PREPARATION  OF  ALUMINIUM,  SODIUM,  ETC. 
419 
which  disengage  ammonia  when  brought  in  contact  with  water, 
and  cause  a  further  destruction  of  aluminium.  It  is  also  neces- 
sary to  avoid  melting  the  aluminium  while  it  contains  sodium,  in 
which  case  it  takes  fire.  It  is  always  better  to  melt  it  together 
with  some  of  the  double  chloride  of  aluminium  and  sodium. 
The  sodium  was  obtained  by  heating  a  mixture  of  carbonate 
of  soda  and  carbon  in  an  iron  vessel.  In  operating  upon  the 
large  scale  coal  is  used,  and  the  proportions  of  the  materials  that 
furnished  the  best  result  were — 
Carbonate  of  soda  (dry)  ...  30 
Coal  13 
Chalk  5 
The  characters  requisite  in  this  mixture  are  that  it  should  not 
melt  at  the  temperature  at  which  sodium  is  liberated,  and  that 
it  should  acquire  a  pasty  consistence  so  as  to  remain  in  contact 
with  the  sides  of  the  vessel  in  which  it  is  heated.  The  high  la- 
tent heat  of  carbonic  oxide  and  of  gaseous  sodium  prevents  the 
iron  vessel  from  being  melted. 
These  substances 
are  dried,  pow- 
dered, and  sifted ; 
then  mixed  and 
sifted  again,  so  as 
to  be  intimately 
mixed.  The  mix- 
ture should  then  be 
calcined.  By  this 
means  its  volume 
is  considerably  re- 
duced, and  when 
the  calcination  can 
be  effected  by  the 
waste  heat  of  a 
furnace,  considera- 
ble advantage  is 
gained. 
