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.

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