The experts to whom the curious device found by the electrician in the little cellar had been sent duly made their report. In their opinion, the butt was all that remained of an ingenious poison-bomb. The original machine consisted of a socket made to fit into an ordinary lamp-holder, but the place of the filament had been taken by a piece of fine wire, round which had been wrapped a thread of gun-cotton. Attached to the socket, and in contact with the gun-cotton, had been a sealed celluloid bulb containing pure prussic acid. The action of such a bomb would be perfectly simple. It had only to be placed in a lamp-holder, and the switch turned on. The current would heat the fine wire, and so ignite the strand of gun-cotton. This in turn would set fire to the celluloid bulb, and its contents would be released in the form of vapour. Although some of this vapour would become ignited, enough would be available to permeate the atmosphere of a room far larger than the small cellar. And, of course, the inhalation of the vapour would rapidly produce death. This opinion, the report stated, was purely speculative, since only the socket of the bomb remained for examination. Inspector Whyland, after consultation with his Chief at Scotland Yard, decided that it had better not be made public. The idea, though it was probably correct, was hypothetical, and altogether too vague to be put before a coroner’s jury. It certainly strengthened Whyland’s conviction that Mr. Martin had been murdered, for no man would go to the trouble of constructing an elaborate poison-bomb with which to kill himself when he might just as easily have inhaled the acid outright. When the electrician left the cellar on the Saturday the bomb had not been there. Whyland was convinced that the man was speaking the truth upon this point. The only two alternatives remaining were that somebody had entered the house between the departure of the foreman and the arrival of Mr. Martin, or that Martin had placed the bomb in position himself. The field of speculation opened by these alternatives was so vast and barren of any clue that their exploration must be a matter of considerable time. As a matter of fact, the Assistant Commissioner was by no means convinced that Mr. Martin had been murdered. “We all know what a difficult job you’re up against in trying to trace the murders of these two fellows, Tovey and Pargent,” he said to Whyland in an interview to which he had summoned him, “But don’t you think you’re rather too much inclined to see the hand of the murderer in everything that happens in that district? Now, honestly, assuming that Martin was murdered, have you the slightest clue which could possibly lead to the conviction of the murderer?” “No, sir, I haven’t,” replied Whyland. “But I’ve got my suspicions of this man Copperdock, whom I’ve mentioned to you.”