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)
