COMMON BLACK WRITING-INK is the tannate of the sesquoxyd of iron mixed with a smaller quantity of the gallate of the sesquoxyd of iron. When in the liquid form, it is generally the tannate and gallate of the protoxyd; but after being long kept, (or put on the paper and drying there,) it absorbs more oxygen from the atmosphere; and thus the saline compounds become the per-tannate and per-gallate, which are blacker than the tannate and gallate of the protoxyd. It is thus and therefore that good modern ink is known by the simple test-quality of darkening by age. On the other hand, when writing becomes yellow, pale or indistinct by age, it is from the decay of the imperfectly combined vegetable astringent,—the marks on the paper or parchment being then little more than the stain of the per-oxyd (that is the sesquoxyd) of iron. If the written surface be then carefully washed or even moistened with the infusion of nut-galls, it will be rendered blacker, and if before indistinct will become legible. This may sometimes be better accomplished by first applying a weak solution of oxalic acid or very dilute muriatic (hydro-chloric) acid, and then delicately laying on the infusion of galls. When the writing paper has been made of inferior rags, bleached with chlorine, the best ink used upon it is liable to become discolored. Nut-gulls or gall-nuts (_Gallæ-tinctoriæ_) 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 “ALEPPO 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 suspended in it, by gum, sugar or some similar substance which gives the quality of viscidity to its solutions.