MANUFACTURE  OF  THE  CARBONATE  OF  MAGNESIA.  369 
from  a  gasometer  in  communication  with  the  pump,  and  a  mer- 
curial guage  is  attached  to  show  'at  all  times  the  amount  of  pres- 
sure within  the  cylinder,  independently  of  the  safety-valve.  In- 
to this  cylinder,  so  fitted  up  as  described,  from  100  to  120  lbs.  of 
the  calcined  limestone  is  introduced,  with  a  quantity  of  pure 
water  nearly  filling  the  cylinder  ;  carbonic  acid  gas  is  next  pumped 
in,  the  agitator  being  constantly  turned  the  while,  and  more  and 
more  gas  forced  in  until  absorption  ceases  under  a  pressure  of 
five  atmospheres.  The  whole  is  allowed  to  remain  in  this  con- 
dition for  three  or  four  hours,  and  the  contents  of  the  cylinder 
are  then  run  off  into  a  cistern  and  allowed  to  settle.  The  clear 
liquor  is  now  a  solution  of  carbonate  of  magnesia  in  water,  im- 
pregnated with  carbonic  acid  gas,  or  a  solution  of  bicarbonate 
of  magnesia  having  a  sp.  gr.  of  about  1028,  and  containing  about 
1600  grains  of  carbonate  of  magnesia  to  the  imperial  gallon. 
Mr.  Pattinson  considers  the  best  mode  of  obtaining  a  solution 
of  bicarbonate  of  magnesia  from  magnesian  limestone,  is  to  ope- 
rate upon  the  limestone  after  being  calcined  at  a  white  heat  in 
the  way  described,  but  the  process  may  be  varied  by  using  in 
the  cylinder  the  mixed  hydrates  of  lime  and  magnesia,  obtained 
by  completely  burning  magnesian  limestone  in  a  kiln,  as  com- 
monly practised,  and  slaking  it  with  water  in  the  usual  manner; 
or,  to  lessen  the  expenditure  of  carbonic  acid  gas,  the  mixed  hy- 
drates may  be  exposed  to  the  air  for  a  few  weeks,  until  the  lime 
has  become  less  caustic  by  the  absorption  of  carbonic  acid  from 
the  atmosphere,  or  the  mixed  hydrates  may  be  treated  with 
water,  as  practised  by  some  manufacturers  of  Epsom  salt, 
until  the  lime  is  wholly  or  principally  removed,  after  which 
the  residual  rough  hydrate  of  magnesia  may  be  prepared  for  so- 
lution in  the  cylinder  by  dissolving  magnesian  limestone  in  hy- 
drochloric acid,  and  treating  the  solution,  or  a  solution  of  chlo- 
ride of  magnesium,  obtained  from  sea-water  by  salt-makers  in 
the  form  of  bittern,  with  its  equivalent  quantity  of  hydrate  of 
lime,  or  of  the  mixed  hydrates  of  lime  and  magnesia  obtained  by 
completely  burning  magnesian  limestone  and  slaking  it  as  above. 
Mr.  Pattinson  states  that,  when  he  uses  this  solution  of  bicar- 
bonate of  magnesia  for  the  purpose  of  preparing  carbonate  of 
magnesia,  he  evaporates  the  solution  to  dryness,  by  which  he  at 
once  obtains  a  pure  carbonate  of  magnesia,  without  the  necessity . 
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