542  Production  of  the  Alkaline  Metals.  {^x0T^6arm' 
The  necessary  cover  for  the  crucible  is  fixed  stationary  in  each  cham- 
ber, and  from  this  cover  projects  the  tube  to  the  outside  of  the  furnace 
into  the  receptacle  for  the  condensation  of  the  metallic  vapors.  When 
operating,  the  crucibles  are  charged  with  the  mixture,  made  as  before 
described,  placed  upon  the  lift,  which  is  situated  directly  below  the 
aperture  made  in  the  bottom  of  the  chamber,  and  raised  into  the 
furnace.  The  edges  of  the  covers  are  made  convex,  while  the  edges 
of  the  crucibles  are  concave,  so  that  when  the  crucibles  are  raised  the 
edges  come  together  and,  being  held  from  below,  form  a  tight 
joint,  preventing  the  exit  of  gas  and  vapor,  excepting  through  the 
tube  provided  from  the  cover.  Gas,  which  is  used  as  a  fuel  in  con- 
nection with  heated  air,  is  allowed  to  enter  the  chamber,  after  the 
crucible  containing  the  charge  is  in  place,  and  the  reduction  and 
distillation  commences  as  soon  as  the  crucible  contents  have  reached 
the  temperature  of  1000°  C,  the  sodium  being  reduced  in  the  crucible 
and  distilled  therefrom  into  the  condenser,  by  passing  through  the 
tube  projecting  from  the  crucible  cover.  As  soon  as  the  operation  is 
finished,  the  crucible  is  lowered  and  a  new  one,  containing  a  fresh 
charge  is  raised  in  its  place,  while  the  crucible  just  removed  is  cooled, 
cleaned  and  recharged.  The  appliances,  which  hold  the  cover  in 
place,  are  so  arranged  that  the  cover  may  be  removed  whenever  ne- 
cessary without  interrupting  the  process,  which  is  practically  continu- 
ous. By  avoiding  the  careful  mixing  and  calcining  of  ingredients 
preparatory  to  charging  the  cylinders,  and  in  carrying  out  generally 
the  mechanical  part  of  the  old  process,  the  details  of  which  require 
the  most  careful  management,  the  method  just  described  will  be  seen 
to  have  many  advantages.  As  the  charge  is  fused,  the  alkali  and  re- 
ducing material  are  in  direct  contact,  and  this  fact,  together  with  the 
aid  rendered  the  carbon  by  the  fine  iron,  in  withdrawing  the  oxygen 
from  the  soda,  fully  explains  the  chemical  reasons  why  the  reduction 
is  accomplished  at  a  moderate  temperature.  Furthermore  by  reducing 
the  metal  from  a  fused  mass,  in  which  the  reducing  agent  remains  in 
suspension,  the  operation  can  be  carried  on  in  crucibles  of  large  di- 
ameter, the  reduction  taking  place  at  the  edges  of  the  mass,  where 
the  heat  is  greatest  and  the  charge  flowing  thereto  from  the  centre  to 
take  the  place  of  that  reduced.  By  the  old  process,  the  heat  required 
to  penetrate  to  the  centre  of  the  refractory  charge  and  reduce  the  soda 
there  situated  in  addition  to  the  heat  absolutely  needful  to  bring  about 
reduction,  necessitates  the  use  of  wrought-iron  vessels,  and  even  when 
