292 Facts fur a History of the Gallic Acid. 



from it very small and very white prismatic crystals ; the 

 mother water furnishes them also, but these are generally 

 a little coloured ; it is sufficient to wash them with water, ■ 

 in order to obtain ihem very white. We obtain by this pro- , 

 cess half an ounce of crystals for each pound of gall-nuts: 

 these crystals are extremely light. 



The processes of Scheele, Deyeux, and Ricbter, have 

 given us the most advantageous results ; but they differ with 

 respect to the purity of their acid. The first, as M. Bcr- 

 tholkt has observed, retains a great deal of tannin ; the se- 

 cond is perfectly v.hite, and the tiiird contains tannin also.' 

 By M. Richter's process, the acid, after having been pu- 

 rified, is of a straw colour. T tried, without effect, to bring 

 ii to that state oF purity pointed out by the author: I ascer- 

 tained that, if we wished to pursue the evaporation, desicca- 

 tion, and afterwards the action of alcqhol, we might each 

 lime decompose a certain quantity of acid, so that the alco- 

 holic liquor, in place of being more transparent, would be- 

 come brown. There is therefore a term at which we must 

 stop, if we wish to preserve the totality of the acid as well 

 as its properties. 



IVI. Berthollet has attempted several methods of purifying 

 Scheele's acid ; the plan which succeeded best, was to treat 

 this acid with oxide of tin recently precipitated from its so- 

 lution by an acid. 



1 repeated this experiment. The following is the manner 

 in which I operated, and the phaenomena I observed : 



After having separated the oxide from the muriate of tin 

 by an alkaline base, it was washed clean with boihng watery 

 it was then boiled for some time in a new quantity of'wafer. 

 It was afterwards treated with gallic acid, and evaporated to 

 the consistence of thick honey. Distilled water was then- 

 added ; the liquor, after having been filtered, was clear and 

 limpid, and without taste or smell : when evaporated to 

 dryness no produce %vas obtained. 



This differenre in the results obtained by M. Berthollet 

 gave me reason to suppose that I had committed an error: 

 1 began the experiment agaiu; and paid every possible atten- 

 tion to it. 



I dissolved 



