J<3$ ON TANIN AND ITS SPECIES. 



Abridgment of a Memoir of Mr. Proust on Tanin and its. 

 Species *, 



Uncertainty of A HE procefs which Mr. Prouft has given for feparating 

 feparating 'tanin Tanin ty muriate of tin is fubjeft to confiderable uncertainty 

 by muriate of when it is ufed, as an Englifh Chemift has lately done to fix 

 the proportion of this principle in vegetable juices. He thinks, 

 therefore, that it may be of utility to announce thefe caufes of 

 error, in order that philofophers, who are engaged in this 

 department of refearch, may place lefs confidence in it, and 

 turn their thoughts to fome more perfect method. To thefe 

 remarks he adds an account of certain varieties which he 

 thinks he has perceived in the genus of Tanin. 

 When the oxide 1. When this muriate is faturated with an aftringent juice, 

 by the unTrTthe ** na PP ens tnat; the muriatic acid takes up in folutiona portion 

 muriatic acid of the tannate of tin, fo that what is collected on the filter re- 

 inftead of being p re f en t s , i n truth, only a part of the tanin principle contained 

 diflblves part of in the plant. This effect is fimilar to what happens in the. 

 the tannate. preparation of ink, the black dye, and in every cafe where a 

 dyeing principle can deprive an acid of the oxide which it 

 held in folution. The affinity oft his acid not being capable 

 of remaining inactive exerts itfelf upon the coloured oxide. 

 This tannate When fmall dofes of alkali are added to the liquor the re* 



may be thrown f ]C j ue f ^ e tannate may be made to precipitate without even. 



down by careful ...-., r * * v •/* i • •'" •■ 



additionof alkali, touching the gallic acid if it be prefent ; and if this point be 



exceeded it may foon be perceived by the green colour which 



the fluid receives from contact of the air. In that cafe a few 



will beove a r nnatCdro P s of acid wiH be fufficient to dif P eI the cloud by'feizing 

 charged with the excefs of alkali and fetting the gallic acid at liberty. But 



oxide if too as ^ on ^ e ther part, it is neceflary at firft to employ an ex- 

 much of the _ ._ . * , . ." . ~ , . Z v , , 

 muriate has been ce * s °* muriate to allure the precipitation or the whole of the 



ufed. tanin, there is danger of loading the tannate with a certain 



excefs of oxide of tin. 



The muriate 2. The prefence of tanin does not exclude the extractive 



precipitate* ex- principle in the juice of a plant, and as the muriate precipitates 



and perhapsother this laft as well as the former, the extractive principle will 



principles along therefore become a new fource of miftake in the eftimate of 

 with the tanin. 



* From the Annalcs de Chemie XLII. 89. 



the 



