366 
MANUFACTURE  OF  CARBONATE  OF  MAGNESIA. 
more  speedily  and  perfectly  will  the  vitriolated  tartar,  which  is 
formed  by  the  union  of  the  alkali  with  the  acid  of  the  sal  cathar- 
ticus,  be  washed  off.  The  neutral  salt  should  be  washed  off  as 
quickly  as  possible,  otherwise,  by  allowing  the  mixture  to  stand 
for  some  time  the  powder  concretes  with  minute  grains,  which, 
when  viewed  with  a  microscope,  appear  to  be  assemblages  of 
needles  diverging  from  a  point.  These  concretions  cannot  be 
redissolved  by  any  washing,  however  long  continued.  Dr.  Black 
orders  four  times  the  quantity  of  water  to  that  of  the  solution  for 
throwing  the  coagulum  into  ;  but  Mr.  Henry  observes  that  this 
quantity  is  much  too  little.  The  water  should  be  pure,  and  dis- 
tilled water  is  the  best.  Hard  or  impure  water  makes  magnesia 
coarse  and  disagreeable. 
In  the  Pharmacopoeias  of  the  London  and  Edinburgh  Colleges 
carbonate  of  soda  is  directed  to  be  employed  in  the  manufacture 
of  carbonate  of  magnesia  from  sulphate  of  magnesia.  The  two 
boiling  hot  solutions  are  run  together  into  large  wooden  vessels, 
while  a  workman  keeps  the  whole  in  continued  agitation  for  some 
time.  The  mixture  is  afterwards  allowed  to  remain  at  rest  for 
an  hour  or  so,  to  permit  the  precipitated  carbonate  of  magnesia 
to  subside.  When  this  deposit  has  taken  place,  the  fine  precipi- 
tate is  repeatedly  washed  with  pure  water,  to  remove  all  traces 
of  the  sulphate  of  soda  formed  by  the  double  decomposition  of 
the  two  salts.  The  carbonate  of  magnesia  is  then  allowed  to 
drain,  until  it  acquires  sufficient  consistency  to  be  formed  into 
small  pieces  of  the  shape  of  parallelopipedons,  and  then  dried  on 
wooden  shelves  in  a  stove  at  a  low  temperature. 
On  account  of  the  great  tendency  of  carbonic  acid  to  form 
soluble  bi-3alts,  it  is  necessary  that  the  solutions  of  sulphate  of 
magnesia  and  carbonate  of  soda  should  be  mixed  boiling  in  the 
manufacture  of  carbonate  of  magnesia.  The  magnesia  alba,  or 
carbonate  of  magnesia  of  commerce,  is  a  subcarbonate,  consisting 
of  a  combination  of  neutral  carbonate  of  magnesia  and  hydrate 
of  magnesia.  The  neutral  carbonate  is  a  crystallizable  salt, 
which  may  be  frequently  observed  depositing  in  the  bottles  con- 
taining solutions  of  the  bicarbonate  in  Murray's  fluid  magnesia. 
This  neutral  carbonate  is  decomposed  when  put  into  water.  If 
the  water  be  cold  it  is  decomposed  into  magnesia  alba  and  bicar- 
bonate ;  if  the  water  be  boiling  it  is  resolved  wholly  into  mag- 
