A week before all London had been mystified and horrified by a most remarkable explosion which had occurred one night in a house in one of the outer suburbs, whereby the place had been set on fire and utterly demolished. Whoever were present in the house had been blown to atoms, for no trace of the occupants, or of what had caused the disaster, had been discovered. At first it was believed to have been caused by an incendiary bomb dropped from the air, but expert evidence quickly established the fact that something within the house had exploded. Was it possible that her father and his dastardly companions possessed knowledge of what had actually occurred there? Suddenly, Drost having handed the tiny sealed tube to Nystrom, the latter proceeded to place it in position within the box, using most infinite care. Then her father turned upon his heel, and came forward to the door behind which his daughter was standing. In a second Ella had shrunk back noiselessly into the cupboard, which the old man passed in the darkness, and descended the stairs. He had passed the door of Ella's room when, having gained the bottom of the stairs, he paused and whistled softly. In a few seconds Nystrom came forth. "Come, Fridtjof," he urged in a low whisper. "Let us drink to the success of our expedition to-night, and the victory of our dear Fatherland," an invitation which his visitor at once accepted. Ella heard the two men descend, making but little noise, and a moment later she crept into the long, well-lit laboratory where, upon the table, stood the big official-looking despatch-box. A second's glance was sufficient to reveal the truth even to her, a woman unversed as she was in such things. It was a most ingeniously-constructed infernal machine which would detonate the quantity of high-explosive which she saw had been placed within. Though her father had taken the greatest precaution to conceal from his daughter the exact line of his chemical experiments, yet, if the truth be told, Ella and her lover had watched carefully, and Kennedy--who had shared his well-beloved's suspicions--had ascertained, without doubt, that Drost and Nystrom had been engaged in that long, low room beneath the roof, in treating toluene with nitric and sulphuric acid for several days under heat thus producing tri-nitro-toluene--or trotul--that modern high-explosive, of terrible force, which was rapidly superseding picric acid as a base for shell-fillers. At a glance Ella saw that the square steel bomb, fashioned like an official despatch-box, was filled with this highly dangerous explosive, and that the thin glass tube which, when broken, would explode it, had already been placed in position. Such a bomb, on exploding in a confined space, must work the most terrible havoc. In those few seconds the girl verified the suspicion which Kennedy had entertained. Some desperate outrage was to be committed. That was quite certain.