80        ON  GALLIC  AND  GALLHUMIC  (METAGALLIC)  ACID. 
iron,  but  the  proto-sesquioxyd  (black  oxyd).  A  reduction  takes 
place,  therefore,  the  oxygen  transforming  some  of  the  carbon  of 
the  gallic  acid  into  carbonic  acid,  which  is  freely  evolved  during 
the  ebullition. 
To  a  portion  of  gallic  acid,  dissolved  in  water  and  heated  to 
ebullition,  a  solution  of  sesquichlorid  of  iron  was  carefully  added 
in  small  quantities  and  the  mixture  heated  again  after  each  addi- 
tion. This  treatment  was  continued,  until  a  drop  of  the  solu- 
tion mixed  with  a  little  water  ceased  to  give  the  characteristic 
bluish-black  precipitate  of  gallic  acid  with  sesquichlorid  of  iron. 
A  solution  of  carbonate  of  soda  was  then  added  in  slight  excess 
and  the  black  precipitate  separated  by  filtration,  A.  portion  of 
the  filtered  dark-brown  liquor,  after  being  exactly  saturated  with 
hydrochloric  acid,  deposited  a  voluminous  black  precipitate, 
which,  if  dried,  formed  a  black  shining  mass,  but  when  freshly 
precipitated,  was  easily  redissolved  by  free  muriatic  acid.  Such 
a  solution,  containing  but  little  free  muriatic  acid,  produced 
black  insoluble  precipitates  with  limewater,  with  the  different 
salts  of  lime  and  baryta,  with  sulphate  of  zinc  and  sulphate  of 
copper.  Another  portion  of  the  filtered  liquor  super-saturated 
with  acetic  acid,  caused  precipitates  of  a  black  color  in  solutions 
of  acetate  of  lead  and  nitrate  of  silver.  From  the  silver  pre- 
cipitate, metallic  silver  was  soon  separated. 
The  lead  precipitate  was  carefully  washed  with  distilled  water, 
and  after  being  dried  in  an  air-bath  at  a  temperature  not  ex- 
ceeding 200°  F.  (94°  C.)  for  ten  hours,  it  was  heated  over  a 
spirit  lamp,  until  the  organic  matter  was  perfectly  destroyed. 
The  residue,  consisting  of  a  mixture  of  oxyd  of  lead  and  metallic 
lead,  was  treated  with  acetic  acid,  and  from  it  the  whole  quan- 
tity of  oxyd  of  lead  was  calculated. 
1-052  gram,  gave  0-662  of  the  mixture  of  PbO+Pb,  which 
left  after  being  treated  with  acetic  acid  0*037  metallic  lead,  a 
quantity  corresponding  to  0-041  oxyd  of  lead.  The  acetic  acid 
extracted  0-625  oxyd  of  lead,  which  quantity  added  to  the  above 
found  0-041,  gives  0-666.    This  is  equal  to  63-30  per  cent. 
Gallhumic  (metagallic)  acid,  which  was  detected  by  Pelouze 
in  the  residue  of  distillation,  when  gallic  acid  was  suddenly 
heated  to  480°  F.  (249°  C.)  shows  the  same  reactions,  and  its 
lead  salt,  2PbO,  C12H303,  contains  63-04  per  cent  of  the  oxyd 
