Invisible Yellow Ink
Steep marigold flowers seven or eight days in clear distilled vinegar.
Press the flowers and strain the liquor, which is to be kept in a
bottle well corked. If you would have it still more clear, add, when
you use it, some pure water.
To make the characters visible, which you write with this ink, pass a
sponge over the paper, dipped in the following solution:
Take a quantity of flowers of pansy, or the common violet, bruise them
in a mortar with water, strain the liquor in a cloth, and keep it in a
bottle.