TheCodeBreakers
This was almost as
incriminating as actual development of a secret-ink letter, for who but a
spy would wet a letter? The seesaw battle between the chemists of
Germany, traditionally world leaders in that science, and those of the
Allies reached a stalemate when both sides discovered the general
reagent—one that would develop any secret ink at any time, even on
moistened paper. Formulas differ slightly, but all use a mixture of iodine,
potassium iodide, glycerine, and water, dabbed on with cotton. The liquid
concentrates in the more disturbed fibers and reveals the writing. By the
time this general reagent appeared, MI-8's secret-ink subsection was
testing 2,000 letters a week for invisible writing and had discovered 50 of
major importance. Among them were letters that led to the capture of
Maria de Victoria, a beautiful German spy who was planning to import
high explosives for sabotage inside the hollow figures of saints and the
Virgin Mary!
annaabi.ee 
