St. Croix availed himself of this state of things to make sure of the fruit of his crimes. He obtained from the marchioness two promissory deeds, one for thirty thousand livres in his own name, and another for twenty-five thousand livres in the name of Martin, one of his familiars. The sum at first sight appears a small one, amounting only to about two thousand three hundred pounds sterling; but the immense difference in the value of money since the seventeenth century must be taken into account. Such, however, at all events, was the price paid by this demon for the death of her two brothers.
Meanwhile the elder d'Aubray became worse and worse; he could take no sustenance, and vomited incessantly. The three last days of his life he felt a fire in his stomach, which seemed to be consuming its very substance. At length he expired on the 17th of June 1670. On being opened, his stomach and duodenum were black, and falling to pieces, as if they had been put on a large fire; and the liver was burnt up and gangrened. It was evident that he had been poisoned: but on whom could suspicion fall?—there was no clue whatever to guide it. The marchioness had gone to the country. St. Croix wrote her that the magistrate was dead, and that, from his brother's situation, he must soon follow. It so turned out. The unfortunate counsellor died, after having lingered three months in excruciating torments; and he was so far from suspecting La Chaussée of any hand in his death, that he left him a legacy of three hundred livres, which was paid.
These three murders were still insufficient. There was yet a sister who kept from the marchioness the half of the successions which she wished to gain by the death of her father and brothers. The sister's life was repeatedly attempted in the same way; but the shocking occurrences in her family had made her suspicious, and her precautions preserved her.
The poor Marquis de Brinvillier was intended by his fury of a wife for her next victim. "Madame de Brinvillier," says Madame de Sevigné in another of her letters, "wanted to marry St. Croix, and for that purpose poisoned her husband repeatedly. But St. Croix, who had no desire to have a wife as wicked as himself, gave the poor man antidotes; so that, having been tossed backward and forward in this way, sometimes poisoned, and sometimes unpoisoned, (désempoisonné), he has, after all, got off with his life."
Though everybody was convinced that the father and his two sons had been poisoned, yet nothing but very vague suspicions were entertained as to the perpetrators of the crime. Nobody thought of St. Croix as having had anything to do with it. He had for a long time ceased, to all appearance, to have any connexion with Madame de Brinvillier; and La Chaussée, the immediate agent, had played his part so well, that he was never suspected.
At last the horrible mystery was discovered. St. Croix continued to practise the art which had been so useful to him; and, as the poisons he made were so subtle as to be fatal even by respiration, he used to intercept their exhalations while compounding them by a glass mask over his face. One day the mask by accident dropped off, and he fell dead on the spot; "a death," says the French writer who mentions this occurrence, "much too good for a monster who had inflicted it by long and agonizing pangs on so many valuable citizens."[11] Having no relations that were known, his repositories were sealed up by the public authorities. When they were opened and examined, the first thing which was found was a casket, in which was a paper in the following terms:
"I earnestly request those into whose hands this casket may fall, to deliver it into the hands of Madame la Marquise de Brinvillier, residing in the Rue Neuve St. Paul, seeing that all that it contains concerns and belongs to her only, and that it can be of no use to any person in the world except herself; and, in case of her being dead before me, to burn it, and all that it contains, without opening or meddling with anything. And should any one contravene these my intentions on this subject, which are just and reasonable, I lay the consequences on their head, both in this world and the next; protesting that this is my last will. Done at Paris this 25th May, afternoon, 1672. (Signed) De Sainte Croix."
The casket contained a number of parcels carefully sealed up, and some phials containing liquids. The parcels were found to contain a variety of drugs, which, having been submitted to the examination of physicians, were found to be most subtle and deadly poisons. This was ascertained by many experiments made upon pigeons, dogs, cats, and other animals, all which were detailed in a formal report made on the subject. It is stated in that report that no traces of the action of the poison, either external or internal, appeared on the bodies of the animals which had perished by it, and that it was impossible to detect its existence by any chemical tests. It would appear, therefore, that St. Croix had by his studies greatly increased in skill since the deaths of the d'Aubray family. The poisons administered to them were of a comparatively coarse and ordinary kind; they burnt up the stomach and bowels, produced horrid torment, and left unequivocal marks of their operation when any suspicion caused these marks to be sought for. But, with the skill subsequently acquired, this hateful pair might have destroyed thousands of their fellow-creatures with absolute impunity. It is impossible to suppose that St. Croix could have been constantly engaged, for a long series of years, in the composition of these secret instruments of death without making use of them; and there is no saying to what extent his work of destruction may have been carried.
The same casket contained ample evidence of the marchioness's share in these transactions. There were a number of letters from her to St. Croix, and the deed of promise which she had executed in his favour for thirty thousand livres.
When the marchioness heard that St. Croix was dead, and that his repositories had been sealed up, she showed the utmost anxiety to get possession of the casket. At ten o'clock at night she came to the house of the commissary who had affixed and taken off the seals, and desired to speak with him. Being told by his clerk that he was asleep, she said she had come to inquire about a casket which belonged to her, and which she wished to get back, and would return next day. When she came back, she was told that the casket could not be given up to her. Thinking it high time, therefore, to take care of herself, she went off during the following night, and took refuge in Liege; leaving, however, a power to an attorney to appear for her and contest the validity of the promise she had given to St. Croix. La Chaussée, too, had the impudence to put in a claim to certain sums of money, which, as he pretended, belonged to him, and which were deposited, in places which he mentioned, in St. Croix's study. This proved that La Chaussée was acquainted with the localities of a place into which it was to be presumed that St. Croix admitted none but his confidants and confederates; and La Chaussée was arrested on suspicion, which was greatly strengthened by the confusion he betrayed when informed of the discoveries made at the removal of the seals.