CRYPTOGRAPHY, the art of writing in cipher, or with sympathetic ink. Amongst the methods which Ovid teaches young women, in order to deceive their guardians, when they write to their lovers, he mentions that of writing with new milk, and of making the writing legible by means of coal dust or soot; from which it appears that the use of sympathetic ink was known to the ancients.
Tuta quoque est, fallitque oculos, e lacte recenti
Litæra: carbonis pulvere tange; leges.
Ausonius proposes the same means to Paulinus in the two following verses:
Lacte incide notas; arescens charta tenebit
Semper inaspicuas; prodentur scripta favillis.
But it would appear that the commentators on this poet have mistaken the meaning of the word favilla, which is used here to signify fuligo, or soot; and in the same sense it is often employed by other poets. Columella, speaking of the method of preserving plants from insects with soot, calls it nigra favilla. In another place he mentions the same practice, and says fuliginem quæ supra focos tectis inheret. Other glutinous juices besides milk may be employed for the same purpose. Pliny recommends the milky juice of certain plants, and particularly mentions that of lettuce.
It is now well known that several metallic solutions may be employed for this purpose; and that these, on being exposed to the action of certain vapours, become visible, and thus exhibit the characters which had been written with them. This effect was perhaps accidentally discovered; but it does not appear to be of great antiquity. In a book De Secretis, compiled by Wecker, from Porta, Cardan, and some other old writers, and printed in 1592, there is no mention of it; nor is it even noticed by Capnarius in his book De Atramentis, printed in 1619. The first receipt given for the preparation of a sympathetic ink is in a book by Peter Borrel, printed at Paris in 1653, where it is called "magnetic water which acts at a distance."