TANNIC ACID. 939 



by loosening the stopper above and admitting air. It FIG. 95. 

 is found to divide into two liquids, of which the 

 denser syrupy and yellowish one, is a very concentrat- 

 ed solution of tannic acid in water, and the lighter, 

 which is coloured green, an ethereal solution of gallic 

 acid and other matters. Additions of ether are made 

 to the gall-nuts, so long as two different liquids flow 

 from the lower orifice. The aqueous ether employed 

 is obtained by agitating common ether with water. If 

 the gall-nuts be moistened with water before the 

 addition of the ether, the solution of tannin which 

 comes off is highly coloured ; but if the gall-nuts be 

 merely exposed to steam, and washed in the perco- 

 lator with anhydrous ether, the process succeeds 

 equally well as with the aqueous ether. j/ I 



By the evaporation of the solution of tannin, a yellow 

 light mass is generally obtained, which is purified by so- 

 lution in water, and evaporation in vacuo over sulphuric acid. It 

 then forms a mass colourless or slightly yellow, which is not crys- 

 talline but resembles dried gum, and becomes somewhat deeper 

 in tint in humid air, but is not otherwise altered. It is dis- 

 solved easily by water and in large quantity ; the taste of the 

 solution is purely astringent without bitterness, it reddens vege- 

 table blues, and decomposes alkaline carbonates with effer- 

 vescence. Tannic acid is soluble in aqueous alcohol, but only 

 very slightly soluble in ether. Its solution is affected by air, 

 particularly at a high temperature, oxygen being absorbed and 

 an equal volume of carbonic acid evolved, while the tannic acid 

 is transformed into gallic and ellagic acids. But the solution of 

 tannic acid keeps without change in close vessels. A mode- 

 rately strong solution of tannic acid gives with sulphuric, hydro- 

 chloric, phosphoric, arsenic or boracic acid, a thick white pre- 

 cipitate, which is a compound of the two acids mixed, and is 

 very soluble in pure water and in alcohol. When the solution 

 of tannic acid is precipitated hot by sulphuric acid, a resinous 

 mass is formed, which dissolves in dilute sulphuric acid at 

 the point of ebullition, of a deep tint, and after being boiled for 

 some minutes is converted, without any evolution, of gas, into 

 gallic acid, which crystallizes on the cooling of the solution. 



