She was a girl in a thousand, there was no doubt about that! Mark evidently thought so, and, since he compared all women with Mary, his standard was about as high as any one could reasonably demand. How enchantingly pretty she had looked as they had sat opposite to each other at dinner. He had only to shut his eyes and---- Hullo! What the devil was that? The sound had come from somewhere down below--a queer, half-deadened noise, like the distant crash of breaking glass. In a second Colin was out of bed and had flung open the door. The lights were still burning exactly as he had left them, and, striding to the banisters, he peered over into the hall. Nothing seemed to be stirring; except for the steady ticking of the grandfather clock the whole house was as silent as a tomb. With a momentary feeling of relief he moved toward the staircase. It was probably only some small accident; the Professor had most likely dropped a tumbler or broken a retort, and in the stillness of the night the noise had been naturally exaggerated. All the same, it would be just as well to have a look round. Running lightly down in his bare feet, he crossed the hall and knocked at the laboratory door. "It's I--Gray," he called out. "Anything wrong, sir?" As he spoke he turned the handle, and the next moment he found himself standing in the open doorway, staring blankly in front of him. The room was empty. For the first time a real sense of misgiving suddenly took possession of him. He wheeled sharply round, and, hurrying back through the hall, rapped loudly at the door of the study. "Mr. Carter," he shouted, "are you there?" There was no answer. He caught hold of the brass knob, only to make another and still more ominous discovery. Somebody had turned the key from inside. With a quick breath he stepped back a couple of paces, and then, hunching up his shoulder, hurled himself against the panel. Under the impact of twelve stone and a few odd pounds the lock gave with a splintering crash which echoed through the house. The door swung open, and at the same moment the shrill scream of a terrified woman rang out from the top landing. Clutching the broken woodwork to steady himself, Colin fumbled for the switch. His fingers closed on it in the darkness and, half prepared as he was for some horror, an involuntary cry broke from his lips as the whole room flared suddenly into light. Face upward, in the centre of the French windows, lay the huddled figure of the Professor. One arm was twisted under him, and his white hair was dabbled in a stream of blood which still oozed slowly from a gaping wound in his forehead. CHAPTER FIVE At the sight of that hideous injury, all Colin's professional training instinctively asserted itself. Letting go the switch, he sprang forward, and, heedless of the blood and broken glass, dropped down on one knee beside the prostrate body.