OX COFFEE. 



123 



riatic ; their action in general being in the ratio of their 



acidity. 



I treated a precipitate of sulphate of iron, obtained by The precipitate 



means of gallic acid, in the same manne?, and the results of iron was a 



, gallat. 



were no way different from the preceding. 



The remainder of the decoction of coffee I precipitated Precipitated by 

 by muriate of tin. This salt occasioned a very copious munatc oftin ' 

 sediment in the liquor, which I washed with water, till no 

 marks of acidity were perceptible in it. I afterward put 

 this metallic compound into a tubulated phial, and poured on 

 it a considerable quantity of distilled water. I then and the tin 

 adapted the phial to a Woulfe's apparatus, so as to pass b^^retted 

 sulphuretted hidrogen gas over the precipitate. The first hidrogen, 

 portions of gas changed the mixture brown, and this colour 

 grew deeper and deeper, in proportion as the liquor be- 

 came saturated with the gas. The precipitate was de- 

 composed: a hidrosulphuret of tin was formed: and the 

 disengaged acid was taken up by the liquor. The liquor, 

 being first filtered, was evaporated by a gentle heat, till it 

 was reduced to one eighth. This product, considered by 

 Mr. Paysse as coffic acid, appeared to me to be nothing but Not a peculiar 

 gallic acid. I not only subjected it to the action of all the a °jj*» but the 

 reagents comparatively with acid obtained from galls in the Proofs, 

 usual way; but, that I might leave no doubt on this head, 

 I treated galls by the same process. In this decoction the 

 muriate of tin formed a more abundant precipitate than in 

 the decoction of coffee : the precipitate, decomposed like 

 the preceding by sulphuretted hidrogen gas, afforded me an 

 acid of the same colour, the same taste, possessing the 

 same properties, and differing only in quantity. I think, 

 therefore, I may conclude, that the coffic acid does not 

 exist; but that coffee contains less gallic acid than nut- 

 galls. 



It is possible, that this gallic acid may exhibit in its com- May have some 

 binations and compounds some slight shades of difference f er a en e c s e °^ h e~ 

 from the acid obtained from the gall of the oak, but it is obtained from 



nevertheless of the same nature. 



galls, 



The immediate materials of vegetables, as is well known, as is the case 

 though of the same kind, and perfectly analogous, are not with other ini- 

 strictly identical. The gums and sugars exhibit differences cipies of ^ge- 

 lt 2 in tables < 



