His benefactresses’ patience becoming exhausted, he left their house, and was apprenticed to a tinman at Chartres.  His master died, and an ironmonger of the same town took him as shop-boy, and from this he passed on to a druggist and grocer.  Until now, although fifteen years old, he had shown no preference for one trade more than another, but it was now necessary he should choose some profession, and his share in the family property amounted to the modest sum of three thousand five hundred livres.  His residence with this last master revealed a decided taste, but it was only another evil instinct developing itself:  the poisoner had scented poison, being always surrounded with drugs which were health-giving or hurtful, according to the use made of them.  Derues would probably have settled at Chartres, but repeated thefts obliged him to leave the town.  The profession of druggist and grocer being one which presented most chances of fortune, and being, moreover, adapted to his tastes, his family apprenticed him to a grocer in the rue Comtesse d’Artois, paying a specified premium for him.

Derues arrived in Paris in 1760.  It was a new horizon, where he was unknown; no suspicion attached to him, and he felt much at his ease.  Lost in the noise and the crowd of this immense receptacle for every vice, he had time to found on hypocrisy his reputation as an honest man.  When his apprenticeship expired, his master proposed to place him with his sister-in-law, who kept a similar establishment in the rue St. Victor, and who had been a widow for several years.  He recommended Derues as a young man whose zeal and intelligence might be useful in her business, being ignorant of various embezzlements committed by his late apprentice, who was always clever enough to cast suspicion on others.  But the negotiation nearly fell through, because, one day, Derues so far forgot his usual prudence and dissimulation as to allow himself to make the observation recorded above to his mistress.  She, horrified, ordered him to be silent, and threatened to ask her husband to dismiss him.  It required a double amount of hypocrisy to remove this unfavourable impression; but he spared no pains to obtain the confidence of the sister-in-law, who was much influenced in his favour.  Every day he inquired what could be done for her, every evening he took a basket-load of the goods she required from the rue Comtesse d’Artois; and it excited the pity of all beholders to see this weakly young man, panting and sweating under his heavy burden, refusing any reward, and labouring merely for the pleasure of obliging, and from natural kindness of heart!  The poor widow, whose spoils he was already coveting, was completely duped.  She rejected the advice of her brother-in-law, and only listened to the concert of praises sung by neighbours much edified by Derues’ conduct, and touched by the interest he appeared to show her.  Often he found occasion to speak of her, always with the liveliest expressions of boundless devotion.  These remarks were repeated to the good woman, and seemed all the more sincere to her as they appeared to have been made quite casually, and she never suspected they were carefully calculated and thought out long before.