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.