GENERAL CHARACTERS. 
23 
to contain two tannins; when such is the case it will 
sometimes be found that ether extracts one, and the 
other may then be removed by water, or occasionally 
cold water will extract but one and the other may then 
be removed by the use of hot water. 
The tannins as a class are amorphous, odorless, 
slightly acid, and strongly astringent compounds, of a 
color varying from dark brown to pure white. They 
are, with few exceptions, soluble in water, alcohol, ether, 
and glycerin, and insoluble in chloroform, benzol, pe¬ 
troleum ether, carbon disulphide, fixed and volatile 
oils. 
In addition to the above reactions given with iron 
salts, most if not all the tannins give brown precipitates 
with potassium bichromate. Lime-water gives precip¬ 
itates, which, however, vary in color; at first they are 
white, but become rapidly darker, some red and others 
blue. Strong solutions of the alkaline carbonates also 
precipitate them. All the tannins are precipitated by 
gelatin or albumen; solutions of alum and ammonium 
chloride render the precipitation more complete. Sat¬ 
urated solutions of sodium chloride, calcium chloride, 
potassium acetate, and a number of other salts precipi¬ 
tate the tannins from solution ; the same is true of 
moderately concentrated sulphuric and hydrochloric 
acids. Concentrated sulphuric acid dissolves the dry 
tannins with a yellow color, which on heating become 
dark-red rufigallic acid and then black metagallic acid. 
Nitric acid does not precipitate the tannins from solu¬ 
tion, but it dissolves the dry compounds with a yellow 
color, and decomposes them with the formation of 
oxalic acid. Other oxidizing reagents, as potassium 
