Black Nov 5 Boston, Ms cds with red manuscript "40" on 1803 folded letter to Philadelphia.

France and England were at war from 1793-1815. It was unclear to the French as to where official American sympathies would finally rest; as they suspected that our Post was opening certain important mails, they sometimes would write a secret message in lemon juice, which was invisible to the eye. When these letters were received, a small chunk would be removed and soaked to bring out a trace of a message; if none appeared, so be it. But if there was a message, the entire letter would be soaked.

This 1803 letter, addressed to Philadelphia, is an example of this practice.