84 
THE TANNINS. 
fully filtering, recovering as much of the alcohol as 
possible, and completing the evaporation in a vacuum. 
It is then broken into pieces which resemble rosin, and 
sent into commerce in that form, or it is first powdered 
by porcelain rollers, which operation gives it a much 
lighter color. The product should be completely solu¬ 
ble in water, indicating its freedom from resin. 
Water-tannin is extracted similarly to the ether-tan¬ 
nin, but after filtering, instead of mixing with ether, 
the solution is run directly into a vacuum-pan and con¬ 
centrated to dryness. It usually comes in commerce in 
the powdered form, which operation is accomplished 
similarly to the alcohol-tannin. 
Purification .—Tannic acid prepared by the foregoing 
methods is contaminated with fat, wax, resin, coloring- 
matter, gallic acid, and glucose. For most purposes the 
small amounts of these impurities are no objection, but 
at times the chemist desires a product free from more 
than traces of impurity, especially when the object is to 
determine its ultimate composition. The fat, wax, some 
coloring-matter, and the odorous principle may be re¬ 
moved by percolation with petroleum ether. 
A number of processes have been suggested for re¬ 
moving the other impurities, all of which, however, 
are attended with considerable waste. One method is 
to take the best ether-tannin and treat it with stronger 
ether, to which just sufficient water is added to effect 
solution. This is best accomplished by using 100 
grammes of the tannic acid, 150 cc. of stronger ether, 
and 100 cc. of water. Three layers are formed, the 
lowest of which is a concentrated solution of nearly 
pure tannic acid. Considerable loss occurs by this pro- 
