Military restrictions now were somewhat relaxed. Leaves of absence were more easily obtained, but it was some time before the Army Boys were able to arrange things so that all their leaves fell on the same night. That time came at last, however, and they started out soon after nightfall with the determination once for all to solve the mystery of the alley. The night was extremely dark, and as the moon would not rise till late they had comparatively little difficulty in seizing an opportunity when the street was practically deserted to slip into the alley unobserved. Their task was rendered easier by the fact that there was no longer ice to hinder their raising of the trap door. It creaked under the straining of their arms, but it yielded, and, using the utmost caution, they descended into the yawning chasm. They had provided themselves with stout sticks that they felt sure would enable them to ward off any attack by rats, though they devoutly hoped that these would not be needed. Nor were they, for Billy's conjecture that the part infested by them was beyond the lighted corridor proved correct. With the stealth of Indians they moved along the narrow passage, darting glances into every opening that seemed to branch off from the main corridor. For some time nothing greeted their eyes but impenetrable blackness, and they began to think that either the light had been extinguished or that they had inadvertently passed it by. "Hist!" came from Billy's lips, and they halted. "There it is," he said in a low tone. They clustered about him as he pointed to the left. There, sure enough, was the electric bulb glowing, and behind it the outline of a door. Turning into the passage and inwardly thankful that as yet no rats had been encountered, they made their way toward the light. The door, as revealed by the light, was of heavy oak. There was no crack or crevice in it anywhere. Standing close to the door they listened intently for any sound from the other side. Everything was absolutely quiet. All that they could hear was their own excited breathing. Frank put his hand on the knob of the door and flashed a look of mute inquiry at his comrades. They nodded understandingly, and inch by inch Frank noiselessly drew the door open. There was no light in the room beyond, but a ray from the electric bulb outside fell on a row of bottles and retorts that indicated a chemical laboratory. Frank had drawn his flashlight from his section pocket and was about to turn it upon the room, when suddenly the room became radiant with a perfect flood of light. At the same time there was the sound of a quick step in the hall beyond the room, the click of a door knob, and Frank had just time to push the heavy oaken door nearly to, when the further door opened and a man came into the room. Through the crack of the door Frank caught a glimpse of the man's face and started back in surprise. CHAPTER XXIV THE DEADLY PHIAL