True or false: in 1781, patriot spies used invisible ink?

True!

Explanation:

One form of secret writing used by both the British and American armies was invisible ink. During the Revolutionary War invisible ink usually consisted of a mixture of ferrous sulfate and water. The secret writing was placed between the lines of an innocent letter and could be discerned by treating the letter with heat or a chemical substance. The recipient placed the paper over the flame of a candle or treated it with a chemical reagent, such as sodium carbonate, which would reveal the letter's hidden contents.