Fads for a History of the Gallic Acid. 293 



I dissolved Gl o-rammes ot" gallic acid, confusedly crystal- 

 lized and still very hrf)vvn, in live hcctograinnies ot' boiling 

 water. I kcp« a niirt of this solution in order to compare it ; 

 the rest was submllted to ebullition with fil grains of oxide 

 of tin well washed and'Still humid; I evaporated until there 

 remained about half of the liquor only, and I afterwards 

 added a sufficient quantity of water to restore it to its pri- 

 mitive weight : I then compared them ; the latter had lost 

 its colour considerably. The difference in the degree of 

 acidity was not sensible ; it still precipitated glue. The 

 precipitate of it was yellow and flocculent, while that by 

 the unpurified liquor was brown, muddy, and more abun- 

 dant ; it even cono^ealed into a mass. We see that the acid 

 is not yet decomposed ; but I could not obtain, like M. Ber- 

 thollet, crystals so white and pure as those given by the 

 process of sublimation. 



Being desirous of knowing if a new quantity of oxide of 

 tin would entirely deprive this acid of tannin, I added to 

 the liquor 30 grammes of oxide of tin, and evaporated 

 until there only remained about 100 grammes of liquor, 

 which was poured off clear without colour ; it did not pre- 

 cipitate either sulphate of iron or glus. I could not 'obtain 

 gallic acid by evaporation. 



This experiment proves that it is very difficult to deprive 

 the gallic acid completely of tannin ; and that, by pursuing 

 the action of the oxide of tin, we decompose the acid. It 

 is certainly in this manner that M. Proust has proceeded ; 

 because this chemist has observed in his Memoir upon Tan- 

 nin, printed in ihe Anvales da Chimie, tom. xlii. that the 

 <.xide of tin he used ibr purifying the gallic acid, only gave 

 hiui as a product a liquor without taste or colour, making 

 not (tie slightest impression upon the solutions ot iron or 

 tincture of turnsole. 



As to the methods proposed by M. Bartholdi, I do not 

 think they can be employed. However, as the author has 

 neglected to examine the products of his operations, I 

 thoi-.ght it was neeessary to repeat his experiments, and lo 

 . determine the matter of the results to which they might give 

 birth. For this purpose f p^mred a solution of galhc acid 

 T 3 "P"" 



