ON VEGETABLE ASTRINGENTS. 



213 



though it is generally stated, that gallic acid is less soluble 

 than tan*, and it is upon this princi pie, that Mr. Biggins 

 founded his process *br ascertaining the relative proportion 

 of tan and gallic acid in the different substances employed 

 in the formation of leatherf. 



The solubility of the tan and the extract, so far as their Tan and ex- 

 presence is indicated by jelly and the oximuriate of tin, ap- ^ll "in" y 

 pears to be nearly equal. This is also contrary to the gene- solubility, 

 rally received opinionf, but I ground my position upon the 

 following experiment. A quantity of galls was employed 

 in the state of coarse powder, in order that it might more 

 readily subside from the infusion. The proportion of the 

 galls, and the length of time occupied in the infusion, were 

 gradually diminished, until 1 found, that by infusing the 

 galls in fifty times their weight of water for only five mi- 

 nutes, a fluid was obtained, capable of forming precipitates 

 with jelly and the oximuriate of tin, which w ere bareiy visi- 

 ble, but as far as could be judged by the eye, equal to each 

 Other. 



Beside jelly, the muriate of tin, and the oxisulphate of Other reagents 

 iron, there have been other agents employed in the analysis em P l °y ed ' 

 of galls. Of these the principal are the sulphuric and mu- 

 riatic acids, the carbonated fixed alkalis, the aluminous 

 ealts, lime water, and the acetate of lead. The acids have Acids. 

 been considered as acting principally upon the tan, and with 

 this view have been proposed as a means of separating it 

 from the other ingredients of the infusion§. They are, 

 however, less delicate tests than jelly ; for I have found, in 

 the successive infusions, that jelly still throws down a consi- 

 derable precipitate, when they have ceased to act ; of the 

 two, the sulphuric is the more delicate. Nearly the same 

 remarks apply to the carbonated alkalis and to lime water, Limewater. 

 as to the acids ; they both throw down copious precipitates 



* Seguin ami Chaussier, Journ. Polyt. IV, 678. 



t Phil.Trins. 1799, 261. Thomson's Fourcroy, III, 93. 



X Davy, Phil. Trans. 1803, p. 234. 



§ The proposal seems to have been first made by Mr. Dize, but Proust 

 and Vauquelin both agree, that the Van may be completely separated by 

 the acids. Ann. de Chim. XXXV, 37. 



from 



