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.
Invisible Red Ink.
To the pure spirit of vitriol or nitre, add eight times as much water.
Use the above solution of violets to make visible the characters written with this ink.
Invisible Green Ink.
Dissolve salt of tartar, clean and dry, in a sufficient quantity of river water. Use the violet solution to render it visible.