SS ' EXrERIMl;JJTS ON ASTRINGENT VEGETABLES. 



of afting upon re-agents, but all precipitable by gelatine. 

 This opinion is fufficiently conformable to the fads generally 

 known concerning the nature of the fubftances which are pro- 

 duced in organifed matter ; but it cannot be confidered as 

 proved, till the tannin in different vegetables has been ex- 

 amined in its pure or infulated ftate. In all the vegetable 

 infufions which have been fubjeded to experiment, it exifts 

 in a Hate of union with other principles; and its properties mud 

 neceifarily be modified by the peculiar circumftances of its 

 combination. 

 The fpecific From the experiments that have been detailed it appears, 



'un'Sn]nl]l in- ^^^^ the fpecijic agencies of tannin in all the different aftringent 

 fufi;.i s are infufions are the fame. In every inflance, it is capable of en- 

 tbe fame. tering into union with the acids, alkalis, and earths ; and of 



forming infoluble compounds with gelatine, and with fliin. The 

 infufions of the barks affefl the greater number of re-agents in a 

 manner fimilar to the infution of galls; and, that this lafl fluid 

 ' is rendered green by the carbonated alkalis, evidently depends 



upon the large proportion of gallic acid it contains. The infu- 

 lion of fumach owes its charaderiflic property, of being pre- 

 cipitated by the cauftic alkalis, to the prefence of fulphate of 

 lime; and, that the folutions of catechu do not copioufly pre- 

 cipitate the carbonated alkalis, appears to depend upon their 

 containing tannin in a peculiar fiate of union with extra6live 

 matter, and uncombined with galHc acid or earthy falts. 

 Its affinities and ^^^ njaking fome experiments upon the affinities of the tanning 

 habitudes. principle, I found that all the earths were capable of attra6ting 



it from the alkalis : and, fo great is their tendency to combine 

 with it, that, by means of them, the compound of tannin and 

 gelatine may be decompofed without much difficulty ; for,, 

 after pure magnefia had been boiled for a few hours with 

 this fubfiance diffufed through water, it became of a red-brown 

 colour, and the fluid obtained by filtration produced a diflineS 

 precipitate with folution of galls. The acids have lefs affinity 

 for tannin than for gelatine ; and, in cafes where compounds 

 of the acids and tannin are a6ted on by folution of gelatine, an 

 equilibrium of affinity is eftablifned, in confequence of which, 

 by far the greatef} quantity of tannin is carried down in the in- 

 foluble combination. The different neutral falts have, com. 

 parativeiy, feeble powers of attradion for the tanning principle ; 

 JDut, that the precipitation they occafion in 



