560         NEW  PROCESS  FOR  PREPARING  FORMIC  ACID. 
engagement  is  produced ;  this  is  pure  oxide  of  carbon.  The 
liquid  distilled  throughout  the  operation  thus  conducted,  does  not 
contain  one-tenth  part  of  the  formic  acid  which  might  be  ob- 
tained bj  operating  as  I  have  said  above. 
7.  This  new  phenomenon — disengagement  of  oxide  of  carbon 
—is  due  to  the  decomposition,  at  200°C.  (392°F.),  of  the  formic 
acid  retained  in  solution  by  the  glycerine,  in  the  same  way  as 
ammoniacal  gas  is  dissolved  by  water.  Indeed  pure  formic  acid, 
heated  for  some  hours  to  between  200°  and  250°C.  (392°  and 
482°F.),  in  sealed  tubes,  is,  for  the  most  part,  decomposed  into 
water  and  oxide  of  carbon  ;  the  glycerine  exerts  scarcely  any 
accelerating  influence  on  this  decomposition.  These  observa- 
tions may  be  turned  to  account,  in  the  preparation  of  oxide  of 
carbon  by  oxalic  acid ;  when,  if  we  heat  oxalic  acid,  mixed,  not 
with  sulphuric  acid,  but  with  glycerine,  we  obtain,  successively 
and  separately,  the  two  gases  which  sulphuric  acid  mixed  in 
equal  volumes  produces  ;  first,  carbonic  acid,  and  then,  carbonic 
oxide.  This  last  body  may,  therefore,  be  thus  prepared  pure, 
without  washing  with  alkali. 
8.  However,  a  considerable  interval  of  temperature  separates 
these  two  successive  phenomena;  decomposition  at  100°C. 
(212°F.),  of  the  oxalic  acid  into  carbonic  acid  and  formic  acid, 
in  contact  with  glycerine,  then,  further  decomposition  at  200°C. 
(392°  F.),  of  formic  acid,  into  water  and  carbonic  acid.  No- 
thing is  easier  than  to  govern  the  reaction,  and  to  obtain,  by 
successive  additions  of  water,  the  whole  of  the  formic  acid  which 
oxalic  acid  can  furnish. 
Formic  acid  thus  prepared,  is  very  pure  and  completely  free 
from  oxalic  acid.  Saturated,  by  means  of  carbonate  of  lime, 
baryta,  or  lead,  it  furnishes  by  the  first  crystallisation,  pure 
formiate  of  lime,  baryta,  or  lead.  500  grammes  of  commercial 
oxalic  acid  produced  about  500  grammes  of  pure  formiate  of 
lead. 
It  will  be  remarked,  that  the  glycerine  is  found  entire  in  the 
retort,  at  the  end  of  each  operation  (with  the  exception  of  a  very 
small  quantity — about  1  gramme  per  litre — volatilised  with  the 
water),  exactly  like  sulphuric  acid  in  the  preparation  of  ether. 
London  Chemist,  from  Comptes  Rendus,  March  3,  1856. 
