TheCodeBreakers
dry but that react to form a visible compound when treated with another
chemical, called the reagent. For example, when a spy writes in iron
sulfate, nothing will be visible until it is painted over with a solution of
potassium cyanate, when the two chemicals will combine to form ferric
ferrocyanide, or Prussian blue, a particularly lovely hue. The colorless
writing of lead sub-acetate will turn into a visible brown compound when
moistened with sodium sulfhydrate. Copper sulfate can be developed
with ammonia fumes, and it may have been this chemical that was used
for the secret writing on the handkerchief of George Dasch, leader of the
eight Nazi spies who landed by submarine on Long Island in 1942 to
blow up American defense plants, railroad bridges, and canal locks. The
red letters that appeared as if by magic when the pungent ammonia
reached it spelled out the names and addresses of a mail drop in Lisbon
annaabi.ee 
