196 
PREPARATION  OF  HEAVY  CALCINED  MAGNESIA. 
marked  No.  6,  7,  and  8.  The  former  of  these,  as  mentioned  here- 
after, is  the  best  product  1  have  succeeded  in  preparing,  and  is 
offered  as  approaching  in  fineness,  softness  and  specific  gravity,  to 
the  best  article  of  heavy  calcined  magnesia  manufactured. 
Third  Series  of  Experiments. 
(a.)  Precipitated  a  solution  of  chloride  of  magnesium  with 
caustic  soda.  The  precipitate  while  wet  and  in  the  recent  condi- 
tion, wras  very  soft  and  beautiful  ;  but  on  drying,  it  assumed  the 
granulated  and  rough  appearance  spoken  of  in  another  place  as 
pertaining  to  the  hydrate. 
(6.)  Strong  solutions  of  chloride  of  magnesium  and  carbonate  of 
soda  were  mixed  together.  The  precipitated  carbonate  was  well 
washed,  and  after  being  partially  dried,  was  introduced  into  a 
crucible,  and  calcined  till  free  from  carbonic  acid.  The  produce 
wTas  of  a  fine  white  color,  and  entirely  free  from  harshness.  The 
density  of  this  article,  however,  was  but  little  more  than  that  of 
the  ordinary  commercial  variety. 
Wishing  to  ascertain  the  effect  of  water  on  magnesia,  in  modify- 
ing its  density  and  smoothness,  I  passed  steam  into  a  bottle  con- 
taining the  result  of  the  above  process  until  about  ten  per  cent, 
was  absorbed.  This  rendered  it  much  more  dense  ;  but  impaired 
in  some  degree  its  original  softness. 
Fourth  Experiment. 
It  having  been  suggested  to  me  by  a  friend,  that  perhaps  a  good 
quality  of  magnesia  could  be  made  by  calcining  the  regular  mono- 
carbonate  of  magnesia  (often,  though  improperly  called  the  bicar- 
bonate) I  determined  to  try  the  experiment,  and  proceeded  in  the 
following  manner  :  Dissolved  the  carbonate  of  magnesia  of  the 
shops  (which  is  a  subcarbonate)  in  an  aqueous  solution  of  carbonic 
acid  HO,  C02.  Exposed  the  solution  in  a  shallow  vessel  for  a 
fewT  days,  that  the  excess  of  C02  might  escape,  when  the  salt  was 
deposited  in  small  crystals.  These  were  dried  and  calcined,  yield- 
ing a  very  inferior  article,  which  was  very  light,  adhering  and 
caking  under  a  slight  pressure,  and  requiring  long  calcination  to 
free  it  entirely  from  carbonic  acid. 
Remarks. 
I  propose  offering  a  few  remarks  on  some  of  the  foregoing  pro- 
