the art of writing in cipher, or with sympathetic ink. Among the methods which Ovid teaches young women 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 foot; from which it appears, that the use of sympathetic ink was known to the ancients.

Tuta quoque eft, falliique oculos, è latte recenti Litera : carbonis pulvere tange; leger. De Arte Amandi, lib. iii.

Austonius proposes the same means to Paulinus in the two following verses:

Laetè incide notas; arescens charta tenebit Semper inaspicuas; prodentur scripta favillis.

Epift. xxiii.

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 foot; and in the same sense it is often employed by other poets. Columella, speaking of the method of preserving plants from insects with foot, calls it nigra favilla. In another place he mentions the same practice, and says fuliginem que supra focos teftis inhaberet. Other glutinous juices besides milk may be employed for the same purpose, as they will equally hold fast the black powder strewed over them. Pliny, therefore, recommends the milky juice of certain plants, and particularly mentions that of lettuce, to produce this effect.

It is now well known that several metallic solutions may be employed for a similar purpose, and being exposed to the action of certain vapours, the characters which are written with them 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 even is it noticed by Canearius in his book de Atramentis, printed in 1619. The first receipt given for the preparation of a sympathetic ink is in a work by Peter Borel printed at Paris in 1653, where it is called magnetic water which acts at a distance. Beckmann, Hist. of Invent. See Chemistry Index, Cipher, and Ink.