Dissolve fine silver in aqua-fortis, and add some distilled water to the solution, in the same manner as in the gold ink; then, whatever is written with this ink, will remain invisible for three or four months, if it be kept close from the air; but if it be exposed to the sun, it will appear in about an hour, of a gray colour, like that of a slate.

Sympathetic Inks of the Third Class, - or such as become visible by having any fine powder strewed over them, - may be composed of the glutinous and colourless juice of any vegetable, the milk of animals, and several other sub-stances.

Sympathetic Inks of the Fourth Class, - are made by diluting acid of vitriol with about three times its weight of common water, or as much as will prevent it from corroding the paper. The juice of lemons, or onions, will answer the same purpose; but either of them requires more heat than the first, and will not keep so long.