If we give this name to any fluid, which when written with, will remain invisible till after a certain operation, such liquids were known in very early periods. Among the methods, with which Ovid teaches young women to deceive their guardians, when they write to their lovers[260], he mentions that of writing with new milk, and of making the writing legible by coal-dust or soot. Ausonius proposes the same means to Paulinus[261]; but his commentators seem not to have fully understood his meaning; for _favilla_ is not to be explained by _favilla non modice calida_, as Vinetus has explained it, but by _fuligo_. That word is often employed by the poets in the same sense. As a proof of it, Columella, speaking of the method, not altogether ineffectual, and even still used, of preserving plants from insects by soot, calls it _nigra favilla_; and afterwards, when mentioning the same method, free from poetical fetters, he says _fuliginem quæ supra focos tectis inhæret_[262]. It may be easily perceived, that instead of milk any other colourless and glutinous juice might be employed, as it would equally hold fast the black powder strewed over it. Pliny, therefore, recommends the milky sap of certain plants for the like purpose[263]. There are several metallic solutions perfectly colourless, or, at least, without any strong tint, which being used for writing, the letters will not appear until the paper be washed over with another colourless solution, or exposed to the vapour of it; but among all these there is none which excites more astonishment, than that which consists of a solution of lead in acetic acid, and which by sulphuretted hydrogen gas becomes black, even at a considerable distance. This ink, which may be employed by conjurers, proves the subtlety of this gas, and the porosity of bodies; as the change or colouring takes place, even when the writing is placed on the other side of a thin wall. This effect presented itself perhaps accidentally to some chemist; but the discovery is not of great antiquity. Wecker, who compiled his book De Secretis from Porta, Cardan, and several old writers, and printed it for the first time in 1582, and gave a third edition in 1592, must have been unacquainted with it; else he certainly would not have omitted it in the fourteenth book, where he mentions all the methods of secret writing. Neither would it have been unnoticed by Caneparius, whose book De Atramentis was printed at Venice, for the first time, in 1619.