RECIPROCAL  ACTION  OF  GLYCERIN  AND  OXALIC  ACID.  471 
of  avoiding  the  almost  imperceptible  losses,  and  of  the  presence 
of  impurities  in  the  oxalic  acid.  I  have,  however,  used  the  same 
glycerin  for  several  months,  the  operations  continuing  inces- 
santly night  and  day. 
II.  Formic  Acid  at  75  Centiemes. — By  making  dehydrated 
oxalic  acid  act  on  saturated  glycerin,  I  obtain  formic  acid  at  an 
average  standard  of  75.  The  heat,  however,  requires  very  care- 
ful management  to  avoid  frothing.  The  decomposition  of  oxalic 
acid  commences  at  50°. 
Ill  Monohydrated  and  Cry stallis able  Formic  Acid. — Mono- 
hydrated  formic  acid  has  hitherto  been  obtained  by  decomposing 
formiate  of  lead  by  sulphuretted  hydrogen,  a  long  and  difficult 
operation,  which  in  unpractised  hands  is  rarely  successful.  I 
have,  in  the  first  place,  substituted  formiate  of  copper,  com- 
paratively a  very  soluble  salt,  for  formiate  of  lead,  it  being  more 
easily  crystallised,  dehydrated,  and  decomposed  by  sulphuretted 
hydrogen,  and  giving  the  theoretical  quantity  of  formic  acid. 
This  is  probably  not  the  only  case  in  which  salt  of  copper  would 
be  preferable  to  salt  of  lead  for  the  preparation  of  organic  acids. 
It  occurred  to  me,  however,  that  the  25  per  cent,  of  water  might 
be  eliminated  from  the  75  per  cent,  acid  by  the  use  of  anhydrous 
oxalic  acid.  On  making  this  acid  react  at  70°  on  formic  acid, 
the  temperature  rises,  the  mixture  becomes  liquid  when  carefully 
heated,  and  when  left  to  stand  it  crystallises  ;  by  decanting  and 
distilling  to  separate  the  dissolved  oxalic  acid,  formic  acid,  at  a 
standard  of  about  100,  is  obtained,  which,  by  a  proper  lowering 
of  the  temperature,  gives  crystallisable  formic  acid. 
IV.  Various  Experiments. — In  the  course  of  these  researches 
I  have  observed  the  following  facts  : — 1.  That  of  all  the  formiates, 
formiate  of  copper  alone  when  decomposed  by  heat,  gives  formic 
acid  of  only  a  certain  degree  of  concentration — 82.  2.  What- 
ever precautions  I  have  taken,  I  have  rarely  obtained  acids  at 
70  by  the  action  of  sulphuric  acid  on  formiates,  and  those  always 
in  relatively  small  quantities.  With  biformiate  I  have  been  not 
more  successful.  8.  The  splitting  up  of  ordinary  or  dehydrated 
oxalic  acid  under  the  influence  of  acetic  and  formic  acids,  may  be 
utilised  in  the  preparation  of  formic  acid.  4.  Dehydrated  oxalic 
acid,  submitted  to  the  moderate  action  of  heat,  furnished  in  one 
