TheCodeBreakers
secret writing in any kind of ink, even clear water. Crystals of iodine,
heated gently, sublimated into fumes of a beautiful violet hue that settled
more densely in those fibers of paper that had been disturbed by any
kind of wetting action, thus tracing the pen's course. The Germans
replied by writing in a sympathetic ink and then moistening the entire
sheet. The Allies struck back with a chemical streak test that would
show whether the paper surface had been dampened. 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
annaabi.ee 
