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, me pure spirit of vitriol, of 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 pansiss, 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.

Or, you may write with a strong solution of roch-alum only, and when the writing is dry, pour a small quantity of water over it, and it will appear of a white colour, like that of the paper before it was wetted.

Also all saline liquors, such as vitriolic, nitrous, and marine acids, diluted with water, the liquor of fixed vegetable alkalis, and even vinegar, will produce the same effect.

If a little aqua-fortis be mixed with the water, the writing will dry well, and not run out of its form when the paper is wetted.

A curious Recreation with Sympathetic Ink, called the Book of Fate.

Make a book, consisting of seventy or eighty leaves, and in the cover at the end of it, let there be a case which opens next to the back, that it may not be perceived. At the top of each right-hand page, write any question you please; and at the beginning of the book, let there be a table of those questions, with the number of the pages in which each is to be found. Then write with common ink on separate papers, each about half the size of the pages, the same questions that are in the book; and under each of them, write the answer with the ink made with the litharge of lead, or the solution of bismuth.

Soak a double paper in the vivifying ink, made of quicklime and orpiment, or the liver of sulphur; and just before you make the experiment, place it in the case that is in the cover of the book.

Having done this, deliver some of the papers on which the questions are written, to the company; and after they have chosen such as they wish to have answered, let them put them into those leaves where the same questions are contained; then shutting the book for a few minutes, the sulphureous spirit, with which the paper in the cover of the book is impregnated, will penetrate the leaves, and make the answer visible, which will be of a brown colour, and more or less deep, in proportion to the time the book has been closed.