198 
PREPARATION  OF  HEAVY  CALCINED  MAGNESIA. 
sure  a  much  finer  product,  than  the  sudden  application  of  a  high 
heat  for  a  short  time. 
The  above  fact  will  appear  much  clearer  on  an  examination  of 
one  of  the  specimens  presented — No.  9.  This  sample  bid  fair  at 
the  beginning  of  the  process  to  be  of  superior  quality,  but  the 
crucible  containing  the  subchloride  was  placed  in  a  strong  air 
furnace  and  raised  quickly  to  a  white  heat,  which  was  continued 
three  hours.  After  the  fire  had  cooled  down,  the  contents  of  the 
crucible  was  removed,  and  very  much  to  my  surprise  and  disap- 
pointment, found  to  be  in  a  hard  semifused  mass  of  a  cellular  form, 
and  colored  w7ith  iron  derived  from  the  crucible. 
It  is  a  difficult  matter  to  procure  crucibles  that  combine  all  the 
requisite  qualities,  viz.,  cheapness,  durability,  and  freedom  from 
iron  or  other  substances  easily  acted  on  by  the  chlorine.  I  found 
the  white  Hessian  crucibles,  when  well  glazed  inside,  to  be  supe- 
rior to  any  other  kind  that  I  could  procure.  I  employed  a  small 
crucible  of  the  above  description  in  eight  operations  before  it  be- 
came useless.  Berlin-ware  crucibles  would  be  the  best  to  use, 
wrere  it  not  for  their  expensiveness  and  liability  to  fracture  on  the 
sudden  application  or  withdrawal  of  heat. 
The  specimen  marked  No.  7  is  part  of  a  lot  that  was  kept  at  as 
high  a  heat  as  a  German  porcelain  evaporating  dish  would  bear, 
for  eight  hours,  when  it  was  found  to  be  in  a  granular  powder, 
almost  free  from  chlorine.  This  powder  was  removed  to  a  filter, 
and  washed  well  with  hot  water,  dried,  and  again  exposed  to  heat 
for  a  short  time.  This  treatment  rendered  it  somewhat  finer,  but 
the  crystalline  condition  could  not  be  entirely  overcome,  even  by 
long  trituration. 
The  above  sample  is  the  one  analyzed,  and  contains  over  95  per 
cent,  of  pure  magnesia.  No.  8  is  a  portion  of  the  same  specimen 
which  has  not  been  triturated,  and  shows  very  clearly  the  granular 
condition  just  mentioned  in  connection  with  No.  7. 
Specimen  No.  6  exceeds  so  much  in  fineness  and  density  the 
others,  that  the  question  naturally  arises,  what  is  the  cause  of  so 
great  a  difference  in  these  three  samples  made  by  the  same  pro- 
cess. In  one  case  the  crystalline  character  is  almost  entirely 
broken  up,  while  in  the  others  the  very  opposite  results  were  pro- 
duced.   I  must  confess  my  inability  to  offer  a  satisfactory  solution 
