24 
THE TANNINS. 
bichromate and permanganate, decompose them very 
easily into carbon dioxide and some other unknown 
compound. When heated dry with arsenic acid, some 
of them are converted into ellagic acid. Most tannins 
are precipitated by alkaloids, notwithstanding the fact 
that both frequently exist in the same plant without 
forming an insoluble compound. The action of dilute 
acids and alkalies is one of considerable importance in 
determining their character; some, for instance, when 
heated to 100° with a one or two per cent, hydro¬ 
chloric acid solution, yield crystalline compounds as 
gallic and ellagic acid, while others yield dark-colored 
amorphous compounds allied to the phlobaphenes; 
glucose is formed at the same time in nearly all the 
decompositions, thus showing their glucosidal nature. 
Another method of determining the character of 
tannins is by the application of heat. When a tan¬ 
nin is heated to 120° no decided change takes place 
except a slight darkening in color; on continuing the 
heat the color gradually becomes darker, until at 160° 
decomposition takes place. With some tannins the 
products are metagallic acid and pyrogallol, and with 
others metagallic acid and catechol. It has been sug¬ 
gested to divide the tannins into two classes according 
as they yield, when heated, pyrogallol or catechol. 
Those which produce a brownish-colored deposit on 
leather, called “bloom,” and form blue compounds with 
ferric salts yield, when heated, a sublimate of pyro¬ 
gallol, while those which do not form a “ bloom” on 
leather and give green compounds with ferric salts 
yield, when heated, a sublimate of catechol. To the 
former class belong the tannins from galls, sumac, 
