THE PROPERTIES OF TANNIC ACID. 89 
ellagic acid; by prolonging this action carbon dioxide 
aud oxalic acid also result. 
When subjected to heat tannic acid changes but little 
until 120° is reached, when it assumes a grayish color, 
without, however, undergoing apparent decomposition, 
at 140° it is darker, and at from 160° to 215° there 
are formed carbon dioxide, pyrogallic acid, water, and 
metagallic acid. The last is not volatile, while the 
others are volatile; and this admits of a ready method 
of separating the meta-acid, which remains behind as 
a-black, amorphous mass, from the pyro-acid, which 
sublimes in acicular crystals. Since the latter vola¬ 
tilizes at 210°, it is natural to suppose that, in order to 
obtain it from tannic acid, its volatilizing temperature 
must be reached; but it is found that the decomposi¬ 
tion begins at 160°, and, in the escape of carbon dioxide 
and water, pyrogallic acid is carried off and condensed 
in the cooler parts of the vessel. The best yield is ob¬ 
tained by allowing the temperature to rise very slowly 
to 215°, keeping it between 190° and 210° for one- 
half hour. If the heat be increased rapidly to 215°, 
there will be a small quantity of the pyro-acid and a 
larger amount of the meta-acid produced. 
On heating with a 2-per-cent, solution of absolute 
hydrochloric acid, tannic acid is converted into gallic 
acid: 
QuHxoOg -f- H 2 0 = 2C 7 H 6 0 5 . 
The pure acid yields gallic acid only, but that usually 
met with gives variable amounts of glucose and ellagic 
acid at the same time. 
With solutions of the alkalies gallic acid is formed, 
8 * 
