EXPERIMENTS ON ASTRINGENT VEGKTABLfeS. ^P 



ts not fimply owing to the circumlUnce of their uniting to a 

 portion of the water which iield the vegetable fabftances in fo- 

 lution, is evident fronn many fads, befides thofe which have 

 been already fiated. The folutions of alum, and of fome 

 other falls which are lefs foluble in water than tannin, pro- 

 duce, in many aftringent infiifions, precipitates as copious as 

 the more foluble faline matters ; and fulphate of lime, and other 

 earthy neutral compounds, which are, comparatively fpeaking, 

 infoluble in water^ (peedily deprive them oi' their tanning 

 principle. 



From the different facts that have been fiated, it is evident ^^ '^ not always 

 that tannin may exifl: in a ftate of combination in different fub- gelatine! ^ ^ 

 fiances, in which its prefence cannot be made evident by 

 means of folution of gelatine; and, in this cafe, to deteft its 

 exiftence, it is neceflary to have recourfe to the a61ion of the 

 diluted acids. 



In confidering the relations of the diifereiit fa6ts that have Skins in tanning 

 been detailed, to the proceiTes of tanning and of leatlier-making, ^,°4r^'' '^ '*^ 

 it will appear fufficiently evident, that when (kin is tanned in 

 aflringent infufions that contain, as well as tannin, extractive 

 matters, portions of thefe matters enter, with the tannin, into 

 chemical combination with the (Icin. In no cafe is (here any 

 reafon to believe that gallic acid is abforbed in this procefs ; 

 and M. Seguin's ingenious theory of the agency of this fub- 

 ilance, in producing the deoxigenation of (kin, feems fup- 

 ported by no proofs. Even in the formation of glue from 

 fnin, there is no evidence which ought to ihduce us to fuppofe 

 that it lofes a portion of oxigen ; and the effe€t appears to be 

 owing merely to the feparation of the gelatine, from the finall 

 quantity of albumen with which it was combined in the orga- 

 nifed form, by the folvent powers of water. 



The different qualities of leather made with the fame kind ^"^ }^^ leather 

 of (kin, feem to depend very much upon tUe different quantities aft^dedTyit- 

 of extradive matter it contains. The leather obtained by means 

 of infufion of galls, is generally found harder, and more liable 

 to crack, than the leather obtained from the infufions of barks j 

 and, in all cafes, it contains a much larger proportioii of tannin^ 

 and a fraaller proportion of extractive matter. 



When (kin is very (lowly tanned in weak folutions of the Soft durable 

 barks, or of catechu, it combines with a confiderable proportioi) ^^^^ " ^ "''^ 

 of extractive matter ; and, in thefe cafes, though the increafe 



