378 
VARIETIES. 
shrink  less  in  drying,  and  retain  more  cohesion  and  solidity  in  the  fire.  In 
this  case,  of  course,  lixiviation  is  necessary. 
In  any  case  the  lime  and  felspar  must  be  in  a  state  of  the  finest  division 
before  they  are  mixed  together.  The  author  does  not  think  it  necessary  to 
say  anything  about  the  proportionate  weights  beyond  what  has  been  already 
stated  ;  it  is  impossible  to  give  any  definite  numbers,  as  they  would  be  dif- 
ferent for  each  raw  material,  for  which  reason  a  preliminary  analysis  is 
necessary.  So  much  lime  must  always  be  added,  that  3  or  4  equivs.  of  base 
may  be  presented  to  1  equiv.  of  acid.  It  is  to  be  observed,  however,  that 
as  the  materials  are  obtained  in  the  form  of  a  fine  mud,  the  amount  of  mois- 
ture in  them  must  be  determined,  when  the  proper  quantities  may  be  ar- 
ranged by  measure  upon  this  basis.  Measuring  in  this  way  is  more  exact 
and  convenient  than  weighing. 
The  intimate  mixture  of  the  materials  is  most  conveniently  effected  by 
means  of  a  clay-mill,  the  usefulness  of  which  is  now  well  known.  The 
paste  is  allowed  to  pass  through  until  it  is  perfectly  homogeneous.  As 
soon  as  this  is  the  case,  the  mixture  issuing  from  the  clay-mill  is  cut  by 
the  machine  itself  into  cylindrical  pieces  of  5  to  6  inches  in  length,  and  2 
to  2§  inches  in  diameter.  These  are  slowly  dried,  and  then  put  in  the 
furnace  to  be  burnt. 
The  best  furnace  for  burning  the  mass  is  the  porcelain  furnace,  because 
a  more  uniform  heat  can  be  attained  in  all  parts  of  it  than  in  the  ordinary 
tile-furnaces.  The  latter  may,  however,  be  employed.  A  blast-furnace 
with  a  permanent  blast  would  also  be  suitable,  although  inequalities  of 
temperature  are  very  liable  to  occur  in  it  in  different  parts.  The  porcelain 
furnaces  may  be  2  or  3  stages  high,  and  be  furnished  with  4  or  6 
charges.  Any  fuel  may  be  used,  as  the  ashes  carried  on  by  the  draught 
cannot  produce  the  same  injury  here  as  in  the  burning  of  porcelain.  The 
necessary  temperature  is  a  bright  red  heat,  but  it  should  be  ascertained  for 
each  material  by  some  preliminary  trial  burnings,  as  the  greater  or  less 
degree  of  fusibility  plays  an  important  part,  and  fusion  is  not  necessary. 
The  cylinders  contract  considerably  by  burning,  and  are  partly  broken  up. 
They  are  ground,  and  then  mixed  with  water  in  the  steam-kettle,  in  which 
the  decomposition  is  to  take  place. 
For  the  sake  of  simplicity  and  easy  management,  several  kettles  are 
heated  by  the  steam  of  one  generator.  It  is  then  unnecessary  to  moderate 
the  fire  during  the  emptying  of  the  kettles,  but  the  refrigeration  necessary 
for  emptying  and  filling  them  may  be  produced  by  simply  shutting  off  the 
steam.  A  double  bottom  is  also  unnecessary,  as  a  solidification  of  the  mass 
and  consequent  overheating  of  the  wall  of  the  kettle  cannot  take  place. 
The  powder  is  put  into  the  kettle  by  a  suitable  arrangement;  the  necessary 
quantity  of  water  is  let  in,  and  then  the  connexion  with  the  steam-genera- 
tor is  established.  Fluid  may  be  drawn  off  by  a  cock,  to  ascertain  the  quan- 
tity of  alkali  dissolved.  When  the  decomposition  is  completed,  the  solution 
