The repugnance felt by Sainte-Croix for his fellow-prisoner did not last long, and the clever master found his pupil apt.  Sainte-Croix, a strange mixture of qualities good and evil, had reached the supreme crisis of his life, when the powers of darkness or of light were to prevail.  Maybe, if he had met some angelic soul at this point, he would have been led to God; he encountered a demon, who conducted him to Satan.

Exili was no vulgar poisoner:  he was a great artist in poisons, comparable with the Medici or the Borgias.  For him murder was a fine art, and he had reduced it to fixed and rigid rules:  he had arrived at a point when he was guided not by his personal interest but by a taste for experiment.  God has reserved the act of creation for Himself, but has suffered destruction to be within the scope of man:  man therefore supposes that in destroying life he is God’s equal.  Such was the nature of Exili’s pride:  he was the dark, pale alchemist of death:  others might seek the mighty secret of life, but he had found the secret of destruction.

For a time Sainte-Croix hesitated:  at last he yielded to the taunts of his companion, who accused Frenchmen of showing too much honour in their crimes, of allowing themselves to be involved in the ruin of their enemies, whereas they might easily survive them and triumph over their destruction.  In opposition to this French gallantry, which often involves the murderer in a death more cruel than that he has given, he pointed to the Florentine traitor with his amiable smile and his deadly poison.  He indicated certain powders and potions, some of them of dull action, wearing out the victim so slowly that he dies after long suffering; others violent and so quick, that they kill like a flash of lightning, leaving not even time for a single cry.  Little by little Sainte-Croix became interested in the ghastly science that puts the lives of all men in the hand of one.  He joined in Exili’s experiments; then he grew clever enough to make them for himself; and when, at the year’s end, he left the Bastille, the pupil was almost as accomplished as his master.

Sainte-Croix returned into that society which had banished him, fortified by a fatal secret by whose aid he could repay all the evil he had received.  Soon afterwards Exili was set free—­how it happened is not known—­and sought out Sainte-Croix, who let him a room in the name of his steward, Martin de Breuille, a room situated in the blind, alley off the Place Maubert, owned by a woman called Brunet.

It is not known whether Sainte-Croix had an opportunity of seeing the Marquise de Brinvilliers during his sojourn in the Bastille, but it is certain that as soon as he was a free man the lovers were more attached than ever.  They had learned by experience, however, of what they had to fear; so they resolved that they would at once make trial of Sainte-Croix’s newly acquired knowledge, and M. d’Aubray was selected