CRYPTOGRAPHY, the art of writing in secret cha-
acters, or with sympathetic ink. Amongst the methods which Ovid recommends to young women when they wish to write secretly to their lovers, is that of writing with new milk, and of making the writing legible by sprinkling it with coal dust or soot; from which it appears that the use of sympathetic ink was not unknown to the ancients.
Tuta quoque est, fallitque oculos, e lacte recenti
Littera: carbonis pulvere tange; leges.—Art. Amer. iii.
Ausonius proposes the same means to Paulinus in the two following verses:
Lacte incide notas; arescens charta tenebit
Semper inspicuas; prodentur scripta favillis.—Epist. xxiii.
Other glutinous juices besides milk may be employed for the same purpose. Pliny (xxvi. 8) recommends the milky juice of certain plants, and particularly that of lettuce.
It is well known that several metallic solutions may be employed for this purpose; and that letters written with these, on being exposed to the action of certain vapours, become visible. 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 Caneparius 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." See INK, Sympathetic.