Infujton of Gatls.-'fheory and Formatien of My C3c. 353 



becomes precipitated as the pot-afli unites Itfelf to the acid, and the fulphate of pot-afli, 

 which has been formed, diflblves in the water. In order to obtain the whole of this fub- 

 ftance, the liquid muft be reduced by evaporation, and the depofit muft be fufFered to 

 become cold, and waflied with cold water. It is the pure tanning principle. 



After defcribing the procefles, which we have juft related, for feparating the principle* 

 of the nut-gall, namely, the tanning principle and the gallic acid, M. Prouft next examines 

 what takes place between thefe fubftances and iron in the making of ink. He remarks, 

 firft, that the reafon why ink is not formed with a folutlon of iron at the minimum of 

 oxidation is, becaufe the tanning principle and the gallic acid have lefs affinity with the 

 iron than the fulphuric acid has. 2dly, That when a folution of iron at the maximum of 

 oxidation immediately produces a blaclc colour with an infufion of galls, the reafon is, that 

 in this cafe the red oxide of iron has a greater attraction to the principles of the gall-nut 

 than to the fulphuric acid. He proves this by diflblving the combination of iron with the 

 tanning principle in fulphuric or muriatic acid, which gives a blueifli tranfparent liquid, 

 which would lofe its colour if decompofition took place. He, therefore, confiders ink as a 

 folution of the tannate and the gallate of iron in fulphuric acid, sdly. That when ink, 

 fpread upon paper, quickly becomes black, it is becaufe it abforbs an additional quantity of 

 ojiygen, which renders it more infoluble in water. The combination of the oxide of iron 

 with the gallic acid and with the tanning principle, which forms the bafis of ink, contrafts 

 only a flight adherence with the acids, and feparates itfelf from them by repofe at the end 

 of fome time. In this cafe it is affefted nearly in the fame manner as metallic falts diflblved 

 in a foreign acid ; and for this reafon it is that when a plate of iron is put into a folution of 

 Ink, this metal precipitates the black particles, abfolutely in the fame manner as irort 

 feparates the phofphate of iron from acids. 



Alkalis, cautioufly added to the folution of ink, precipitates its bjack parts, but an excefs 

 of alkali diflblves the atramentary combination, and produces a fluid of the colour of wine, 

 more or lefs intenfe. 



Gallic acid produces no effeft on ink, becaufe the oxide of iron is faturated. The fame 

 acid is not very a£live in fhewing the prefence of red oxide in a folution containing an 

 excefs of acid, becaufe the combination is diflblved in proportion as it is formed. But 

 by carefully faturating the excefs of acid, the liquid is rendered black. 



The combination of the gallic acid, and the tanning principle with iron, does not become 

 changed into pruGiate of iron by means of prufllate of pot-afli, even with the aflTiftancc of 

 heat. The nitric acid diflblves the ink, and efl^efts no change in its nature in the cold j but 

 by heat it dcftroys it, and the yellow oxalate of iron is precipitated by the addition of am- 

 moniac. Whenever iron filings and an infufion of galls are mixed together, a difengage- 

 ment of hydrogen gas takes place 5 which proves, fays M. Prouft, that the oxidation of 

 iron begins at the expence of the water, and that it is completed by the air, an event which 

 alfo takes place with vegetable acids, which, not fufiiciently attracting the oxide of iron 

 oxided to the minimum, cannot counterpoife the affinity of the oxigen which tends to ad- 



VoL. IV. — November. 1800. Zz vance 



