use.  This  fil- 
tration or  percolation  of  the  liquid  through  the  precipitate  also 
serves  to  efi'ect  the  decomposition  of  any  remaining  portion  of 
carbonate  of  potash,  if  there  should  be  such.  I  have  sometimes 
completed  the  decomposition  in  this  way  by  passing  the  solution 
through  the  precipitate  several  times,  and  have  found  it  an  ex- 
peditious mode  of  operating. 
Liquor  Potassae  prepared  by  this  process  will  be  as  free  from 
impurity  as  the  nature  of  the  substances  used  for  its  production 
"will  admit.  The  alumina  always  present  in  the  lime,  although 
dissolved  to  a  considerable  extent  in  the  early  part  of  the  pro- 
cess, before  the  decomposition  of  the  carbonate  of  potash  is 
complete,  is  ultimately  precipitated  by  the  excess  of  lime  with 
which  it  forms  a  compound  insoluble  in  the  cold.  To  some  ex- 
tent this  precipitation  is  effected  when  heat  is  employed ;  but  in 
operating  with  heat,  a  little  alumina  is  retained  in  solution.  In 
this  case,  even  repeated  agitation  after  the  liquor  has  cooled 
does  not  entirely  remove  the  alumina,  unless  some  fresh  milk  of 
lime  be  added. 
The  absence  of  alumina  is  an  important  feature  in  the  Liquor 
Potasses  made  by  the  cold  process,  and  one  that  will  be  appre- 
ciated by  those  who  have  occasion  to  use  it  for  chemical  purpo- 
ses. To  ensure  the  absence  of  this  contamination,  however,  it 
is  essential  that  excess  of  lime  should  be  used,  and  this  is  one 
of  the  objects  contemplated  in  augmenting  the  quantity  of  lime 
ordered,  and  endeavoring  to  ensure  its  good  quality. 
The  silica  which  commercial  carbonate  of  potash  contains, 
will  be  dissolved  by  the  caustic  alkali,  and  this  constitutes  one 
of  the  usual  impurities  of  Liquor  Potassae.  It  is  stated  by  some 
authors  that,  in  the  process  of  boiling,  the  silica  forms  an  in- 
soluble compound  with  lime,  and  is  thus  removed ;  but  I  do  not 
find  that  this  separation  is  complete.  There  is  a  little  silica  re- 
tained in  solution,  whichever  process  is  adopted,  and  I  cannot 
discover  any  difference  in  the  quantity  present  in  solution  pre- 
pared from  the  same  materials  by  the  different  processes. 
