368         MANUFACTURE  OF  THE  CARBONATE  OF  MAGNESIA. 
portion  of  it  used  is  a  quantity  containing  about  thirty-seven 
parts  of  pure  hydrochloric  acid  to  every  twenty-eight  parts  of 
lime  in  the  calcined  magnesian  limestone,  and  the  same  for  every 
fifty  parts  of  carbonate  of  lime  in  the  uncalcined  stone. 
The  magnesia  or  carbonate  of  magnesia  thus  obtained,  Dr. 
Richardson  employs  in  the  magnesian  salts,  as  noticed  in  the  ar- 
ticle on  "Epsom  Salt,"  in  the  last  number  of  the  Pharmaceuti- 
cal Journal.  He  also  manufactures  carbonate  of  magnesia  by 
causing  a  stream  of  carbonic  acid  to  be  forced  through  vessels  con- 
taining magnesia  obtained  as  before  mentioned,  diffused  through 
water.  A  bicarbonate  is  thus  produced,  which  enters  into  solu- 
tion with  the  water,  and  this  is  subsequently  converted  into  car- 
bonate, which  is  obtained  as  a  precipitate  by  the  application  of 
a  gentle  heat  to  the  vessel  containing  the  solution  of  bicarbon- 
ate. 
The  best  process  for  the  manufacture  of  carbonate  of  magne- 
sia is,  undoubtedly,  that  patented  by  Mr.  H.  L.  Pattinson,  Sept. 
24th,  1841.  The  process  is  as  follows  : — Magnesian  limestone, 
as  rich  as  possible  in  magnesia,  is  reduced  to  powder,  and  sifted 
through  a  sieve  of  forty  or  fifty  meshes  to  the  linear  inch.  It  is 
then  heated  red-hot  in  an  iron  retort  or  reverberatory  furnace 
for  two  or  three  hours,  when,  the  carbonic  acid  being  expelled 
from  the  carbonate  of  magnesia,  and  not  from  the  carbonate  of 
lime,  the  whole  is  withdrawn  from  the  retort  or  furnace  and  al- 
lowed to  cool.  The  magnesia  contained  in  the  limestone  is  now 
soluble  in  water  impregnated  with  carbonic  acid  gas,  and  to  ef- 
fect this  solution  Mr.  Pattinson  proceeds  as  follows  : — An  iron 
cylinder  lined  with  lead,  of  any  convenient  size,  say  four  feet 
long  by  two  feet  and  a  half  in  diameter,  is  provided,  furnished 
with  a  safety  valve  and  agitator,  which  latter  may  be  an  axis  in 
the  centre  of  the  cylinder,  with  arms  reaching  nearly  to  the  cir- 
cumference— all  made  of  iron  and  covered  with  lead..  This 
cylinder  is  placed  horizontally,  and  one  extremity  of  this  axis  is 
supported  within  it  by  a  proper  carriage,  the  other  extremity 
being  prolonged,  and  passing  through  a  stuffing-box  at  the  other 
end  of  the  cylinder,  so  that  the  agitator  may  be  turned  round  by 
applying  manual  or  other  power  to  its  projecting  end.  A  pipe 
leading  from  a  force-pump  is  connected  with  the  under  side  of 
the  cylinder,  through  which  carbonic  acid  gas  may  be  forced 
