It was the famous physician, the man whose hate for Americans was so notorious, the man with whom they had already had unpleasant encounters, the man who had so often shot venomous looks at Frank and his comrades as they passed and yet who of late had worn an air so jubilant. It was his house then to which this mysterious passage afforded secret entrance, that entrance which the Army Boys had felt sure was used by conspirators and assassins. What did it all mean? The doctor approached one of the retorts in which some concoction was bubbling and examined it carefully, reducing the heat a little as he glanced at the thermometer. Then he walked over to a row of phials on one of the shelves and handled them almost caressingly. One of them he pressed with an almost rapturous gesture to his breast, at the same time breaking out in a strain of mingled eulogy and denunciation. The eulogy seemed to be for the phial, the denunciation for the "accursed Americans," which phrase Frank heard him repeat several times. The doctor then replaced the phials on the shelf and picked up an evening paper printed in German that was lying on a chair. He looked over the headlines which ran all the way across the page, and indulged in a chuckle. He read the article through, then threw down the paper and walked to and fro in the room, rubbing his hands and evidently in the highest spirits. The paper had been thrown down in such a way that Frank could plainly see the flaring headlines. They ran thus: "MYSTERIOUS DISEASE STILL UNABATED More Americans Stricken." This then accounted for the doctor's elation. Frank's eye glanced from the paper to the phial and back again to the paper. Suddenly a terrible conviction struck him with the force of a blow. At that moment a bell rang somewhere outside. The doctor stopped in his pacing, listened a moment, and then with a gesture of impatience strode to the door and passed out into the hall, closing the door after him. Like a flash, Frank was in the room and had possessed himself of the mysterious phial. Then he was back again among his companions, who had gazed after him in wonder. "Quick!" he directed as he closed the heavy door. "Back to the alley as fast as we can." "What's the big idea, Frank?" asked Bart, as the boys hurried after their leader. "Can't stop to talk about it now, old fellow. Tell you later what I think I've stumbled on. I think I know now what my hunch meant. I'm streaking it straight for headquarters as fast as my legs will carry me." Bart saw how wrought up he was, and followed him without further questioning.