The heat revealed what had been invisibly written in lemon juice... the complete working drawings of a new French war tank.

9 seconds sound clip from the The 39 Steps (1937) classic radio play.

You can hear this line at 00:42:10 in the radio play.

Quote context

[...]

- It concerns a Swiss opera singer who volunteered in the French Red Cross as an entertainer.

- For two years she made periodic visits to a home in Switzerland, each time using the same route, over the French border.

- One day she started to cross and found a new officer in charge.

- He took a good look at her and immediately ordered his women assistants to search her.

- They returned with the petticoat of this famous singer. Suspecting that it might contain invisible writing, Lieutenant called for hot irons and himself applied them to the petticoat.

- The heat revealed what had been invisibly written in lemon juice... the complete working drawings of a new French war tank.

- The reason Lieutenant had suspected Madame was that before the war he had been a style designer in Paris.

- One of the first things he observed about the singer was that while she was dressed in the height of fashion, she wore a starched petticoat and starched petticoats had been out of style for ten years.

- One of the truest utterances of the great French general, Marshal Foch, was his statement to a group of his espionage agents. 'You will die a thousand deaths, before oblivion comes, while the man in the trenches dies but once.'

- And no better example can be found, than that patriot of revolutionary war, John Honeyman. Too old to carry arms, he became a confidential spy at Washington's urgent request and posed as a Tory.

[...]