The man who was interested in the chemistry of explosives apparently wasn't in the least afraid of being followed, for he took a bus uptown, alighted at Eighty-third Street, and vanished into one of the innumerable small apartment houses in that section of the city. Walters kept close behind him, and he entered the lobby of the apartment house in time to hear his quarry ascending to the fourth floor. Then he signaled to the four men who had followed him up the Avenue in a government-owned machine--men who had been stationed outside the Library in the event of just such an occurrence--and instructed two of them to guard the rear of the house, while the other two remained in front. "I'm going to make this haul myself," Walters stated, "but I want you boys to cover up in case anything happens to me. No matter what occurs, don't let him get away. Shoot first and ask questions afterward!" and he had re-entered the house almost before he finished speaking. On the landing at the third floor he paused long enough to give the men at the rear a chance to get located. Then--a quick ring at the bell on the fourth floor and he waited for action. Nothing happened. Another ring--and still no response. As he pressed the button for the third time the door swung slowly inward, affording only a glimpse of a dark, uninviting hall. But, once he was inside, the door closed silently and he heard a bolt slipped into place. Simultaneously a spot light, arranged over the doorway, flashed on and Dick was almost dazzled by the glare. Out of the darkness came the guttural inquiry: "What do you want?" "Not a thing in the world," replied Walters, "except to know if a man named Simpson lives here." "No," came the voice, "he does not. Get out!" "Sure I will if you'll pull back that bolt. What's the idea, anyhow? You're as mysterious as if you were running a bomb factory or something--" As he spoke he ducked, for if the words had the effect he hoped, the other would realize that he was cornered and attempt to escape. A guttural German oath, followed by a rapid movement of the man's hand toward his hip pocket was the reply. In a flash Dick slipped forward, bending low to avoid the expected attack, and seized the German in a half nelson that defied movement. Backing out of the circle of light, he held the helpless man in front of him--as a shelter in case of an attack from other occupants of the apartment--and called for assistance. The crash of glass at the rear told him that reinforcements had made their way up the fire escape and had broken in through the window. A moment later came the sound of feet on the stairs and the other two operatives were at the door, revolvers drawn and ready for action.