32 
THE TANNINS. 
diluted with ten times its volume of water, will pro¬ 
duce a brilliant red color with solutions of the tannins, 
even when highly diluted. 
Other characteristic tests for the tannins will be found 
under the chemical properties of the different varieties. 
The methods for the quantitative determination of 
the tannins may conveniently be considered in four 
groups: 
1. Miscellaneous methods, not belonging to any of 
the following. 
2. Precipitation by metallic salts. 
3. The gelatin or hide methods. 
4. The permanganate methods. 
1. Under the first class is the process of Commaile, 
which was based on the reduction of iodic acid by 
tannin in the presence of hydrocyanic acid. The iodic 
acid not consumed was estimated by reduction with 
sulphurous acid and precipitation by silver nitrate. 
This method has been unfavorably criticised by Cecil 
and by Watts. 
P. Becker suggested the use of methyl violet, 0.250 
gramme in one litre. This was standardized by a one- 
per-cent. tannin solution, and it could then be used on 
tannin solutions of unknown strength. 
Mittenzwey recommended a process in which the 
volume of oxygen absorbed by an alkaline solution of 
tannin was measured. Cecil found this to give unsat¬ 
isfactory results, but Watts, after improving some 
details in the apparatus, considered it a fairly accurate 
method. 
Wagner proposed titrating the tannin solution with 
cinchonine sulphate, using rosaniline acetate as an indi- 
