to participate in good deeds while really immersed in crime.' Though he was an officer and married, Sainte-Croix sometimes assumed the garb and the title of Abbé. Sainte-Croix was a brilliant and gallant cavalier; and the Marquise de Brinvilliers, with her blue eyes and dainty figure, was the most charming creature in the world. 'Lady Brinvilliers,' observes Vautier the advocate, 'did not make a mystery of her amour; she gloried in it in society, whence there resulted much _éclat_.' She gloried in it also before her husband, who responded by boasting of his own love for other ladies; but as she ventured also to brag about it before her father, the civil lieutenant, a man of the old school, he, strong in the rights with which ancient customs endowed a father, obtained a _lettre de cachet_ against his daughter's lover. On March 19, 1663, Sainte-Croix was arrested 'in the marquise's own carriage as he sat by her side,' and was thrown into the Bastille. Various writers who have dealt with these facts depict Sainte-Croix as the prison companion of the famous Exili, from whom he learnt the secret of Italian poisons. Restored to liberty, Sainte-Croix is said to have handed the terrible prescriptions to his mistress and others, who in their turn spread them through France. We find this opinion expressed in the documents of the time, among others in the speech delivered by Maître Nivelle before the Parlement, on behalf of Madame de Brinvilliers. Exili, whose real name was Eggidi or Gilles, was an Italian gentleman attached to the service of Queen Christina of Sweden. It is true that he was confined in the Bastille at the same period as Sainte-Croix. He remained there from February 2 to June 27, 1663; Sainte-Croix was there from March 19 to May 2. A captain of police named Desgrez--who will play an important part in the sequel--met Exili on leaving prison with an order to conduct him to Calais and embark him for England; but, whether Exili gave him the slip on the way, or that he had no sooner reached England than he returned to France, we soon find the Italian again in Paris, and in the house of Sainte-Croix himself, with whom he stayed for six months. After all, it was not Exili who trained Sainte-Croix in the 'art of poisons,' to adopt the phrase of the time. Long before he entered the Bastille the young cavalry officer had acquired a knowledge of poisons which far exceeded that of Exili. He owed it to a celebrated Swiss chemist named Christophe Glaser, who had set up an establishment in the Faubourg Saint-Germain, where he had attained a considerable standing, after the publication in 1665 of a _Treatise on Chemistry_, which had a noteworthy success at the time, and was often reprinted and translated. Glaser was apothecary in ordinary to the king and Monsieur,[1] and demonstrator in chemistry to the Jardin des Plantes. He was, moreover, a scientist of real merit. Sulphate of potassium, which he discovered, long bore his name.