S a n d y W r o t e:
Mike [mmotyka@lsil.com] wrote:
I would bet that there is SOMETHING that is dissolved by liquid freon. Just mark your letters with the stuff and look for the integrity of the mark at the other end.
Or... is there something that REACTS to freon in an interesting way...?
IIRC in 'The Codebreakers' Kahn states that invisible ink can always be read by exposing the paper to iodine vapor, since the 'ink' distorts the fibers of the paper. Even plain water can be used to write messages. This can be defeated by wiping the surface of the paper with a damp sponge, requiring Mallory to find the correct chemical to react with the ink and cause a color change. However, _this_ can be detected by the smear left on the fibers of the paper. Perhaps this also occurs with sprayed on solvents? This could work on 'unprepared' mail. As an added bonus, the iodine vapor is _supposed_ to fade out afterwards, unless you use too much of it as we did in science class trying to develop fingerprints. The Baker Street Irregulars