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The most effectual way of preventing any ill effectsfrom this redundant acid seems to be by an addition ofiron itself; part of which, in proportion as the iron of thevitriol is extricated, will be dissolved in its place, and thuscontinue both to satiate the acid, and supply one of theessential ingredients of the ink. It should seem that inthis method, a much less proportion of acid, that is, aless quantity of vitriol, would suthce, than is otherwisenecessary; the same acid serving to combine with thegalls fresh quantities of the iron : and in such case, keep-ing for a length of time, as a year or more, would im-prove the ink. Of this I have not yet had full experience;but a friend informs me, that he has seen writings ofmore than eighty years standing, which continued of afull black colour, without any tendency to yellow orbrown ; that the ink was made in the common mannerwith vitriol and galls, and long kept with pieces of ironin the vessel. Possibly boiling for a little time mightanswer the same purpose as long keeping; for boilingremarkably promotes the separation of iron from vitriol,and consequently the action of the acid on fresh iron.

Gum arabic is added to inks, to give a greater con-sistence to the fluid, and enable it better to keep the co-louring matter suspended: it contributes, perhaps, to pre-vent the black matter from concreting into particles largeenough to settle by their weight, as well as to prevent orretard their settling after they have so concreted; for wehave seen in a foregoing part of this essay, that the coa-lition into sensible particles is successive, and that beforeit happens, the black matter is in so subtile a state, thatit may be considered as being in actual dissolution.

The gum appears also to be of another advantage, pre-venting the ink from spreading upon the paper, so thata greater quantity of the fluid, and consequently a greater

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