on the constituent Parts of astringent Vegetables. 249 
to the metallic oxide ; but, from the observation of different pro- 
cesses 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 filtre still acted upon gelatine, and 
seemed to contain no excess of acid; for it gave a precipitate 
to carbonate of potash, without producing effervescence. The 
solid compound, when decomposed by sulphuretted hydrogen, 
after the manner recommended by Mr. Proust, was found 
strongly to redden litmus-paper, and it copiously precipitated 
nitrate of silver ; whereas, the primitive infusion only rendered 
it slightly turbid ; so that there is every reason to believe, that 
the precipitate contained muriatic 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, con- 
tained in four folds of filtrating paper, I obtained a light 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 sulphate of iron, with superabundance of me- 
tallic salt. I have already mentioned that gallic acid, when in 
very small proportion, does not precipitate the oxygenated salts 
of iron ; and Mr. Proust, in his ingenious Paper upon the Dif- 
ference of the Salts of Iron, has supposed that, in the formation 
of ink, a portion of the oxide of iron in union with gallic acid 
is dissolved by the sulphuric acid of the sulphate. This comes 
near to the opinion that they form a triple compound : and, in 
reasoning upon the general phenomena, it seems fair to 
MDCCCIII. K k 
