VARIETIES. 
377 
manufacture  of  potash  would  appear  to  be  remunerative.  Of  course,  in 
this  case,  the  chemical  composition  is  to  be  taken  into  consideration,  and  the 
amount  of  lime  added  to  be  modified  accordingly.  All  that  has  to  be  done 
is  to  establish  the  proportion  of  3  or  4  equivs.  of  base  to  1  equiv.  of  acid, 
in  which  potash,  soda,  lithia,  lime,  and  magnesia  are  to  be  regarded  as 
bases;  and  silica,  alumina,  and  oxide  of  iron  as  acids.  Any  chlorine  or 
fluorine  that  may  exist  has  no  influence  ;  and  magnesia,  instead  of  being 
injurious,  has  been  found  to  be  preferable  to  lime  for  the  separation  of  pot- 
ash. Moreover,  it  is  well  known  that  mica,  which  would  play  an  impor- 
tant part  when  granite  is  employed,  is  far  more  easily  decomposed  than 
felspar,  for,  as  Mitscherlich  has  lately  discovered,  it  is  even  completely  de- 
composed by  muriatic  acid  in  a  glass  tube  at  212°  F. 
The  points  to  be  observed  in  carrying  out  this  process  on  a  large  scale 
are  now  to  be  referred  to  ;  these,  however,  may  easily  require  modification 
by  local  and  other  circumstances. 
As  the  abundant  result  in  potash  depends  especially  upon  the  complete 
decomposition  of  the  felspar,  and  the  latter  can  only  be  effected  by  a  very 
intimate  mixture  with  lime,  the  greatest  attention  is  to  be  paid  to  the  fine 
division  of  the  substances  to  be  employed,  in  order  that  in  the  intermix- 
ture the  portions  of  felspar  may  be  in  contact  with  the  lime  in  many  places. 
The  felspar,  or  the  mineral  containing  felspar  (of  course,  only  granites  with  a 
small  proportion  of  quartz  will  be  operated  on),  is  burnt  in  a  furnace  which 
works  uninterruptedly,  or  in  any  reverberatory  furnace,  taken  out  of  the 
fire  whilst  still  red-hot  and  thrown  into  water.  By  this  treatment  it  will 
be  split  in  every  direction,  and  rendered  sufficiently  soft  for  further  divi- 
sion. It  is  then  powdered  under  stampers  or  between  cast-iron  crushing 
rollers,  and  afterwards  ground  with  water  upon  mill-stones.  The  bottom 
stone  and  the  runner  must  be  of  quartz  or  granite,  and  possess  consider- 
able weight.  The  finely  ground  powder  is  then  passed  through  sieves  into 
the  lixiviating  apparatus,  very  finely  lixiviated,  and  conducted  into  pits  to 
settle.  It  is  of  the  greatest  importance  only  to  employ  fine  lixiviated 
powder  in  the  manufacture,  as  this  greatly  facilitates  and  hastens  the 
decomposition  by  ignition,  and  causes  a  saving  in  fuel.  The  time  occupied 
in  lixiviation  is  not  so  considerable,  as  it  might  appear  at  the  first  glance, 
as  the  rule  adopted  in  the  porcelain  factories  is  not  to  be  applied  here. 
The  greater  specific  gravity  of  the  felspar  causes  it  to  settle  far  more 
rapidly  than  clay  ;  it  is  unnecessary,  as  in  porcelain  factories,  to  bestow 
great  care  on  purity,  on  the  exclusion  of  dust,  iron,  &c,  so  that  the  simplest 
arrangement  is  sufficient  for  the  purpose.  The  coarser  powder  is,  of  course, 
ground  again. 
A  similar  fine  division  is  required  for  the  lime,  and  when  this  is  employ- 
ed in  the  burnt  state,  it  is  most  completely  effected  by  slaking.  Neverthe- 
less, when  circumstances  admit  of  the  employment  of  carbonate  of  lime, 
the  latter  is  to  be  preferred,  because  the  balls  or  cakes  prepared  with  it 
