Action of Gallic and Tannic Acids in Dyeing. 267 



ing it from exerting its reducing action on the iron mordant, 

 which would interfere with the dyeing properties of its tannic 

 acid. 



I was also anxious to ascertain the difference of the action 

 of gallic and tannic acids on alumina. I accordingly took two 

 pieces of calico, of 100 square inches each, previously mordanted 

 with alumina, aged and dunged, and placed them in separate 

 baths, one of which contained 20 grains of gallic acid, and the 

 other 20 grains of tannic acid ; and during 2 J hours gradually 

 carried the whole to the boiling point. These pieces were then 

 taken out, washed in distilled water, and subsequently dyed 

 with madder. It was found that the piece which had been in 

 the gallic acid bath was almost colourless ; while that from 

 the tannic acid had acquired a deep red tint. The same re- 

 sults were obtained on dyeing a piece of calico mordanted with 

 alumina, in a bath, composed of 1J pint water, 12 grains peach- 

 wood, and 8 grains garancine, together with 20 grains tannic, 

 or 20 grains of gallic acid. To leave no doubt as to the true 

 action of these acids on alumina, I introduced into two tubes 

 pure hydrate of alumina, with a solution of each of them ; and 

 after a few days' contact, I found, on examining the supernatant 

 fluids, that the gallic acid alone had dissolved alumina, the 

 tannic acid not having acted at all, so that the latter may be 

 considered, if not a neutral substance, at all events a very 

 feeble acid. 



I also attempted to obtain reds and blacks, with an extract of 

 sumach, which had been kept some time, but failed, owing no 

 doubt to the transformation of its tannin into gallic acid; as 

 the results obtained were identical to those furnished by the 

 above free acids. This rapid transformation of tannin into 

 gallic acid in the extract of sumach, is remarkable, when it is 

 remembered that it takes only a few weeks or months in the 

 case of the extract, whilst it requires years when the tannin is 

 confined in the plant. These differences are no doubt due to 

 the presence of water, which facilitates chemical actions. This 

 rapid deterioration of tanning matters in the form of extract is 

 the reason why their substitution for the solid substances them- 

 selves has not been adopted by the silk dyers or tanners. I 

 therefore deemed it advisable to make a series of experiments, 



