As he walked through the dark garden and saw the great square block of the house looming up before him, it was with a quickening sense of anticipation and pleasure. All the worries of his life were momentarily over and done with, he was coming back to his great passion, to his life work, the service of science! It was about ten o'clock, and as he opened the front door and came into the hall, everything was silent and still. He lifted up the padded stick which hung beside the dinner-gong and struck the metal, standing still while the deep booming note echoed mournfully through the house. The butler did not answer the summons. Sir William realized that the man must be out; Wilson Guest had probably given the servants an evening's holiday for some purpose of his own. He crossed the dimly-lit hall, pushed open the baize door which led to the study, and entered his own room. The fire was burning brightly, the electric lights glowed, but the place was quite empty. On his writing-table were a pile of letters, on a round table set beside the fire was a cold chicken and a bottle of claret. Obviously his first surmise had been right, and the servants were out. He left the study, proceeded onwards down the passage and unlocked another door, a door through which no one but himself and Guest were allowed to penetrate, a door that was always kept locked, and which led to the laboratories, mechanical rooms, and invention studios, which had been built out at the back of the house over what were once the tennis lawns, and occupied a considerable area. Locking the door behind him, Sir William went on down a short passage. The first door on the right had the letter "A" painted on it in white. He opened this door and looked in. The room was empty, though it was brilliantly lit. It was a place filled with large tables, on which there were drawing instruments, sheets of figures and tracings. Guest was not there. Closing the door again and passing onward, Sir William entered the chemical laboratory, a long, low place, lit by a sky-light in day, and by electricity at night. As he opened the door quietly, he heard sounds of movement. And then immediately, at the far end of the laboratory, he saw the man he was looking for. The place was in entire darkness save at one end, where two incandescent bulbs glowed above an experiment table. The assistant was bending over a Bunsen burner above which a large glass tube was clamped, in which some liquid was boiling. Suddenly he heard Sir William's advancing footsteps, and leapt up. For a single moment the grey-pink hairless face was suffused with furtive terror at the sound. It shone out in the light of the lamps clear and distinct, though the lower part of the body was hidden by the darkness. "Here you are then," Gouldesbrough said. "The whole house seems deserted." Guest sighed with relief, and then began to titter in his curious, almost feminine, way--