Synthetically  it  may  be  prepared  by  the  oxidation  of  gallic
acid  by  means  of  arsenic  acid,  or  better  by  oxidizing  gallic
acid  in  acetic  acid  solution  with  potassium  persulphate  and
sulphuric  acid.*

Ellagic  acid  is  a  yellow  microcrystalline  solid  which  is
very  slightly  soluble  in  water,  and  therefore  readily  separates
from  aqueous  solutions  in  which  it  is  formed  ;  it  is  also  very
slightly  soluble  in  alcohol  or  ether,  but  dissolves  somewhat
readily  in  boiling  pyridine.  The  dried  substance  treated  with
1-2  drops  of  nitric  acid  gives,  on  dilution  with  10-20  drops  of
water,  a  blood-red  colour  (Griessmayer's  reaction).
Its  constitution  is,  according  to  Graebe,  best  represented  by
the  formula —


\o  .  Co/


from  which  it  will  be  seen  that  it  may  be  considered  to  be  pro-
duced by  the  abstraction  of  two  molecules  of  water  from  two
molecules  of  gallic  acid  with  simultaneous  oxidation  or  removal
of  two  atoms  of  hydrogen,

Ellagic  acid  is  used  to  some  extent  as  a  dye-stuff,  being
sold  under  the  name  of  "  Alizarine  yellow  in  paste,"  for  use
with  chromium  mordants.
Catellagic,  Metellagic,  and  Flavellagic  acids  are  the  names
given  by  I'erkin  to  artificially  synthesized  acids  obtained  by
him.  They  are  closely  related  to  ellagic  acid,  but  have  not,
as  yet,  been  found  to  occur  naturally.
*  Perkin  and  Nierenstein;  "J,  Chem.  Soc,  Lond.,"  1905,  87,  1415.
2o8  TANNINS

PHLOBAPHENES.
