EXPERIMENTS ON ASTRINGENT VEGETABLE!. QQQ 



tained in four folds of filtrating paper, I obtained a light olive- 

 green fluid, in which there was no excefs of fulphuric acid, 

 and which I am inclined to fuppofe was a folution of the com- 

 pound of gallic acid and fulphate of iron, with fuperabundance 

 of metallic fait. I have already mentioned that gallic acid, 

 when in very fmall proportion, does not precipitate the 

 oxigenated falts of ire i; and Mr. Prouft, in his ingenious 

 paper upon the difference of the falts of iron, has fuppofed 

 that, in the formation of ink, a portion of the oxide of iron in 

 union with gallic acid is ditTolved by the fulphuric acid of the 

 fulphate. This comes near to the opinion that they form a 

 triple compound : and, in reafoning upon the general pheno- 

 mena, it feems fair to conclude, that, in the cafe of the pre- 

 cipitation of tannin by the falts of tin and of iron, compounds 

 are formed, of tannin and the falts ; and that, of thefe com- 

 pounds, fuch as contain tin are (lightly folubie in water, whilfi 

 thofe that contain iron are almoft wholly infoluble. 



In examining the aclion of animal fubftances upon the infu- Animal matter* 

 lion of galls, with the view of afcertaining the compolition of 3 r * ein * ufi< * 

 the compounds of gelatine, andoflkin, with tannin, I found 

 that a fatu rated folution of gelatine, which contained the 

 folubie matter of 50 grains of dry ifinglafs, produced from the 

 infufion a precipitate that weighed nearly 91 grains ; and, in 

 another inftance, a folution containing 30 grains of ifinglafs, 

 gave about 56* grains ; fo that, taking the mean of the two ex- 

 periments, and allowing for the fmall quantity of infoluble 

 matter in ifinglafs, we may conclude, that 100 grains of the Precipitate fey 



compound of gelatine and tannin, formed by precipitation e elatin e cootai»» 



- r , / ,. , . 54 gelatine and 



from faturated folutions, contain about 54- grains or gelatine, 46 tanin nearly. 



and 46 of tannin. 



A piece of dry calf- (kin, perfectly free from extraneous A (kin quickly 

 matter, that weighed 180 grains, after being prepared ^SStttt^MwS 

 tanning by long immerfion in water, was tanned in a portion ikin and one 

 of the infufion, being expofed to it for three weeks. When ve S etable matter* 

 dry, the leather weighed 295 grains : fo that, confidering 

 this experiment as accurate, leather quickly tanned by means 

 of an infufion of galls, con fi ft s of about 61 grains of (kin, and 

 39 of vegetable matter, in 100 grains. 



After depriving a portion of the infufion of all its tanning J n « exhaufted 

 matter, by repeatedly expofing it to the adtion of pieces of ed Jefs | ( j c 

 ikin, I found (hat it gave a much (lighter colour to oxigenated acid j probably 



fulDhate fromd?com P ofi ' 



