“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. “I had expected that Ted Copperdock’s first act upon discovering his father’s body would be to call me. I thought that this would furnish me with an opportunity for adding yet another mystification for the police to solve. I placed a numbered counter on my own mantelpiece, and waited. Sure enough, young Copperdock came across to me. I met him in the shop, and we went over together, I taking care not to close the door behind us. I have no doubt that the scene in Mr. Copperdock’s room had been described to you in detail. But there is one incident which seems to have escaped everybody’s recollection. I was left alone in the room while Ted went for the doctor and Waters telephoned to Inspector Whyland. “I had meant all along to contrive to be alone in the room sooner or later, and was prepared for the opportunity. I had in my pocket the handle of the paper-maker’s knife and one of the blades. I placed the handle in Mr. Copperdock’s right hand, and pressed the fingers round it. Then I put the blade and handle in the pocket of one of his overcoats, satisfied that sooner or later it would be found there, with his finger-prints upon it. A few minutes later, a chance remark of Inspector Whyland’s gave me a further idea. If the suicide theory were correct, the hypodermic syringe ought to be found. I had, I confess, overlooked this point, but it was readily rectified. Directly he left me, I looked out an old syringe, fitted the other half of the needle to it, and dropped it by the side of the road under Mr. Copperdock’s window. Waters found it within half an hour. “Meanwhile, Inspector Whyland had returned here at my suggestion. He was the very witness I required to my finding of the counter. The door left open afforded an obvious suggestion of the means by which it had reached my mantelpiece. I did not care what theory he evolved as to the identity of the person who had entered the house. My only object was to confuse the issue as much as possible.