The diabolical art which she so widely practised was learned from St. Croix, a young officer, who was her paramour. He was a friend of her husband, who, in opposition to her real or feigned remonstrances, made him an inmate of his house. A criminal intimacy soon took place between the wife and the friend. The husband, a man of dissipated habits, seems to have been regardless of their intrigue; but her father was so disgusted by its shameless publicity that he obtained a lettre-de-cachet, and St. Croix was lodged in the Bastile, where he continued for twelve months. There St. Croix was placed in the same apartment with Exili, an Italian, who was confined on suspicion of being, as he really was, a compounder and vender of poisons. Exili taught St. Croix all his detestable secrets, and the latter communicated them to the marchioness, who was a willing scholar. St. Croix died suddenly in 1672, and, as he had no relatives, the government took possession of his effects. Among them was a small box, which was importunately claimed by the marchioness. It was opened, and found to contain a note, desiring that it might be delivered, without the contents being disturbed, to Madame de Brinvilliers. The box was filled with poisons of all kinds, some of the marchioness’s letters to him, and a note of hand to him, for 30,000 livres, bearing her signature. Disappointed in all attempts to gain possession of the box, and finding that suspicion began to fall heavily upon her, Brinvilliers took flight. After having visited England, she fixed her residence at Liege. Fresh presumptions of her guilt having arisen, it was resolved to arrest her. Desgrais, the exempt of police, was accordingly despatched to Liege. He disguised himself as an Abbé, pretended to be enamoured of her, insinuated himself into her good graces, and ultimately succeeded in seizing the lady and her papers, and conveying them to Paris.