28 
THE TANNINS. 
The ethereal solution which removed gallic acid is 
evaporated, the residue, dissolved in warm water, al¬ 
lowed to cool, and filtered, when gallic acid, if present 
in the filtrate, may be detected by the red coloration 
produced by the addition of a few drops of potassium 
cyanide solution. 
Since glucose occurs as a mixture in most tannins, it 
is necessary first to get rid of it before applying the 
above process. This may be accomplished by repeated 
purification of the tannin by precipitation with lead 
acetate; but such a process is very slow and tedious: 
so it has been suggested to use the lead salt for heating 
with the acid, in which case the sediment which sepa¬ 
rates when the liquid cools will consist in part or all of 
lead chloride, which may be collected on a filter with 
the ellagic acid and treated with boiling water, in which 
the lead chloride will dissolve. 
It has been found by the author that the shortest 
method is to determine the amount of glucose in a 
tannin by removing the latter with lead acetate, and 
the excess of lead with hydrogen sulphide. The fil¬ 
trate is then heated on a water-bath until every trace 
of hydrogen sulphide is driven off, neutralized with 
sodium hydrate, filtered, and the glucose estimated in 
the filtrate gravimetrically with Fehling’s solution. 
Another portion of the sample is then heated with 
hydrochloric acid, and, after carrying out the process 
of hydrolysis, the glucose is estimated. Any excess of 
glucose in the second estimation over that in the first 
indicates that much glucose existed in the tannin as 
glucoside. 
The action of alkalies is determined either on a sample 
