338 
PREPARATION  OP  PYROGALLIC  ACID. 
ON  THE  PREPARATION  OF  PYROGALLIC  ACID. 
By  Prof.  Liebig. 
The  author  has  already  called  attention  to  the  advantages 
of  the  employment  of  pyrogallic  acid  in  photography ;  it  has 
since  almost  entirely  taken  the  place  of  gallic  acid.  He  now 
gives  a  process  for  its  preparation,  which  he  regards  as  the  most 
advantageous,  and  which  he  has  employed  for  the  last  three 
years. 
The  material  required  is  crystallized  gallic  acid ;  when  decom- 
posed by  heat,  it  furnishes  the  most  beautiful  pyrogallic  acid  in 
very  large  quantity.  There  is  no  saving  in  the  employment  of 
dried  gall-nuts  or  their  dry  extract. 
For  this  purpose  the  gallic  acid  is  strongly  dried,  mixed  with 
twice  its  weight  of  coarsely  powdered  pumice-stone,  and  exposed 
to  its  temperature  of  decomposition  in  a  current  of  carbonic  acid. 
The  mixture  of  gallic  acid  and  pumice-stone  is  put  into  a  tubu- 
lated retort,  of  which  it  does  not  fill  more  than  one-fourth  ;  the 
retort  is  placed  in  a  sand-bath,  and  surrounded  with  sand  nearly 
up  to  its  tubulure.  A  glass  tube  is  inserted  into  the  tubulure. 
through  an  india-rubber  tube,  and  connected  with  an  apparatus 
for  the  evolution  of  carbonic  acid ;  the  tube  reaches  deeply  into 
the  belly  of  the  retort,  so  that  its  opening  is  about  \  inch  from 
the  mixture. 
The  neck  of  the  retort,  which  must  be  tolerably  wide,  reaches 
about  8  inches  beyond  the  margin  of  the  sand-bath,  and  is  loosely 
inserted  into  a  corresponding  receiver,  so  that  the  latter  may  be 
easily  removed.  The  object  of  this  arrangement  is  easily  under- 
stood. Dry  gallic  acid  is  decomposed  at  a  high  temperature  into 
pyrogallic  acid,  metagallic  or  melanogallic  acid,  carbonic  acid 
and  water  ;  the  author  has  supposed  that  from  2  atoms  of  gallic 
acid,  C28  H12  O'20,  there  are  formed  2  atoms  of  pyrogallic  acid 
and  1  atom  of  metagallic  acid,  C12  II4  O4,  with  elimination  of  4 
atoms  of  carbonic  acid  and  2  atoms  of  water,  so  that  100  parts 
of  dry  gallic  acid  should  furnish  39  parts  of  pyrogallic  acid. 
But  as  pyrogallic  acid  is  decomposed  into  water  and  metagallic 
acid  nearly  at  the  same  temperature  at  which  gallic  acid  is  de- 
composed, the  procuring  of  39  per  cent,  of  pyrogallic  acid  de- 
pends essentially  on  the  rapidity  with  which  the  acid  is  removed 
