The astringent principle is a peculiar kind of tannic acid, called by chemists quercitannic, which, yielding more stable compounds with gelatine than other forms, gives oak bark its high value to the tanner.
This substance differs from the mucins by being precipitated by tannic acid but not by acetic acid, and being endowed with a higher proportion of sulphur.
Since the introduction of iron ships teak has supplanted oak, because it contains an essential oil which preserves iron and steel, instead of corroding them like the tannic acid contained in oak.
From tannic acid is also made gallic acid, which resembles tannic acid but has no astringent taste.
The treatment of strychnine poisoning is to immediately evacuate the stomach with a stomach-pump or emetic, chloroform being administered to allay the spasms. If the patient can swallow, draughts of water containing tannic acid may be given.
In the intestine tannic acid controls intestinal bleeding, acting as a powerful astringent and causing constipation; for this reason it has been recommended to check diarrhoea.
The action of tannic acid is strictly local, and depends upon its power of precipitating albumen and of destroying germs. It thus acts as an astringent on all mucous membranes.
The results of applying tannic acid are to harden the pelt and discolour and weaken the fur.
Unlike tannic acid, gallic acid does not precipitate albumen or salts of the alkaloids, or, except when mixed with gum, gelatin.
In these the variety of tannic acid is not exactly the same, but although there are slight chemical differences, they all possess the power of tanning raw hides and of preserving albuminous tissues.