This was part of his list of the 20 different methods of secret communications in a book called On the Defense of Fortifications. He mentions it in discussing how to survive under siege but does not indicate the type of ink to be used. One of the earliest writers to mention an invisible ink is Aeneas Tacticus, in the 4th century BC. Invisible ink is one form of steganography. Invisible ink, also known as security ink or sympathetic ink, is a substance used for writing, which is invisible either on application or soon thereafter, and can later be made visible by some means, such as heat or ultraviolet light. Handwriting by Peggy Shippen Arnold is interspersed with coded communication in Arnold's hand.
A coded letter from Benedict Arnold, originally written in invisible ink.
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