PREPARATION  OF   HEAVY  CALCINED  MAGNESIA. 
197 
cesses  and  products,  which  may  aid  others  who  feel  disposed  to 
investigate  the  subject.  A  pure,  heavy  and  smooth  magnesia  may 
be  obtained  either  by  precipitating  a  hot  concentrated  solution  of 
sulphate  of  magnesia  with  a  like  solution  of  carbonate  of  soda,  or 
by  the  decomposition  of  chloride  of  magnesium  by  heat. 
It  is  difficult  to  say  which  of  these  processes  is  on  the  whole 
preferable.  The  difficulty  of  separating  iron  from  the  commercial 
sulphate  of  magnesia  is  an  objection  to  the  first  process,  but  the 
heat  in  this  need  not  be  so  carefully  regulated  as  in  operating  upon 
the  chloride.  It  has  also  the  advantage  of  giving  off  no  corrosive 
gas  in  the  process  of  calcination.  The  precautions  to  be  used  in 
executing  the  formula  before  given,  in  which  the  chloride  is  pre- 
pared from  commercial  muriatic  acid  and  carbonate  of  magnesia, 
and  afterwards  decomposed  by  heat,  pertain  mainly  to  the  separa- 
tion of  iron  and  the  regulation  of  the  heat. 
As  commercial  muriatic  acid  always  contains  more  or  less  iron, 
it  is  important  to  find  the  best  and  cheapest  method  of  freeing  it 
from  that  impurity  without  resorting  to  the  process  of  redistillation. 
The  nitric  acid,  which  in  the  formula  is  directed  to  be  added  to 
the  solution  of  chloride  of  magnesium,  answers  the  purpose  ad- 
mirably. The  acid  acts  by  peroxidizing  the  iron,  which  is  almost 
entirely  precipitated  and  separated  by  neutralizing  the  nitric  acid 
with  an  excess  of  carbonate  of  magnesia,  and  then  filtering  the 
solution,  during  which  it  will  often  be  necessary  to  add  a  little 
water  to  prevent  its  congealing.  During  the  evaporation,  the  fire 
must  be  under  the  complete  control  of  the  operator,  or  the  vessel 
of  such  capacity  as  to  hold  considerably  more  than  he  is  operating 
upon,  for  without  this  precaution  the  solution,  as  it  becomes  con- 
centrated, will  froth  up  so  as  to  be  spirted  out  of  the  capsule. 
Towards  the  close  of  the  evaporation,  and  as  the  chloride  be- 
comes more  and  more  decomposed,  it  sinks  to  the  bottom  of  the 
Capsule  in  the  form  of  a  hard  cake  of  subchloride,  from  which,  as 
the  decomposition  proceeds,  the  chlorine  escapes  through  minute 
holes,  which  give  the  mass  somewhat  the  appearance  of  a  honey 
comb. 
In  regard  to  the  intensity  and  duration  of  the  heat  in  the  cal- 
cination, I  have  come  to  the  conclusion,  after  many  trials,  that  a 
low  heat,  just  approaching  to  redness  and  long  continued,  will  en- 
