Page:The History of Ink.djvu/24

18 are excrescences growing upon the leaves or twigs of oak trees, (especially the Quercus infectoria,) caused by the puncture of an insect (the Cynips gallæ-tinctoriæ) which deposits its eggs in the perforations thus made. The Quercus infectoria is most abundant in Persia, Mesopotamia, Syria and Asia Minor, from which countries the galls are brought in large quantities to the manufactories of Europe and America. The best are called " galls," from the name of the Syrian city which is the chief original market for them. Those from Smyrna are also highly esteemed.

They contain the vegetable astringent principle called tannin in greater abundance than any other known substance. This is chemically resolved into the acids known as the tannic and gallic. All the woods and barks employed in the manufacture of leather by the tanning of hides contain this astringent matter in various degrees. The oak and the hemlock, for instance, are in extensive and familiar use for this purpose in the United States. The blackness of ink, as has been already indicated, is derived from the combination of these two acids with oxydized iron in saline compounds which are insoluble in water, and are therefore precipitated or deposited at the bottom of the fluid, unless held mechanically