470     RECIPROCAL  ACTION  OF  GLYCERIN  AND  OXALIC  ACID. 
shows  that  126  grammes  of  oxalic  acid  furnish  82  grammes  of 
aqueous  formic  acid,  which  ought  to  contain,  and,  in  fact,  does 
contain,  56  per  cent,  of  true  formic  acid.  The  existence  of  this 
limit  is  the  result  of  the  repeated  and  successive  combination  of 
the  formic  acid,  held  back  by  the  glycerin,  with  this  polyatomic 
alcohol, — a  combination  rendered  especially  evident  by  the  fact 
that  the  quantity  of  water  eliminated  from  the  glycerin  is  equiva- 
lent to  the  quantity  of  formic  acid  fixed  in  each  of  the  successive 
phases  of  the  reaction. 
In  one  series  of  experiments  the  aqueous  formic  acid  from  each 
kilogramme  of  oxalic  acid,  added  in  portions  of  250  grammes, 
titrated  24,  44,  53 ;  in  a  second  series,  17,  33,  41,  46,  50,  and 
51*5  ;  the  standard  was  raised  more  rapidly  at  the  commence- 
ment of  the  operation.  The  excess  of  water,  shown  by  these  ex- 
periments to  exist  in  the  first  products,  appears,  then,  to  be  the 
result  of  the  formation  of  a  compound  of  glycerin  and  formic 
acid,  the  acid  being  regularly  produced  only  after  this  compound 
has  ceased  to  be  formed.  The  limit  of  saturation  has  been  found 
to  correspond  to  70  centiemes  of  the  quantity  of  formic  acid  which 
would  have  been  necessary  to  saturate  the  glycerin,  giving  a 
monoformine. 
This  method  of  preparing  formic  acid  of  56  per  cent,  is  so  con- 
tinuous and  regular  that  it  is  one  of  the  easiest  of  chemical  opera- 
tions. No  attention  need  be  paid  to  the  temperature,  the  disen- 
gagement of  carbonic  acid  forming  the  initial  and  final  phases  of 
the  operation.  Starting  with  1  kilogramme  of  glycerin,  and  by 
successively  adding  250  grammes  of  oxalic  acid,  we  obtain,  for 
each  kilogramme  of  acid  added  at  one  time,  650  grammes  of 
formic  acid  at  56  per  cent.  It  is  moreover  evident  that  under 
these  conditions  an  equal  weight  of  formic  acid  at  25  per  cent, 
costs  no  more  than  the  oxalic  acid  which  has  served  to  produce 
it ;  for  by  the  addition  of  water,  forming  a  litre  of  each  quantity 
of  formic  acid  collected  in  the  second  series  of  experiments,  I 
obtained  the  standards  21,  26,  31,  33,  34.  The  glycerine  being 
saturated,  1  kilogramme  of  oxalic  acid  furnished  1*5  kilog.  of 
formic  acid,  at  25.  The  operation  may  be  prolonged  to  any  ex- 
tent, and  if  after  being  used  a  long  time  it  is  found  necessary  to 
change  the  glycerin,  it  will  be  on  account  of  the  impossibility 
