ammonia. Perkin [Footnote: _Proc. Chem. Soc._, 1905, 21, 212.] obtained
a substance very similar to ellagic acid by electrolysis of gallic acid
in sulphuric acid solution; on oxidising gallic acid in concentrated
sulphuric acid solution, Perkin and Nierenstein [Footnote: _Ibid._,
1905, 21, 185.] obtained flavellagic acid. Ellagic acid is also
obtained by heating luteic acid in a 10 per cent. soda solution.

Ellagic acid thus prepared crystallises with 2 molecules of water as
yellow micro-crystalline rhombic prisms or prismatic needles. The
crystals lose this water when heated to 100 C., and it is possible that
it is water of constitution, in which case the substance would be
hexoxydiphenylcarboxylic acid, and the substance left after drying at
100 C., the dilactone.[Footnote: _Arch. d. Pharm_., 1907, 244, 575.]
Ellagic acid is slightly soluble in water, alcohol, and ether, but is
easily soluble in caustic potash. With concentrated nitric acid the
product assumes a red colour, which appears to be due to the presence of
impurities; ellagic acid is commercially known as "alizarin yellow."

The constitution of ellagic acid was uncertain for a long time, and
different structural formulae were proposed which more or less
corresponded to its properties. The most satisfactory structural formula
was proposed by Graebe--[Footnote: _Chem. Ztg_., 1903, 129.]

---CO.O---
^ -------- ^ OH
| | | |
HO V --O.CO-- V OH
OH

This would represent a tetroxydiphenylmethylolide.