Lavenne possessed a vast fund of special knowledge without which, despite his genius and fertility of resource, he would have been lost a hundred times in the course of a year. Not only had he a quick mind to receive knowledge, he had also a memory to retain it. Again, that kindness and rectitude of spirit which made so many men his friends, opened for him a living library in the Hôtel de Sartines. For instance, he had learned much of the science of Cryptography from Fremin. Jondret, who would certainly have been hanged some day as a housebreaker, had not de Sartines recognized his genius and drawn him into the police, had taught him the science of picking locks, whilst Cabuchon, a little old man, who in the year 1767 had placed his dirty forefinger on the poisoner of M. Terell, the haberdasher of the Rue St. Honoré, had taught him many of the tricks of poisoners. The art of poisoning, first studied in Europe seriously by the Italians, had been imported into France in the days of the infamous Catherine de Medicis. The Revolution put its heel definitely on the last remnants of this fine product of the Middle Ages, but in the time of the fifteenth Louis there were still a few practitioners of the business, as witness the case of M. Terell poisoned by a candle. Cabuchon had disclosed many of the secrets of this horrible science to the eager Lavenne. He had not only given him considerable knowledge of the methods used by the practitioners of the Italian art, such as the poisoning of gloves and flowers, but he had also given his pupil an insight into the psychology of the poisoner who uses recondite means, showing clearly and by instance that these people develop a passion for the business, and are sometimes held under the sway and fascination of the demon who presides over it so firmly that they will poison their fellow men and women for the slightest reason, and sometimes for no perceptible reason at all. It was this knowledge derived from Cabuchon that disclosed to Lavenne at one stroke the poisoner of Atalanta, and the intending poisoner of Madame Camus. It was the knowledge derived from Jondret that was now guiding his dexterous hand in the use of the _crochet_. Feeling and exploring the wards, examining the construction of the lock, using the delicacy and gentleness of a surgeon who is probing a wound, he worked, till, assured of the mechanism, with a powerful and sudden turn of the wrist he forced the bolt back and the door was open. He entered the room, shut the door, and proceeded to examine the lock. The bolt was a spring bolt, that is to say, that whilst it required a key to open, it required none to lock it again. He pressed the door to, and it closed with a click scarcely audible and speaking well for the perfection of the mechanism.