A Green Ink of the Fifth Class.—Take zaffre in powder, and let it remain dissolved in aqua-regia for twenty-four hours; after which pour the liquor off clear, and, adding to it as much common water, keep it in a bottle well corked. Then, if any characters be written with this ink, and exposed to the fire, or strong rays of the sun, they will appear of a lively green.
It is the peculiar property of this ink, that as soon as the paper becomes cold again, the letters will disappear; and this alternate appearance and disappearance may be repeated a great number of times, provided the heat be not too great.
Other Sympathetic Inks.
A Yellow Ink of this kind may be made, by steeping the flowers of marigolds seven or eight days in clear distilled vinegar, and then pressing them out, and keeping the liquor well corked in a bottle for use.
For a Red invisible Ink,—take the pure spirit of vitriol, or that of nitre, and add to it eight or ten times as much water, according as you would have it more or less red.
For a Green Ink of this sort,—dissolve salt of tartar, the clearest and driest you can procure, in a sufficient quantity of river water; and for a Violet sympathetic Ink, express the juice of lemons, and keep it in a bottle well corked.
Then, if any characters be written with one of these inks, they will appear in their proper colours, the paper having been dipped in the following liquor.
Take a sufficient quantity of the flowers of pansies, or common violets, and after adding some water to them, strain the liquor through a cloth, and keep it in a bottle for use.
A Sympathetic Ink which appears by being wetted with Water.
Mix alum with a sufficient quantity of lemon juice; then, if any letters or characters be written with this mixture, they will be invisible till they are wetted with water, which will make them appear of a grayish colour, and quite transparent.