16 VEGETABLE SUBSTANCES. 



of a particular construction gives the substance called 

 Spanish black. 



The Oak from which the nut-galls of commerce 

 are procured (Quercus infectofia) is minutely de- 

 scribed by M. Olivier, in his travels. The species is 

 very common in Asia-Minor ; but, .till the time of 

 this traveller, Europeans had very little information 

 on the subject, although the galls were a considerable 

 article of commerce. It is a shrub, seldom exceeding 

 six feet in height ; and it has not only been accurately 

 described by M. Olivier, but was introduced by him 

 into France, where it is cultivated as a garden shrub, 

 and grows well in the open air. 



The gall is a morbid excrescence produced by the 

 puncture of a winged insect, to which Olivier has given 

 the name of DiploJepis Gallce Tinctoriae. This ex- 

 crescence is of a globular form, with an unequal and 

 tuberculous surface. It is developed on the young 

 shoots of the tree, and contains within it the eggs which 

 the insect has deposited. The best galls are gathered 

 before the transformation of the insect, because in 

 that state they are heavier, and contain more of the 

 tannin principle. When the insect has left them, they 

 are pierced from the interior to the surface. The best 

 galls come from Aleppo. The substance of which 

 they are composed is peculiarly astringent ; of which, 

 according to Sir Humphry Davy, five hundred parts 

 contain a hundred and eighty parts of soluble matter, 

 principally formed of tannin and gallic acid. One 

 hundred and seventy-four tons of galls were im- 

 ported into the United Kingdom in 1827. 



The instinct by which certain insects choose for the 

 nests of their future offspring the substance of vari- 

 ous vegetable bodies, is one of the most curious pro- 

 visions in the economy of nature. After having 

 pierced those bodies, they deposite their eggs, which, 

 being hatched, produce /arce, that are more or 



