GALLIC ACID. 153 



should produce such varied results, and why one 

 excrescence should be the oak-apple, another the 

 spangle, a third the artichoke, and a fourth the 

 nut, is indeed a mystery. But that these small 

 insects do really produce the excrescences in 

 question, is an ascertained fact, and a most im- 

 portant one in the commercial world. 



Nut galls contain a large quantity of the vege- 

 table principle called tannin, being the astringent 

 property for which oak-bark is in so much repute. 

 They also abound in an acid called from thence 

 gallic acid, which is the important ingredient in 

 black dyes, and in fixing and improving several 

 other colours, as well as in the composition of ink. 



