Page:The History of Ink.djvu/45

Rh cobalt, form sympathetic inks,—the first, blue, and (with the addition of nickel,) green; the second, red. Chloride of copper gives a gamboge yellow; bromide of copper, a fine rich brown.

Letters written with a solution of acetate of lead, are invisible until exposed to the action of sulphuretted hydrogen, which makes them distinct, with the lustrous greyish black of sulphuret of lead, the same substance which is called galena when it occurs as lead-ore. A weak infusion of galls or other vegetable astringent, will, if applied to paper in the form of letters, become legible when touched with any solution of iron. If written with a solution of ferro-cyanide of potash, letters will remain invisible until touched with a solution of sulphate of iron.

Astle speaks very impressively and justly on this point; and we contribute to this part of our subject calling attention to facts almost daily occurring or brought to notice in this country, especially in the older cities and states, where town-records, parish-registers, and other documents of ancient date, and of high importance in