On the life of Oak Bark in dyeing Black. 1 77 



bark. Twelve hats in each were marked : they were of the 

 fame ftuff and the fame fize, had been prepared with all the 

 precautions which each of the two methods required, and 

 the whole procefs was carefully obferved by a commiflioner 

 u ho attended for the purpofe. After all thefe hats had been 

 properly dried, cleaned, and brufhed, they were placed in-, 

 dii'criminatelv on a table. Several of the moft expert dyers 

 of Paris were invited to fele£l, from the twenty-four hats, the 

 twelve which (liould appear to them to be the beft dyed. 

 Thefe dyers arrived feparately at two different times, fo that 

 there were two fele6tions 5 and in both cafes, one hat ex- 

 cepted, thefe dyers pointed out as the beft dyed thofe hat^ 

 which had been treated with oak bark. Was^ any thing 

 more neeeifary to determine which method deferved the 

 prefeience ? The fate of this new procefs however, which 

 to fuccefs in the operation of dyeing, unites economy, and 

 the advantage of freeing us from paying tribute to a foreign 

 nation, has remained in a ftate of uncertainty. 



" One of the ftrongeft obje«Stions which has been made to 

 d\eing black with oak bark, is the confiderable difference 

 between the quantities of the precipitates ; and we muft fay, 

 for the information of thofe unacquainted with the fubjeft, 

 that the black matter of the dye, of which we here fpeak, 

 refuUs from the combination of an acid, called the gallic, 

 with iron; and in -the operation we are defcribing, the quan- 

 tity of this acid is generally determined by knowing how 

 n)anv parts of iron (furnifhed by fulphat of iron, commonly 

 called green copperas,) have gone into a known quantity of 

 the precipitate : the remaining parts which are neceffary to 

 make up the known quantity are counted as gallic acid. 



" If the decoftion of a given quantity of gall nuts be poured 

 on a foUilion of the fulphat of iron, you obtain a black pre- 

 cipitate ; and if the fame operation be performed with a de- 

 co6iion of oak bark, and in the fame quantities, you will 

 have alfo a black precipitate, with this difference, that the 

 gall nuts will have produced eight or ten times as much as 

 the oak bark : but this abundance in the gall nuts is only 

 apparent, and is owing to a fort of feculent matter, or gum, 

 which adheres to the acid and is carried down with the pre- 



VoL. VI. A a cipitate, 



