In a very short time Madame Lafarge discovered that both she and her relatives had been deceived as to the position of her husband, and that instead of being a man of considerable fortune, he was straitened for means. On his representations she bestowed upon him all her fortune, and even wrote letters at his dictation to some of her wealthy friends, asking them to aid him to find money to develop a new method he claimed to have discovered for smelting iron. With these letters of introduction, Lafarge set out for Paris in December, 1839, to raise money to start his new project.

While he was thus away, his wife had her portrait drawn by an artist in Glandier, and determined to send it to her absent husband. She therefore packed it in a box, with some cakes made by his mother, together with an affectionate letter, and despatched them to Paris. This box, which contained nothing but the five small cakes, the portrait, and the letter, was packed and sealed by Madame Lafarge in the presence of several witnesses.

When it reached Paris and was opened by Lafarge, it contained only one large cake, after partaking of which he was suddenly taken ill, and was eventually compelled to return home, where he arrived on January 5, 1840. His sickness continued and increased in severity, and nine days afterwards he died.

Shortly after his death his mother and friends, who were well aware how the widow disliked them and her husband also, who had made her life so unhappy, at once imputed the cause of death to poison administered by his wife in the cake she had sent to Paris, and Marie Cappelle Lafarge was arrested on suspicion.

When the house of the deceased man was searched, certain diamonds were found, which were supposed to have been stolen from the Vicomtesse de Léotaud by Madame Lafarge before her marriage.

The unfortunate woman was therefore charged with the double crime of theft and murder.

Though arrested in January, 1840, the trial of Madame Lafarge did not commence till July 9 of the same year, and the charge of theft was first proceeded with in her absence, and she was found guilty.

While this judgment was still under appeal, she was brought to trial on the graver charge.

The evidence for the prosecution went to prove that the illness of Lafarge commenced with the eating of the cake received from his home. As already stated, when the box arrived in Paris the seals had been broken, the five cakes had disappeared, and a single cake "as large as a plate" had been substituted for them. It was alleged by the prosecution that this single cake had been prepared by Madame Lafarge, and secretly placed in the box; but no evidence could be brought to prove that she ever tampered with the box after it had been sealed. Lafarge's clerk, Denis Barbier, made a clandestine visit to Paris after the box had been despatched, and he was with Lafarge when it arrived in Paris, yet no notice seems to have been taken of this suspicious fact. It transpired, it was he who also first threw out hints on his master's return that he was being poisoned by arsenic, and told a brother employé that his master would be dead within ten days. There was ample proof, however, that there was a considerable quantity of arsenic in the house at Glandier. It was found that Madame Lafarge had purchased some in December, stating she required it for destroying rats; Denis also stated in evidence, that Madame had requested him to procure her some arsenic. He bought some, but did not give it to her. It was further stated that Madame Lafarge was seen to stir a white powder into some chicken broth which had been prepared for her husband, the remains of which, found in a bowl, were said by the analyst to contain arsenic.