22 
PREPARATION  OF  PYROGALLIC  ACID,  ETC. 
and  that  the  carbonic  acid  is  nearly  altogether  driven  off.  We 
then  had  recourse  to  pure  water,  and  the  results  obtained  have 
surpassed  our  hopes. 
We  introduce  into  a  bronze  boiler  gallic  acid,  with  two  or 
three  times  its  weight  of  water.  Elevate  the  temperature  from 
392°  F.  to  410°  F.,  maintain  it  during  half  an  hour,  and  allow 
it  to  cool.  The  operation  extends  through  one  and  a  half  to 
two  hours.  On  opening  the  boiler,  which  contains  a  slightly- 
colored  solution  of  pyrogallic  acid,  it  is  heated  with  a  little  pure 
animal  charcoal,  filtered,  and  evaporated  over  a  naked  fire,  to 
remove  the  water.  By  cooling,  the  pyrogallic  acid  crystallizes 
under  the  form  of  a  hard  mass,  of  a  light  amber  or  sometimes 
rose-color.  To  have  it  quite  white,  it  suffices  to  distill  it  in  a 
vacuum.  The  product  is  equal  to  the  theoretic  yield,  sometimes 
a  little  greater,  owing  to  the  retention  of  a  little  water  by  the 
pyrogallic  acid. 
The  boiler  which  has  served  our  purpose  is  formed  of  a  Pa- 
pin's  digester,  and  we  employed  a  disc  of  pasteboard  as  the 
packing  for  the  cover  joint.  We  were  astonished,  in  our  first 
essays,  not  to  find  the  carbonic  acid,  which  passed  through  the 
joints,  whilst  the  vapor  of  water  is  retained.  We  have  deter- 
mined the  disengagement  of  carbonic  acid  by  enclosing  within 
the  boiler  tubes  of  glass  full  of  lime  and  baryta  waters,  which 
were  transformed  into  carbonates. 
The  distillation  of  pyrogallic  acid  in  a  vacuum  of  two  to  three 
centimetres  is  made  with  great  rapidity,  and  nearly  instanta- 
neously. M.  Dumas  has  often  called  the  attention  of  chemists 
to  the  advantage  and  even  the  necessity  of  distilling  organic 
substances  at  low  temperatures,  by  operating  in  the  vacuum. 
The  new  pneumatic  apparatus  of  M.  Deleuil  could  be  usefully 
applied  in  these  circumstances. 
M.  Bayard,  at  the  request  of  M.  Regnault,  has  first  tried 
this  pyrogallic  acid,  and  M.  Bertall  has  kindly  compared  the 
crude  acid  prepared  by  our  process  with  the  sublimed  acid, 
which  they  generally  use.  The  trials  made  in  their  work-rooms 
by  means  of  the  crude  acid  prove  that  it  is  quite  equal  to  the 
sublimed  acid. 
We  have  studied  the  action  of  pyrogallic  acid  on  bases.  We 
