The Man Who Knew is a 1918 British thriller novel by Edgar Wallace. A detective investigates the death of a South African diamond magnate in London. Excerpt: "The face of the man sitting at the table watching this unsavory stew was hidden behind a mica and rubber mask, for the fumes which were being given off by the fluid were neither pleasant nor healthy. Save for a shaded light upon the table and the blue glow of the Bunsen lamp, the room was in darkness. Now and again the student would take a glass rod…"
