constituent Parts of astringent Vegetables. os' 



standard temperature has been between 60° and 70*' Fah- 

 renheit; and the sokitions oi gelatine that I have used were 

 made by dissolving 120 grains of isinglass in 20 ounces of 

 water. 



In ascertaining the proportions of tannin in astrino-ent 

 infusions, great care must be taken to prevent the presence 

 of any excess of gelatine ; for when this excess exists, I 

 have found that a small portion of the solid compound 

 formed is redissolved, and the results of the experiment 

 otherwise affected. It is not difficult to discover the precise 

 pomt of saturation, if the solution of isinglass be added only 

 m small quantities at a time, and if portions of the clear 

 , liq-or be passed through a filter at different periods of the 

 process. The properties of these portions will indicate the 

 quantities of the solution of gelatine required for the com- 

 pletion of the experiment. 



That the composition of any precipitate containing tan- 

 nin and gelatine may be known with a tolerable decree of 

 precision, it is necessary that the isin2;lass employed" in the 

 solution, and the new compound formed, be brouQ;ht as 

 nearly as jiossible to the same degree of dryness. For this 

 purpose I have generally exposed them, for an equal time, 

 upon the lower plate of a sand-bath, which was seldom 

 heated to more than 150°. This method I have found much 

 better tlian that of drying at the temperatures of the afmo- 

 sphere, as the different states of the air^ with regard to 

 moisture, materially influence the results. 



Mr. Hatchett has noticed, in his excellent paper on Zoo- 

 phytes, &c.*, that isinglass is almost wholly composed of 

 gelatine. I have found that 100 grains of good and dry isin- 

 glass contain rather more than 98 grains of matter soluble in 

 water. So that when the quantity of isinglass, in any so- 

 lution emi)loyed for acting upon an astringent infusion, is 

 compared with the quantity of the precipitate obtained, the 

 difference between them will indicate the proportion of tan- 

 nin, as it exists in the combination. 



After the tannin has been separated from an astringent 

 infusion, for the purpose of ascertaining its other compo- 

 nent parts, I have been accustomed to evaporate the resi- 

 dual liquor very slowly, at a temperature below 200° f' I" 



this 

 '* Philo'iophical Transictions for 1800, p. 327. 



+ M. Deveux Has shown ( irn.tUi dr C imic, tomt- xvii. p. 361. that in 

 the process of tvaporating solutions of grills, ix. gillic ;ic.d is tarried ovt-r 

 by thi: water at a tempei attire, below that of eDullitioi). Mativ astnn- 

 gcnt infusions howe\Lr, lose a portion of their ar.matic principle even' 

 in casts when rhcv nrc not made to boil; but this subotance, though evi- 

 Vo/.. XV'II. No, 6j. H dent 



