CYNIPS QUERCUS FOLII. 



large quantity of tannic acid. The aqueous infusion red- 

 dens litmus. Sir H. Davy found 25 per cent, of tannin, 6.2 

 of gallic acid and extractive, 2.4 of mucilage, and 2.4 of saline 

 and earthy matters, in galls ; but Royer states that he obtains 

 125 grains of pure gallic acid from 500 grains of the galls ; 

 and Dr. Duncan thinks that Sir H. Davy has estimated the 

 quantity of tannin too low ; in one experiment with 500 

 grains of gall-nuts, Dr. Duncan obtained 220 grains, and in 

 another 256 grains of soluble matter. 



The chemist who has most successfully examined galls is 

 Pelouze, who has obtained from them by means of ether from 

 40 to 60 per cent, of tannic acid. He recommends that pow- 

 dered gall-nuts should be introduced into a tube closed at 

 one end with a linen rag, and pouring over them sulphuric 

 ether. The lower end of the tube is then inserted in a com- 

 mon jar or bottle. The ether gradually yields its water to 

 the tannic acid, and forms with it a thick syrup, which is 

 pushed into the bottle by the expansion of the ether above it. 

 This syrup, which consists of water, ether, and tannic acid, 

 being evaporated, leaves the latter in a state of purity. The 

 acid, however, is more easily procured by a process recom- 

 mended by M. Leionnot ; namely, to macerate the powdered 

 galls in ether and submit the pulp to the action of a strong 

 press, repeating the process on the marc several successive 

 times, until the gall-powder is exhausted, and then uniting 

 the solutions, distilling off a portion of the ether at a low 

 heat, and evaporating the residue in a water-bath. 



Tannic acid, when pure, is nearly colorless, apparently but 

 not regularly crystallized, inodorous, and powerfully astrin- 

 gent to the taste. It is unalterable in the air, and is easily 

 pulverized. It is a true acid, capable of decomposing the 

 alkaline carbonates with effervescence, and forming tannates 

 with oxides. It dissolves readily in water, and is soluble in 

 alcohol and ether. When exposed to the air in solution for 

 a considerable time, it absorbs oxygen, which changes it into 

 gallic acid. It is a curious fact, that, although tannic acid 

 is a real acid, yet it is precipitated from its solution by nitric, 

 phosphoric, hydrochloric, and arsenic acids. Protosulphate 

 of iron produces no alteration on the solution of pure tannic 

 acid; but the sesquisulphate immediately precipitates it in 

 combination with the oxide, of a deep bluish-black color; if 

 an excess of the solution be added, what remains undecom- 



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