“My first move was to make the acquaintance of the builder who was carrying out the alterations to Number 407, and from him I learnt in the course of conversation the name of the new tenant. He asked me to come and look at the place, an invitation which I accepted without too great a show of eagerness. In the next few days I visited the place again, more than once. I wanted the builder’s men to get accustomed to the sight of me. For this purpose I was Mr. Ludgrove the local resident, of course. As a matter of fact, most of the builder’s friends went to have a look at the place. There is always an interest in vacant premises in a street like this. “Dr. Morlandson had provided me with a number of celluloid capsules, containing pure prussic acid. I took an ordinary electric lamp, broke it, and connected the ends of the leading-in wires by a piece of fine foil, which would become red hot on the passage of a current. Round this I wrapped a few strands of nitrated cellulose, and to this I fixed the celluloid capsule. The action of this device was very simple. If a current were passed through it, the cellulose would inflame and set fire to the capsule, thus releasing the prussic acid. Some of the vapour of this might catch fire, but enough would be left to be fatal to anybody breathing it in a confined space. And I knew of a confined space most suitably adapted to the purpose. “I wrote a letter to Martin, which I knew could not fail to bring him to Number 407, and to bring him alone. It suggested that somebody had discovered his true occupation, and was prepared to keep the matter secret, for a consideration. Then, on the Saturday morning, just before the men were due to leave, I walked openly into Number 407. I had kept an eye on their habits, and knew that before they left they packed away their tools in the back room on the ground floor. I could hear them there as I went in, and I made my way down to the cellar without anybody noticing me. All I did there was to put my poison device in the lamp holder in the small cellar—I had heard the electrician tell the builder that the current would be connected that day—hang the numbered counter on the switch, and remove the string by which the swing door could be pulled open from within. The trap was set, it remained only to ensure that Martin should have easy access to it.