22 
THE TANNINS. 
under reduced pressure, and the residue is dissolved in 
such a quantity of water as will allow the solution to 
filter readily. This aqueous solution is distilled to dry¬ 
ness under reduced pressure, and the residue dissolved in 
a small quantity of water and rapidly distilled to dry¬ 
ness in as near a vacuum as possible, using in this, as in 
the preceding cases, a water-bath as the source of heat, 
when the tannin will “ puff up” and remain as a light- 
or dark-brown residue, according to the character of the 
original material. It may then be purified according to 
the method given later for the purification of gallotan- 
nic acid. This process may be used for most substances 
which contain five per cent, of tannin or over; but it is 
well to bear in mind that there is no separation of the 
percolate into layers, as in the case of that from galls. 
If less than five per cent, of tannin be present, it is 
more economical to extract with water and precipitate 
the tannin with lead acetate. The resulting lead tan- 
nate is then suspended in water, decomposed by hydro¬ 
gen sulphide, and the filtrate from lead sulphide dis¬ 
tilled to dryness under reduced pressure. Alcohol is 
sometimes used, but where the solvent can be recovered 
it has few if any advantages over ether. When econ¬ 
omy is the first consideration hot water is used, and this 
will later claim especial attention under tannin extracts. 
In a few cases cold instead of hot water must be used 
or the impurities will predominate, as in the case of 
canaigre, which contains a large quantity of starch that 
is dissolved with the tannin by hot water and interferes 
with the use of the extract for many purposes. 
Always when a pure tannin is desired heat and ex¬ 
posure to air must be avoided. Many plants appear 
