ON VEGETABLE ASTRINGENTS* f> 1 g 



ducing tile insoluble residue appear to be diminished. This 

 experiment proves, that the supposed extract of an astrin- 

 gent infusion cannot, according to the common opinion, be 

 separated by one evaporation ; and I think it ma) 7 lead us 

 to doubt, whether the effect is not rather produced upon its 

 solid contents in general, than upon any one part of them. 

 In confirmation of this supposition we may observe, that 

 there are several processes, in which the tan itself is ren- 

 dered insoluble by the addition of oxigen. Such is thought 

 to be the method, in which the oxide of tin operates upon 

 it ; for although what is thrown down is a compound of tan 

 and oxide, yet if the oxide be removed by the action of a 

 hidrosulphuret, the tan still remains insoluble. The same 

 kind of effect is also produced by the nitric and the oxi- 

 muriatic acids, they throw down from the infusion of galls 

 an insoluble compound, and deprive the fluid of the pro- 

 perty of precipitating jelly. The idea, that insolubility 

 after evaporation was a specific characteristic of extract, 

 seems to have originated from the experiments that were 

 performed by Mr. Fourcroy on the bark of St. Domingo ; 

 but ihe constituents of this bark are so different from those 

 of galls, that we are not authorized in extending the ana- 

 logy from one to the other, unless it be supported by some 

 independent facts. So far therefore as 1 may be warranted 

 to draw any conclusion on this subject from my own obser- 

 vation, either of the effect of successively evaporating the 

 same infusion, or of the changes mentioned above, which 

 are produced by long exposure to the atmosphere, 1 should 

 conclude, that the tan itself is rendered insoluble in both 

 these operations. 



The third circumstance, which has been adduced to Not distin- 

 prove the existence of extract, and especially to distinguish gashed from 



1 %ii- tan by its great- 



it from tan, is the greater insolubility of tne latter while er solubility. 



they both exist in their natural state. In analysing vege- 

 table astringents we are told, that by subjecting them to a 

 hasty infusion we shall procure the tan nearly free from ex- 

 tract, while by continuing the infusion for a greater length 

 of time we get a fluid which chiefly consists of this sub- 

 stance. It is true, that in applying water to galls, the first 

 portion takes up more than the subsequent ones; but that 



which 



