Green hazard.
From the book: News that Hambledon is killed in a chemical explosion spreads gloom throughout the British Foreign Office and Military Intelligence. He had been sent to Switzerland to investigate a new explosive being developed by Professor Ulseth, a famous German chemist. Both Hambledon and the Professor had apparently gone up in the blast. Then from British agents inside Germany come reports that a Professor Ulseth is in Berlin, alive and doing well under the care of the Nazis -- but the Herr Professor's fingerprints are those of Tommy Hambledon.