GENERAL CHARACTERS. 
21 
and, on the other hand, an iron-greening tannin may 
be changed to an iron-bluing one by the cautious addi¬ 
tion of alkali. 
A more recent and much more ex act classification is 
that of Wagner into 'pathological tannins, those formed 
on vegetable tissue by the sting of an insect, and 'phys¬ 
iological tannins, those occurring in vegetable tissue. 
To the former class belongs gallotannic acid, and to the 
latter the great number of tannins found throughout 
the vegetable kingdom. The former give blue and the 
latter give both blue and green precipitates with salts of 
iron. It has been found that some of the latter class 
are identical with gallotannic acid ; this was first pointed . 
out to be true of sumactannic acid by Stenhouse, and 
was later confirmed by Lowe. 
The method of preparing a tannin depends very 
much on the character of the material in which it 
occurs. Most of the published processes are based on 
the use of galls as the source, and a process available 
for exhausting them is not adapted to extracting a hard 
woody substance containing only five or ten per cent, 
of tannin. A process will, therefore, be given for each 
tannin, unless it can be referred to the following gen¬ 
eral method, which will be found available in many 
cases. 
The finely-powdered material is packed in a perco¬ 
lator and extracted with commercial ether. The offi¬ 
cial ether of the United States Pharmacopoeia, contain¬ 
ing seventy-four parts ether, twenty-six parts alcohol, 
and a small quantity of water, gives satisfactory results; 
its specific gravity is very close to 0.750. The ether is 
recovered by distillation, the last portions being removed 
A 
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