How to make invisible ink

Invisible Ink in espionage:

"Well, invisible ink is of course one of the key ways of communicating all the way through this period and there is a formula. He did employ an early scientist, a fellow of the Royal Society later, who produces a formula for invisible ink.

But then someone discovered, apparently to Cumming's delight, that semen makes a very good invisible ink and the Head of Station at Copenhagen took to this with some enthusiasm apparently... and his letters arrived stinking of high heaven and he had to be instructed that a fresh operation was required for each communication.

Now we haven't actually tested this but if there are any volunteers out there I'd like to, you know, I'd like to hear from them and perhaps we could arrange some kind of field test for this."

Read the complete interview with the author of The Secret History of MI6