GENERAL CHARACTERS. 
25 
chestnut,, and oak, and to the latter those from hem¬ 
lock, catechu, rhatany, and mangrove. It is known 
not only that the tannins in the two classes have a dif¬ 
ferent composition, but that most of those in the same 
class differ from one another in many particulars. In 
order to assist one in determining the character of a 
tannin it is necessary to study its decompositions by 
heat alone, by acids, and by alkalies. 
Procter, in his “ Text-Book of Tanning,” and Allen, 
in his “ Commercial Organic Analysis,” have given 
similar schemes for carrying out these three processes. 
The decomposition by heat may be effected by heat¬ 
ing the sample in a small glass retort, but this is at¬ 
tended with considerable loss from a portion becoming 
overheated. The decomposition should take place by 
slowly raising the temperature from 160° to 215°, 
keeping it between 200° and 210° for twenty minutes. 
It has been suggested to assist the sublimation of the 
volatile products by leading a stream of carbon dioxide 
into the retort to carry them off as rapidly as formed. 
Thorpe suggested a method of preparing pyrogallol 
for photographic purposes by heating gallic acid in 
glycerin as above directed, and this has since been 
applied to decomposing the tannins. 
The details of the method are to heat one gramme 
of the tannin in 5 cc. of glycerin, slowly raising the 
temperature from 160°, and keeping it for from twenty 
to thirty minutes between 200° and 210°. It is then 
allowed to cool, 20 cc. of water are added, and the 
liquid is shaken with an equal volume of ether with¬ 
out previous filtration. The separated ethereal layer 
is evaporated to dryness and dissolved in 50 cc. of 
3 
