370       MANUFACTURE  OF  THE  CARBONATE  OF  MAGNESIA. 
of  using  a  carbonated  alkali,  as  in  the  old  process  ;  and  from 
this  he  prepares  pure  calcined  magnesia  by  calcination  in  the 
usual  manner,  or,  instead  of  boiling  the  solution  to  dryness  he 
merely  heats  it  for  some  time  to  the  boiling  point,  by  which  ex- 
cess of  carbonic  acid  is  partly  driven  off,  and  pure  carbonate  of 
magnesia  precipitated,  which  is  collected  and  dried  in  the  usual 
way. 
Manufacture  of  Carbonate  of  Magnesia  from  Magnesite. — 
Dr.  Richardson's  more  recent  improvements  in  the  manufacture 
of  carbonate  of  magnesia,  are  contained  in  the  specification  of 
his  patent,  dated  June  14th,  1853.  He  takes  any  impure  hy- 
drate, or  carbonate  of  magnesia,  free  from  lime,  or  nearly  so, 
such,  for  instance,  us. magnesite,  (the  impure  hydrate  of  magne- 
sia, which  is  a  waste  product  in  Ward's  process  of  carburating 
soda  ash,  being  preferred)  and  diffuses  it  through  water,  so  as  to 
form  a  cream  or  milk-like  fluid,  which  he  runs  into  a  large  soda- 
water  machine,  wherein  he  pumps  carbonic  acid,  in  the  same 
manner  as  when  making  soda-water.  Or,  he  partially  fills  a 
wooden  box,  divided  into  cells  by  partitions  running  from  the 
top  and  bottom  alternately,  with  this  milk-like  fluid,  and  by 
means  of  a  steam-jet,  or  air-pump  worked  by  suitable  machinery, 
lie  draws  a  stream  of  carbonic  acid  through  a  whole  series  of  com- 
partments from  a  furnace  filled  with  coke  or  charcoal.  When 
the  liquid  becomes  saturated  with  bicarbonate  of  magnesia,  or 
stands  at  5°  to  llc  on  Twaddel's  hydrometer  (sp.  gr.  1-025  to 
1-055),  it  is  drawn  off,  and  after  standing  about  an  hour  a  small 
quantity  of  a  cream  of  magnesite  is  gradually  added,  which  he 
finds  carries  down  all  the  oxide  of  iron  and  other  impurities, 
leaving  a  clear  pure  solution  of  bicarbonate  of  magnesia.  This 
solution  is  heated  to  expel  the  excess  of  carbonate  precipitates, 
and  is  collected  and  treated  in  the  usual  way,  or  a  quantity 
of  pure  magnesia  in  suspension  in  water  is  added  until  the  whole 
is  precipitated  as  carbonate  of  magnesia. 
The  furnace  employed  for  producing  carbonic  acid  is  filled  with 
coke  or  charcoal)  and  is  supplied  with  air  through  an  opening  at 
the  bottom,  and  another  higher  up,  on  a  level  with  the  top  of 
the  fuel.  The  air  may  be  drawn  through  by  means  of  a  jet  of 
steam  or  otherwise,  or  forced  forwards  by  a  blowing  cylinder  or 
£an-blast?  s.o  regulated  by  valves  or  dampers,  that  the  supply  of 
