Disappearing Ink.
Boil some nut-galls in aqua fortis, and add to the infusion some gum Arabic and a little sulphuric acid. When written on paper this ink is perfectly legible, but will disappear from the paper in a few days.
Invisible Ink.
Invisible inks are those which when first written are not visible, but upon the application of heat or other means the characters are made to appear distinctly. The following are decidedly the best preparations for this purpose:
| Oil of vitriol1 | fluid ounce. |
| Soft water1 | pint. |
This makes a fluid which is perfectly invisible until heat is applied by holding over a lamp or by placing in an oven, when it changes to a permanent black. Write with a clean steel pen. All invisible inks will show on glazed paper, therefore unglazed paper should be used.
Another singular invisible ink is made as follows: Oxide of cobalt, 1/2 ounce; muriatic acid, sufficient to dissolve it; water, 4 ounces; mucilage of gum acacia, 1 drachm. Place in a stained bottle. Characters written with this solution are invisible, but on the application of heat they instantly appear in blue. On cooling they again become invisible.
Ink Erasing Fluid.
| Hydrochloric acid 1/8 | ounce. |
| Water1 | pint. |