MANUFACTURE  OF  THE  CARBONATE  OF  MAGNESIA.  36T 
nesia  alba  or  the  subcarbonate  ;  the  excess  of  carbonic  acid  being 
expelled. 
Heavy  Carbonate  of  Magnesia. — Pereira  gives  the  following  as 
the  method  which  he  has  seen  followed  in  a  large  and  esteemed 
manufactory  : — Add  one  volume  of  a  cold  saturated  solution  of 
carbonate  of  soda  to  a  boiling  mixture  of  one  volume  of  a  satu- 
rated solution  of  sulphate  of  magnesia  and  three  volumes  of  wa- 
ter. Boil  until  effervescence  has  ceased,  constantly  stirring  with 
a  spatula.  Then  dilute  with  boiling  water,  set  aside,  pour  off 
the  supernatant  liquor,  and  wash  the  precipitate  with  hot  water 
on  a  linen  cloth.    Afterwards  dry  it  by  heat  in  an  iron  pot. 
Manufacture  of  Carbonate  of  Magnesia  from  Bittern. — In 
the  manufacture  of  carbonate  of  magnesia  from  bittern,  (chlo- 
ride of  magnesium),  or  the  mother-liquor  remaining  after 
the  extraction  of  common  salt  from  sea-water,  the  crude  car- 
bonate of  ammonia,  obtained  in  the  distillation  of  bones,  or 
the  ammoniacal  liquor  of  gas-works,  is  used  as  the  decom- 
posing agent.  In  this  case  carbonate  of  magnesia  is  precipi- 
tated and  muriate  of  ammonia  obtained  in  solution.  The  after 
treatment  is  the  same  as  that  described  above.  Dolomite  is  also 
employed  for  the  decomposition  of  bittern,  in  the  same  way  as 
that  of  artificial  chloride  of  magnesium.  In  this  process  the 
carbonate  of  lime  of  the  dolomite  becomes  converted  into  chlo- 
ride of  calcium,  leaving  the  carbonate  of  magnesia  behind  as  an 
insoluble  powder. 
Manufacture  of  Carbonate  of  Magnesia  from  Dolomite.— In. 
his  patent  of  January  23d,  Dr.  Richardson  claims  the  manufac- 
turing of  magnesia  and  carbonate  of  magnesia  from  magnesian 
limestone  by  the  employment  of  muriatic  acid.  The  quantity  of 
the  acid  used  should  be  slightly  in  excess  of  the  exact  quantity 
required  to  dissolve  out  the  lime  in  the  limestone  operated  upon. 
This  may  be  either  burnt  to  expel  the  carbonic  acid  and  then 
slaked  in  pits  previous  to  adding  the  muriatic  acid,  or  it  may  be 
employed  in  the  natural  state,  reduced  to  an  impalpable  powder, 
the  product  in  the  former  case  being  magnesia,  and  in  the  latter 
an  impure  carbonate,  which  may  either  be  employed  as  such,  or 
be  deprived  of  its  carbonic  acid  by  the  application  of  heat.  The 
acid  preferred  for  this  purpose  is  the  weak  muriatic  acid,  which 
is  at  present  allowed  to  run  to  waste  in  alkali  works,  and  the  pro- 
