The Mysterious Writing
Write on a piece of paper with common ink any question; then
underneath it write the answer either in invisible silver ink, or the
invisible green ink, made with zaffre and aqua regia, described in
pages 24 and 25.
You give this paper to your friend, and tell him to place it against
the wall, or on his dressing-table, keeping the door locked, that he
may be sure no person has entered his room: he will next day find the
answer written on it.