Vinen, loc. cit.), with gallic and ellagic acids, ligneous fibre, water, and minute quantities of proteids, chlorophyll, resin, free sugar and, in the cells around the inner shelly chamber, calcium oxalate.
The tannin of oak, C/9H16010, which is found, mixed with gallic acid, ellagic acid and quercite, in oak bark, is a red powder; its aqueous solution is coloured dark blue by ferric chloride, and boiling with dilute sulphuric acid gives oak red or phlobaphene.
Oxidizing agents, such as arsenic acid, convert it into ellagic acid, C 14 H 8 0 9 +H 2 0, probably a fluorene derivative, a substance which occurs in gall-nuts, in the external membrane of the episperm of the walnut, and prob ably in many plants, and composes the "bezoar stones" found in the intestines of Persian wild goats.