44 
THE TANNINS. 
treated with permanganate are again decolorized by 
that reagent. The difference in volume of perman¬ 
ganate solution used before and after treatment with 
gelatin gives the amount of permanganate required to 
oxidize this known amount of tannin. 
This tannin solution can then be replaced by an in¬ 
fusion which represents a known quantity of tannin 
material, and the percentage readily calculated. The 
indigo serves as an indicator and at the same time con¬ 
trols the oxidation ; since it is decolorized with some 
difficulty, it indicates that the other substances have all 
been oxidized when the blue color disappears. 
SAMPLING AND PREPARATION OF THE TANNIN 
INFUSION. 
From 500 to 1000 grammes are so taken as to rep¬ 
resent the whole cargo. This sample is then ground in 
a drug-mill or powdered in a mortar until all of it 
will pass through a sieve of 20 meshes to the inch. It 
is not sufficient to discontinue the grinding when 
enough has been obtained to furnish 10 or 20 grammes 
for the infusion, but the whole 500 or 1000 grammes 
must be ground, sieved, and then thoroughly mixed 
before proceeding, since the first portion ground is 
much softer than that reduced last, and therefore repre¬ 
sents a different amount of tannin. After all has been 
ground, sieved, and thoroughly mixed, 5 grammes of 
galls, 10 grammes of sumac, valonia, or myrobalans, 
or 20 grammes of bark are boiled for one-half hour 
with 500 cc. of water, strained into a litre flask, and 
washed and pressed with sufficient water to bring the 
measure of the infusion to one litre. 
