To prove it, however, despite the surrounding circumstances, and to locate and corner Margate again, to say nothing of doing so in time to save the reputations of Senator Barclay and the impulsive American girl who had put herself in a position that threatened to ruin the remaining years of her life--all this was an entirely different proposition. The discernment of Nick Carter, nevertheless, as well as the wisdom of the course he had shaped, appeared in part in the visit of Chick Carter to the laboratory of the eminent Washington chemist, and in what immediately followed his departure. It was nearly noon when Chick introduced himself to Professor Arden and stated his mission. He met with a cordial reception, and the chemist soon began an examination of the small quantity of fluid still remaining in the vial. Chick waited in an adjoining room for more than an hour. Most of this time was passed in reading a magazine found on the table. Ending an article in which he had become interested, Chick replaced the book on the table and glanced incidentially through one of the screened windows overlooking the grounds without and an adjoining side street. A man who was passing at that moment caught the detective’s eye, and his sinister appearance and somewhat stealthy movements quickly aroused Chick’s suspicions. He was a slender, cheaply clad fellow in the twenties, wearing a baggy brown suit and a woolen cap, the latter pulled suggestively low over his brow. He peered from under it while passing a boxwood hedge flanking one side of the grounds, and once he paused nearly back of a clump of shrubbery to gaze intently toward the laboratory windows, though the wire screen prevented any view of the interior. “By Jove, he is sizing up this place,” thought Chick, after intently watching the fellow. “What’s his motive? If it corresponds with his looks, by gracious, it’s sinister enough. What motive can he have in which I do not figure, since he appears to have turned up since I arrived here? If I’m right, and I’d stake a trifle on it, that fellow is a rat that needs watching.” The man had moved on, crossing the side street and turning an opposite corner. He scarce had turned it, however, when Chick, still watching, saw his bullet-shaped head thrust cautiously around the corner building. It was obvious, too, that his ratty eyes were directed toward the taxicab in front of the chemist’s residence, that in which Chick had come there and for whom the chauffeur was waiting. Presently the head vanished--but not the detective’s suspicions. When Professor Arden rejoined Chick a few moments later, he returned the nearly empty vial, saying, with a smile: