“The bullet had possessed sufficient velocity to drive the needle well into Mr. Copperdock’s body, and I knew I could trust it to destroy its own evidence. I need scarcely explain the properties of metallic potassium to you, Dr. Priestley. On exposure to the air it oxidizes, as you know, first to the hydroxide, which is caustic potash, and then to the carbonate. The doctor who was called in quite correctly stated that the presence of the carbonate showed that caustic potash had been applied. But he never guessed that the caustic potash had itself been derived from potassium.