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Welcome to Science Talk, the more or less weekly podcast of Scientific American, posted on April 20th, 2010. I'm Steve Murski. This week on the podcast,

you would write with one chemical and then have to apply a second chemical, which then would make the message appear.

That's John Nakey, author of the new book, Invisible Inc. Spycraft of the American Revolution.

He spoke last month of the American Revolution.

He spoke last month at the famous Revolutionary War hangout the Francis Tavern in lower Manhattan,

and we'll hear an edited version of his talk.

Plus, we'll test your knowledge of some recent science in the news.

John Nagy is a founding father himself, a founder of the American Revolution Roundtable of Philadelphia.

He's written numerous books about the war.

And since spying involves some technology and science, I thought it would be fun to hear about it in our venue.

Nagy begins with a specific case about a captured document written in ciphers and symbols. Then he talks in more general terms about codes and ciphers,

the chemistry employed to send messages in Invisible Inc.,

and some of the psychology involved in fooling your enemy during war.

The American Revolution begins up in Boston

after the battles of Concord in Lexington.

The American Army surrounds the British who are under a virtual siege.

...