f 4 Experiments and Olservat'ions t>n the 



tent, in water, and possessed of the power of acting upon 

 gelatine. From the products given by their incmeration 

 and by their distillation, I am, however, incUned to believe 

 that they contain, besides tannin, a portikin of gallic acid 

 and extractive matter, and a quantity of the salt employed 

 in the primitive solution. 



It is well known th.ii many of the metallic solutions oc- 

 casion dense precipitates in the inlus.on of galls ; and it has 

 hccu aenevally supposed that these precipitates are composed 

 of tamiin and extractive matter, or of those two substances 

 and callic acid united to the metallic oxide; bu. from 'he 

 observation of d'iferent processes of this kind, in which the 

 salts of iron and of tin were employed, I am inclined to 

 believe that they contain also a portion of the acid of the 

 saline compound. 



When the muriate of tin was made to act upon a portion 

 of the infusion, till no more precipitation could be produced 

 in it, the fluid that passed through the filter still acted upon 

 gelatine, and seemed to contain no excess of acid ; for it 

 gave a precipitate to carbonate of potash without producing 

 effeiTCseence. The solid compound, when decomposed by 

 sulphuretted hydiogen, alter the manner recommended by 

 Mr. Proust, was found strongly to redden litmus paper, and 

 it copiously precipitated nitrate of silver ; whereas the pri- 

 mitive infusion only rendered it slightly turbid ; so that 

 there is every reason to believe that the precipitate contained 

 nuiriatic acid. 



By passing the black and turbid fluid, procured by the 

 action of solution of oxygenated sulphate of iron in excess 

 upon a portion of the infusion, through finely-divided pure 

 flint, contained in four folds of filtrating paper, I obtained 

 a lisiht olive-green fluid, in which there was no excess of 

 sulphuric acid, and which I am inclined to suppose was a. 

 solution of the compound of gallic acid and su'phateof iron, 

 with superabundance of metallic salt. I have already men- 

 tioned that gallic acid, when in very small proportion, does 

 not precipitate the oxygenated salt* of iron ; and Mr. Proust, 

 in his ingenious paper upon the Difl'erence of the Salts of 

 Iron, has supposed that, m the formation of ink, a portion 

 of the oxide of iron in union w ith gallic acid is dissolved 

 by the sulphuric acid of the sulphate. This comes near to 

 the opinion that they form a lri[ Ic crmpound ; and, in rea-> 

 soiling upon the general ph;eriomena, it seenis fair to con- 

 clude thai, in the case of the precipitation of tannin by the 

 iji^lts of tin and of iron, compounds are formed of tannin 



ijind 



