Invisible Ink
Put litharge of lead into very strong vinegar, and let it stand
twenty-four hours. Strain it off, and let it remain till quite
settled; then put the liquor in a bottle.
You next dissolve orpiment in quick lime water, by setting the water
in the sun for two or three days, turning it five or six times a-day.
Keep the bottle containing this liquor well corked, as the vapour is
highly pernicious if received into the mouth.
Write what you wish with a pen dipped in the first liquor; and, to
make it visible, expose it to the vapour of the second liquor. If you
wish them to disappear again, draw a sponge or pencil, dipped in aqua
fortis, or spirit of nitre, over the paper; and if you wish them to
re-appear, let the paper be quite dry, and then pass the solution of
orpiment over it.
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