1825. J comparativeTmnin^PmeH of Astringents, 403f^ 



iless in Dublin from entering into such an analysis ; but as the 

 object proposed is truly important, and as the scientific world 

 yet appears to be of opinion that the process should, with proper 

 attention, lead to correct results, it may be well to recapitulate 

 the sources of error, which, in my opinion, render it totally 

 inadmissible. 



1. The degree of concentration of the solutions (of tan and gela- 

 tine) has a decided influence on the quantity of the precipitate 

 formed ; the strongest solutions giving most : so that a bad 

 Sample of bark which only 'partially saturated the quart of 

 water employed, would, on this account, appear (by its deficient 

 precipitate,) worse than it really was. This is a serious cause of 

 inaccuracy, for it is without a remedy. Evaporation, to equalize 

 the strength of the infusions, is here inadmissible, as boiling, or 

 even moderate continued heat with exposure, is found to preci- 

 pitate both tan and extract in an insoluble form. 



Additions of the astringent substance under examination, to 

 bring up the specific gravity of the weaker infusion, afford no surer 

 means of equalizing the tanning matter in both. For the muci- 

 lage present in vegetable astringents, so far influences the specific 

 gravities of their solutions, that their equality in this respect 

 determines nothing to the purpose. 



2. When bad samples (giving weak infusions) are tested> 

 the precipitate is not entirely retained on the filter; but (not- 

 withstanding repeated filtration) is partially carried through 

 \\''ith the residual hquor, in which it remains a long time sus- 

 pended, rendering it turbid and opaque. 



3. The solution of gelatine must be fresh made preparatory 

 to every new set of experiments ; for, if it lie till tainted, its 

 power of precipitating tan will be materially impaired. 



4. The solution of gelatine must be in as high a state of 

 saturation as is compatible with its perfect fluidity ; and to 

 ensure this latter requisite, heat must be applied to keep it at a 

 standard temperature during the experiment. 



5. Great care must be taken to prevent excess of gelatine 

 in the mixed liquors, for when this excess exists, a portion of the 

 solid compound formed is redissolved. 



So far, it may be said, these are only difficulties in practice to 

 the attainment of correct results ; but Sir H. Davy mentions one 

 striking fact, which is, in reality, an objection m principle to the 

 institution of any comparison {by this mode) between astringents 

 not of the same species. He says (Phil. Trans. 1803), " the 

 tanning principle, in diferent vegetables, demands for its satu- 

 ration different proportions of gelatine ;" so that precipitates 

 from valonia and sumach (by gelatine) of equal weight, might 

 contain unequal quantities of tan. 



Since the publication of the two essays above-mentiOned, this 



2i>2 



