COMPOSITION OF TANNIC ACID. 
99 
We must conclude, from our present knowledge, that 
the tannin from galls as found in commerce is capable 
of being purified until it is simply digallic acid. It is, 
however, rarely, if ever, found in this condition, but 
contains variable amounts of glucose in weak combina¬ 
tion ; therefore tannic acid as we know it is either a 
glucoside of digallic acid or a mixture of this glucoside 
with variable proportions of digallic acid. 
