Dissolve in water a small quantity, about as much as will lay on a ten-cent piece, of chloride of cobalt, which is of a bluish-green color, and the solution will be pink; write with it and the characters will scarcely be visible; but if gently heated they will appear in brilliant green, which will disappear as the paper cools.

Dissolve in water a few grains of prussiate of potash; write with this liquid, which is invisible when dry; wash over with a dilute solution of iron, made by dissolving a nail in a little aqua fortis; a blue and legible writing is immediately apparent.