By sympathetic inks, are meant those kinds of liquors, with which if any characters be written, they will remain invisible, till some method is used to give them a colour.

The first class of these inks consists of such as become visible by passing another liquor over them, or by exposing them to the vapour of that liqur.

The second, of those which do not appear so long as they are kept close, but soon become visible on being exposed to the air.

The third, of such as become apparent by strewing or sifting some very fine powder over them.

The fourth, of those which do not become visible till they are exposed to the fire, or heated.

The fifth, like the fourth, of such as appear by heat, but disappear again when the paper becomes cold, or has had a sufficient time to imbibe the moisture of the air.