Purple Sympathetic Ink.
Dissolve cobalt or zaffre in nitric acid, add salt of tartar gradually and in small quantities to avoid too powerful an effervescence, let the precipitate subside, and, having drawn off the super-natant clear liquid, add a sufficient quantity of water, when the ink will be fit for use.
Rose-coloured Sympathetic Ink.
Dissolve zaffre in nitric acid, to which add saltpetre well purified, and you will possess a rose-coloured ink, having the same properties as the preceding.
Application of the Secret Inks.
Write any unimportant matter with common ink, and let the lines be tolerably wide apart; then between these lines write the communication you wish to make with any of the above invisible inks. Your correspondent, by holding the paper before the fire, will be speedily enabled to peruse the letter; the characters will again become invisible when the paper has cooled. The writing in common ink will serve to lull the suspicions of those who might intercept the letter.
With this kind of ink, some very ingenious and amusing tricks may be performed.
A Drawing which alternately represents Winter and Summer Scenes.
Draw a landscape, and delineate the ground, the trunks, and branches of the trees, with the usual water-colours employed for that purpose, tracing the grass and trees with sympathetic ink. By these means you will have a drawing, which, at the common temperature of the atmosphere, will represent a winter piece; but if it be exposed to a proper degree of heat, not too strong, you will see the ground covered with verdure, and the trees suddenly wrapped with a beautiful foliage; replace the picture in the cool, and the dreary aspect of winter re-appears. The colours must be lighter for the back grounds.
Screens painted in this manner were formerly made at Paris. Those to whom they were presented, if unacquainted with the artifice, were astonished to find, when they made use of them, that the views they exhibited were totally changed. Persons not understanding drawing may amuse themselves in this manner, by colouring an outlined engraving representing winter, following exactly the process enjoined for the above.