240      ON  PREPARATION  OF  ALUMINIUM  FROM  CRYOLITE. 
of  aluminium  thus  formed  is  absorbed  so  readily  by  the  chloride 
of  sodium,  that  scarcely  any  is  deposited  at  other  parts  of  the 
apparatus. 
Several  modifications  of  the  preparation  of  aluminium  were 
tried,  but  the  one  described  was  found  most  advantageous. 
When  the  sodium  was  placed  at  the  bottom  of  the  crucible, 
above  it  the  cryolite  powder,  and  then  the  chloride  of  potassium, 
much  of  the  sodium  escaped  in  vapor,  which  was  not  the  case 
when  the  sodium  was  cut  in  thin  slices  and  placed  in  layers  with 
the  cryolite. 
After  the  crucible  begins  to  be  red-hot,  it  suddenly  becomes 
intensely  so  when  decomposition  takes  place.  The  heat  must 
not  be  lessened  then,  but  maintained  for  half  an  hour  only.  If 
continued  for  a  longer  time,  the  reaction  of  chloride  of  potassium 
upon  the  aluminium  would  cause  loss.  Neither  were  the  globules 
of  aluminium  larger  when  the  heat  was  maintained  for  two  hours. 
That  is  the  case  only  when  a  very  intense  heat  is  employed. 
When  the  heat  is  discontinued  after  five  or  ten  minutes  the 
yield  of  aluminium  is  remarkably  small,  because  the  metal  is  then 
pulverulent,  and  burns  while  the  crucible  cools. 
No  larger  product  was  obtained  when  the  cryolite  powder  was 
mixed  with  an  equal  quantity  of  chloride  of  potassium,  or  when 
the  layers  of  sodium  and  cryolite  were  covered  with  double 
chloride  of  aluminium  and  sodium. 
The  use  of  chloride  of  sodium  as  a  flux  necessitates  a  higher 
temperature,  but  the  result  is  not  sensibly  different. 
The  yield  of  aluminium,  obtained  when  the  operation  was  con- 
ducted in  the  manner  described,  was  very  variable.  It  never 
amounted  to  the  whole  of  the  metal  in  the  cryolite  used.  This 
substance  contains  only  13  per  cent,  of  aluminium.  The  largest 
quantity  obtained  from  10  grms.  of  cryolite  was  0.8  grm.,  fre- 
quently only  0.3  grm. ;  and  less  was  obtained,  instead  of  the 
1.3,  which,  by  calculation,  it  ought  to  yield,  if  the  reduction  was 
complete. 
The  most  important  condition  to  which  these  variations  must 
be  ascribed  is  the  temperature.  The  higher  it  is,  the  more  does 
the  aluminium  aggregate  into  globules.  I  once  succeeded  in 
obtaining  almost  the  whole  of  the  reduced  aluminium  in  one 
button,  weighing  0.5  grm. 
