The milk of figs, cows and nuts; lemon juice, orange juice and onion juice; saliva, urine, blood, vinegar, aspirin and laxatives: the list of substances from which invisible ink can be concocted is long and sometimes gross, and as strong a testament as could be imagined to the strength of our intertwined desires for communication and privacy. Why else would someone bother not only to squeeze the juices from a dormouse’s corpse, but also to write a foul-smelling letter with them?
In Prisoners, Lovers, and Spies: The Story of Invisible Ink From Herodotus to al-Qaeda (Yale; $27.50), espionage historian Kristie Macrakis discusses invisible ink (or “sympathetic ink,” as the French call it) within the broader history of steganography, or secret writing: of scrolls stuffed in rabbit bellies, messages inscribed on…