Pdtis for a History of the (Gallic Acid. 33 



thists. In fact, it is difficult not to admit hydrogen in the 

 composition of galHc acid ; and M. Fourcroy has expressed 

 his doubts on this subject in his System of Chemical Know- 

 ledge, but the question is not yet decided by experiment. 



In consequeilcej I heated gallic dcid in a retort. The fire 

 \vas gradually raised till the retort was red-hot. During 

 this action df the fire I obtained sevetal jars full of clastic 

 fluid. The first contained only atmospheric air, the others 

 fcarbonic acid gas : at least, the gas had all the characters of 

 this acid ; but the pha2nomena that occurred during the de- 

 composition of the gallic acid led me to suspect, that, if 

 any hydrogen gas had beeii evolved, it could exist only in a 

 Very small quantity. I was hot satisfied, therefore, with the 

 trial by lime water, and the extinction of a taper in the gas. 

 Having perceived that hydrogen gas mixed with a great deal 

 of carbonic acid gas cannot be burnt> because this acid acts 

 too promptly on the fiame of the taper, I f^assed a little 

 caustic potash into the last jar of gas; agitated it, in order 

 to absorb the carbonic acid gas; and then immersed a taper " 

 in the residual gas, which burnt with flamcj and thus afforded 

 me a proof of the presence of carbonated hydrooen gas. 



The gallic acid, therl, like other vegetable acids, is com- 

 posed of oxygen, hydrogen, arid carbon. If but a small 

 quantity of hydrogcti can be obtained, it is because water" 

 is formed during the decomposition of the acid, so that the " 

 hydrogen passes over only when very little oxygen remains 

 to act on the carbon < 



1 have attempted to show that the' gallic ac'A is a corri- 

 pound. Its formation by Scheele's process appears to me 

 to favour this opinion. In fact, if the quantity of acid ex- 

 tracted from the aqueous infusion, exposed to the air, be com- 

 pared with that aflbrded by siiblirnation, I conceive it is not 

 difHcult to account for the increase. There can be no doubc 

 that acetic acid is formed in the liquor, which, acting on a 

 portion of tannin and extractive matter, constitutes the gallic 

 acid of Scheele : but this combination is rendered more iri- 

 timate, and somewhat different, by the action of caloric j 

 of which we have a proof when the acid is obtained by sub- 

 limation, for not only is the tannin decomposed, but the 



Vol. 29. No. 113. Oct. 1807* C acid 



