54 JtXPEKIMENTS ON ASTRINGENT VEGETABLES, 



I attempted, in different modes, to obtain uncombined gallicS 

 acid from the (olid matter produced by tiie evaporation of the 

 barks, but without fuccefs. When portions of this folid matter 

 were expofed to the degree of heat that is required for the pro- 

 dufilion of gallic acid from Aleppo galls, no cry ftals were form- 

 ed ; and the fluid that came over gave only a brown colour to 

 the folution of falts of iron, and was found to contain mucb 

 acetous acid and empyreumatic oil. 



When pure water was made to a6l, in fucceffive portions, 

 upon oak bark in coarfe powder, till all its foluble parts were 

 taken up, the quantities of liquor lafl; obtained, though they 

 did not aft much upon folution of gelatine, or perceptibly red- 

 den litmus-paper, produced a denfe black with the folution of 

 fuiphate of iron : by evaporation, they furniflied a brown mat- 

 ter, of which a part was rendered infoluble in water by the 

 adion of the atmofphere ; and the part foluble in water was 

 not in any degree taken up by fulphuric ether; fo that, if it 

 contained gallic acid, it was in a ftate of intimate union with 

 extra6^ive matter. 

 Slow tanning Two pieces ofcalf-fkin, which weighed when dry 94- grains 



appears to add ^ach, were flowly tanned ; one by beingr expofed to a weak in- 

 lefs to the weight , ' •' ' J t> r 



cC the leather tufion of the Leicefter willow bark, and the other by bemg aded 

 than quick, and upon by a weak infufion of oak bark. The procefs was corn- 

 more of the mu- , , . , , , , . ,-11, 

 cilage is taken pleted m about three months ; and it was found, that one piece 



"F* of flvin had gained in weight 14 grains, and the other piece 



about 16| grains. This increafe is proportionally much lefs 

 than that which took place in the experiment on the procefs of 

 quick tanning. The colour of the pieces of leather was deeper 

 than that of the pieces which had been quickly tanned ; and, 

 to judge fi'om the properties of the refidual liquors, more of 

 the extradive matters of the barks had been combined with 

 them. 



The experiments of Mr. Biggin * have fliown, that fimilar 

 barks, when taken from trees at different feafons, differ as to 

 the quantities of tannin they contain : and I have obferved, 

 that the proportions of the aftringent principles in barks, vary 

 confiderably according as their age and fize are different ; be- 

 iides, thefe proportions are often influenced by accidental cir- 

 cumftances, fo that it is extremely difficult to afcertain their 

 «liftinct relations to each other. 



* Phil. Tranf. for 1799, page 299. 



In 



