The American Instructor: Of Secret Writing

 Now this is interesting:

Of Secret Writing

"Here it may not be improper to say something of Secret Writing; to which Bishop Wilkins, in his Book of Mathematical Magick, speaks largely; but it is principally concerning Writing in Cypher, which requires great Pains, and an uncommon Share of Ingenuity, both in Writers, and Readers. But however I shall shew two or three particular Ways, that are very pretty and amusing, and also very easy both as to Cost and Pains. And,

"First, If you dip your Pen in the Juice of a Lemon, or of an Onion, or in your own Urin, or in Spirits of Vitriol, and write on clean paper whatever you intend, it shall not be discerned till you hold it to the fire, and then it will appear legible. And if with any of the aforementioned, you write on your Skin, as on your Arm, the Back of your Hand, etc. it shall not be seen till you burn a Piece of Paper, and with the Ashes rub on the Place, and then it will appear very plain. And this I have experienced and try'd, and therefore can say, Probatum est."

Mr. Fisher, in following paragraphs, goes on to recommend writing with milk which will appear a bluish color when held to the fire and offers a couple methods for making a cipher. One day soon cursive may serve better than code for keeping correspondence private.