Chap. 29."} Ink. 169 



m the hiftory of iron, is the effect produced on it by 

 aftringent vegetable fubftances. The principle of 

 aftringency, which is now found to be a peculiar acid, 

 called, in the new nomenclature, gallic acid, refides in 

 a great number of vegetable matters, particularly oak 

 galls, tea, -&c According to M. de la Metherie, 

 however, the acid of galls is only a variety of the co- 

 louring principle. Galls are protuberances on the 

 leaves of the oak, -occasioned by the puncture of a par- 

 ticular infect. Any of thefe fubftances, added to a 

 iblution of green vitriol, precipitates a fine black fecula, 

 which may be fufpended a confiderable time in the 

 fluid by the addition of gum arabic. Oak galls art 

 commonly made ufe of in this prccefs, which is that 

 of making ink. The following proportions of thefe 

 ingredients for making ink anfwer very well. One 

 ounce of martial vitriol to three of powdered galls ; to 

 which one ounce of powdered logwood may be added, 

 to render the ink more permanent, and one ounce of 

 gum arabic to fufpend the colouring matter. Let 

 thefe be infufed in a quart of water or vinegar for ten 

 days, and fhaken occaftonally, when the ink will be fit 

 for ufe. 



The colouring matter of ink feems to be produced 

 from an union of the acid of the galls with fome part 

 of the iron in a calciform ftate. A fmall quantity of 

 any of the mineral acids deftroys the colour of the ink, 

 by diffolving the iron, which was imperfectly precipi- 

 tated ; and this colour is again reftored by the addi- 

 tion of an alkali, which takes away the acid. The 

 black fecula of ink is not magnetical; but it may be 

 converted into a brown magnetic calx by heat. Ink 

 becomes blacker by expofure to the air, which acidi- 

 fies more completely the principle combined with the 

 iron i but ancient writings become more and more 



yellow 



