194 
PREPARATION  OF  HEAVY  CALCINED  MAGNESIA. 
raised  to  the  boiling  point,  transferred  to  a  filter,  and  the  precipitate 
well  washed. 
After  drying  the  precipitated  carbonate  and  calcining  it  at  a 
high  heat,  I  found  the  product  to  be  free  from  carbonic  acid,  about 
twice  the  specific  gravity  of  the  ordinary  magnesia  of  the  shops, 
not  purely  white,  owing  to  the  presence  of  iron,  and  possessed  of 
a  good  degree  of  the  softness  so  characteristic  of  a  fine  article. 
(b.)  A  hot  concentrated  solution  of  sulphate  of  magnesia  was 
mixed  with  a  strong  solution  of  sesqui-carbonate  of  ammonia,  and 
the  precipitated  carbonate  treated  as  in  the  first  experiment,  (a.) 
The  product  was  not  so  heavy  nor  so  smooth  as  the  foregoing, 
and  like  that  was  discolored  by  iron. 
(c.)  A  solution  of  Epsom  salts,  similar  to  the  one  above  described, 
was  treated  with  the  officinal  liquor  potassa?  and  boiled  for  ten 
minutes,  when  a  very  dense  precipitate  of  hydrated  magnesia  was 
the  result.  After  being  washed,  which  required  a  long  time  to  free 
it  from  sulphate  of  potassa,it  was  dried  at  a  moderate  heat,  during 
which  it  assumed  a  granular  consistence  and  became  hard  and 
gritty.  At  a  full  red  heat  this  hydrate  was  not  decomposed  and 
could  not  be  rendered  fine  and  smooth. 
(d.)  Hot  saturated  solutions  of  the  sulphate  of  magnesia  and 
carbonate  of  soda  were  mixed  as  in  the  first  experiment  (a),  and 
evaporated  to  complete  dryness,  stirring  frequently.  Water  was 
now  added,  it  was  raised  to  the  boiling  point  and  the  precipitate 
separated  on  a  filter  and  washed  well  with  hot  water.  The  pre- 
cipitate, after  being  dried  at  a  moderate  temperature,  was  calcined 
at  a  high  red  heat,  which  was  continued  about  two  hours.  A  dense 
though  somewhat  harsh  magnesia  was  obtained,  not,  however,  en- 
tirely free  from  carbonic  acid  and  oxide  of  iron.  In  order  to  free 
it  from  these  impurities  it  was  washed  with  water  acidulated  with 
a  few  drops  of  pure  muriatic  acid.  The  acid  decomposed  the 
traces  of  carbonates  present,  forming  with  them  and  the  iron  soluble 
chlorides,  which  were  dissolved  out  by  the  water,  any  trace  of  chlorine 
remaining  being  driven  off  by  the  subsequent  heating  to  which  the 
magnesia  was  exposed.  This  treatment  rendered  the  product 
whiter,  softer  and  more  dense,  and  freed  it  entirely  from  carbonic 
acid. 
(e.)  Strong  solutions  of  sulphate  of  magnesia  and  caustic  soda 
were  mixed  and  treated  in  the  same  manner  as  (d.)    On  attempt- 
