The Cameleon Spirit.
Put into a decanter volatile spirit, in which you have dissolved copper filings, and it will produce a fine blue. If the bottle be stopped, the colour will disappear; but when unstopped, it will return. This experiment may be often repeated.
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.
Another.
Dissolve bismuth in nitrous acid. When the writing with this fluid is exposed to the vapour of liver of sulphur, it will become quite black.
Another.
Dissolve green vitriol and a little nitrous acid in common water. Write your characters with a new pen.