A widespread legend makes Sainte-Croix' death the result of a chemical experiment; it is said that the glass mask with which he covered his face to protect it from the poisonous vapours had broken. But he really died a natural death after an illness of some months, in the course of which he was visited by several persons who have left their testimony in regard to the matter. In the legendary laboratory of the cul-de-sac there was found indeed a furnace of 'digestion.' Sainte-Croix 'philosophised' there, that is, worked at the philosopher's stone, and more particularly at solidifying mercury, that eternal dream of the alchemists.
Madame de Brinvilliers soon learned of the death of her lover. Her first cry was, 'The little box!'
Sainte-Croix died overwhelmed in debt. His things were all put under seal. The seals were raised on August 8, 1672, by Commissary Picard, assisted by a sergeant named Creuillebois, two notaries, the agent of the widow, and an agent of the creditors. The three first meetings had passed without incident when a Carmelite monk who was present handed to the commissary the key of the private room in which the furnace was kept. Entering, they saw on the table a rolled-up paper bearing the words, 'My confession.' The persons present decided without hesitation to keep the paper secret, and to burn it on the spot. They found, further, at the end of a shelf, a small box, oblong in shape and red in colour, from which hung a key. It contained some phials, some of which were filled with a clear liquid like water, others with a liquid of reddish colour; and in addition, there were the letters addressed by Madame de Brinvilliers to Sainte-Croix, the two promissory notes signed by the marchioness after the poisoning of her father and brothers, and a receipt and power of attorney relating to a sum of 10,000 livres lent by Pennautier, receiver-general of the clergy, to Monsieur and Madame de Brinvilliers through the agency of Sainte-Croix. These two last papers were in a sealed envelope on which was written: 'Papers to be restored to the Sieur Pennautier, receiver-general of the clergy, as belonging to him; and I humbly beg those into whose hands they fall, to be good enough to return them to him at my death, they being of no consequence except to him alone.'