EXPERIMENTS ON ASTRINGENT VEGETABLES. 257 



I. OBSERVATIONS ON THE ANALYSIS OF ASTRINGENT 

 VEGETABLE INFUSIONS. 



The fubftances that have been fuppofed to exift moft gene- Aftringents con- 

 rally in aftringent infufions are, tannin, gallic acid, and ex- JJJnfj^Tanl 

 tractive matter. extractive. 



The prefence of tannin in an infufion, is denoted by the Tannin fhewn 

 precipitate it forms with the folution of glue, or of ifinglafs. J^ "dex- 

 And, when this principle is wholly feparated, if the remain- trad, by faks 

 ing liquor gives a dark colour with the oxygenated falts of iron, aiumTn/mu-* 

 and an immediate precipitate with the folutions of alum and ofriate of tin. 

 muriate of tin, it is believed to contain gallic acid, and extrac- 

 tive matter. 



The experiments of MM. Fourcroy, Vauquelin, and Se- Aftringent folu- 

 guin, have fhovvn that many aftringent folutions undergo a ^° n ^ ry apt t# 

 change by expofure to the atmofphere ; an infoluble matter 

 being precipitated from them. A precipitation islikewife oc- 

 cafioned in them by the action of heat ; and thefe circum- 

 ftances render it extremely difficult to alcertain, with any de- 

 gree of precifion, the quantities of their conftituent parts, as 

 they exift in the primitive combination. « 



After trying feveral experiments on different methods of af- 

 certaining the quantity of tannin in aftringent infufions, I was 

 induced to employ the common procefs of precipitation by 

 gelatine, as being the moft accurate. 



This procefs, however, requires many precautions. The Precip. of tannin 



tanning principle in different vegetables, as will.be feen here- v a r'STs ^""di f-* 



after, demands for its faturation different proportions of gela- ferent vege- 



tine; and the quantity of the precipitate obtained by filtration, tables > 



is not always exactly proportional to the quantities of tannin /}) an a by the 



and gelatine in folutions, but is influenced by the degree of concentration of 

 , . . . _, , ,, , , , , ^ r , . ,, the folutions. 



their concentration. Thus, I found that 10 grains or dry ltin- 



glafs, diflblved in two ounces of diftilled water, gave, with 



folution of galls in excefs, a precipitate weighing, when dry, 



17 grains ; whilft the fame quantity, diflblved in fix oances of 



water, produced, all other circumftances being fimilar, not 



quite 15 grains. With more diluted folutions, the lofs was 



ftill greater ; and analogous effects took place, when equal 



portions of the fame folution of ifinglafs were adted on by equal 



portions of the fame infufion of galls diluted in different de- 



Vol. V. — August. S gree; 



