PREPARATION  OF   TANNIC  ACID. 
447 
would  appear  to  have  aimed  at  an  approximation  to  the  method 
originally  adopted  by  Pelouze,  in  which  crude  ether  was  used  ; 
and  to  have  assumed  that  when  watery  ether  is  used,  the  lower 
layer  of  the  percolated  liquid  is  a  solution  of  tannic  acid  in  water. 
However,  Mohr  found  that  this  is  not  the  case,  but  that  the  lower 
layer  is  a  solution  of  tannic  acid  in  ether;  and  Sandrock  has  ob- 
tained the  same  result  on  repeating  his  experiments.  The  addi- 
tion of  water  to  the  ether  is,  therefore  useless,  and  moreover  inju- 
rious, for  the  solution  of  tannic  acid  in  ether  is  so  thick  that  the 
percolation  goes  on  very  slowly,  and  sometimes  stops  altogether. 
The  use  of  pure  ether  is  open  to  the  same  objection. 
The  extraction  of  the  tannic  acid  from  galls  may  on  the  contrary 
be  effected  with  ease  by  the  crude  ether,  and  on  account  of  the 
small  quantity  of  alcohol  which  it  contains.  The  alcohol  facilitates 
.the  percolation  by  rendering  the  solution  of  tannic  acid  less  viscid. 
Instead  of  crude  ether  a  mixture  of  sixteen  parts  ether  and  one 
part  alcohol  may  be  used  with  equally  satisfactory  results.  The 
percolated  liquid  separates  into  two  layers.  The  lower  one  con- 
taining the  tannic  may  easily  be  separated,  and  yields  a  perfectly 
pure  product  on  evaporation.  The  upper  layer  contains  the  gallic 
acid,  coloring  matter,  and  some  tannic  acid. 
When  a  mixture  of  eight  parts  ether  and  one  part  alcohol  is 
employed,  the  percolate  still  separates  into  twTo  layers,  but  the 
lower  one  is  smaller  than  when  the  proportion  of  alcohol  is  less, 
and  the  upper  layer  contains  a  considerably  larger  quantity  of  tan- 
nic acid. 
Finally,  when  a  mixture  of  four  parts  ether  and  one  part  alcohol 
is  employed,  the  percolate  does  not  separate  into  two  layers,  and  it 
is  difficult  to  separate  the  tannic  acid  from  the  impurities  with 
which  it  is  mixed. 
By  means  of  the  above  process  a  much  larger  product  of  tannic 
acid  may  be  obtained  than  with  either  pure  or  watery  ether.  The 
tannic  acid  remaining  in  the  upper  layer  may  likewise  be  obtained 
by  evaporating  the  liquid  to  dryness,  treating  the  residue  with  pure 
ether,  until  the  lower  of  the  two  layers  into  which  the  liquid  sep- 
arates no  longer  presents  a  green  color.  It  is  then  separated,  fil- 
tered, if  necessary  a  little  alcohol  added,  and  evaporated. 
The  process  recommended  by  Mohr,  of  treating  the  galls  with  a 
mixture  of  alcohol  and  ether  in  equal  volumes,  than  evaporating 
