“Now, above my shop is a room, which I use for storage purposes. From the window of this room you can see into the window of Mr. Copperdock’s bedroom. I had merely to wait for a windless evening, fine enough for Mr. Copperdock to leave his window open, and seize my opportunity when I knew his house was empty. The opportunity came, and I took it.
“It might have been thought that by delivering the counter in advance I had increased my difficulties. But, on the contrary, I had diminished them. The obvious improbability of Mr. Copperdock’s story, that he had found a counter on his bed at a time when it was demonstrable that nobody had entered the house, was merely another link in the chain of suspicion which I was forging round him. I allowed a few days to pass, and then, the following Saturday, I took the final step.
“Now, even you, Dr. Priestley, failed to read the riddle of the broken hypodermic needle and the incrustation of potassium carbonate round the puncture. Yet it was ridiculously simple. I secured a piece of metallic potassium, and moulded it into the shape of a bullet to fit an air-gun which I possessed. Into the head of this bullet I inserted the broken end of a hypodermic needle, which I had already charged with the same drug as had been so effectual in the case of Mr. Colburn. Keeping this prepared bullet in paraffin, to avoid oxidation, I placed the air-gun ready upstairs, and waited for Mr. Copperdock’s return from the Cambridge Arms. He brought two friends home with him, and I was afraid at first that I should be compelled to postpone my attempt. It was essential to my scheme that he should be alone in the house when I fired the gun.
“However, the two friends left after a while, and Mr. Copperdock entered his bedroom and proceeded to undress. I could just see the washing-stand through a gap in his curtains, and I waited until he was bending over this. Then I loaded the gun with my potassium bullet, and pulled the trigger. I saw Mr. Copperdock spin round, and then he passed out of my line of vision. But I knew I had succeeded, and I came quietly down here.
“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.
“Then, Dr. Priestley, Inspector Hanslet came upon the scene, and I knew that the real battle of wits had begun. As I have told you, I had spent years studying the characters of the members of the jury, and among them I had devoted a large part of my time to observing you. I soon discovered that Inspector Hanslet consulted you upon his principal cases, and I knew as soon as Hanslet appeared, that I was at last pitted against the most acute brain in England. I may say, without exaggeration, that the knowledge gave me the keenest pleasure which I had known for years.”
The Black Sailor paused, and smiled at his auditor benignly. “You see, Dr. Priestley,” he continued, “I was in rather a delicate position, where you were concerned. I had purposely left you until the last, because, as foreman of the jury, Dr. Morlandson wished you to have special treatment. You were to be warned, but your fate was to be left hanging over your head as long as possible. I was afraid that you would not attach much importance to the mere receipt of a counter. You are not the sort of person to be so easily frightened. Yet how to deliver a personal warning without giving you a clue which would be fatal to the execution of my design, I could not see.