"This is not, in itself, a provable fact of intentional murder," said Mr Herdman, who is writing a book about Cotton, based on official documents. The earth surrounding corpses contains elements of natural arsenic, which over time can be absorbed into the organs and flesh of a decaying corpse. "Many accounts of her life are hearsay. I have uncovered episodes of completely inappropriate statements, evidence and official inaccuracies." Death was commonplace in Cotton's time. Diseases easily cured now could wipe out whole families. The death certificates for Cotton's supposed victims recorded nothing suspicious. Indeed, the child she was convicted of murdering had been ill before his death. "He was under the care of two doctors," said Mr Herdman. "He was suffering from purges and the chronic diarrhoea often association with TB. "Medicine had been prescribed and, after her arrest, a search of Cotton's house revealed numerous empty medicine bottles prescribed by the doctors. The fact that they were empty, and not thrown away, suggests they were administered by Cotton as directed by the physicians."
She was arrested in July 1872 for the murder of the seven-year-old youngster, but maintained her innocence until the day she was hanged. Part of the prosecution's case was that Cotton possessed arsenic in a soap mixture designed to kill bed-bugs and lice and had the knowledge to use it. But while Cotton admitted to scrubbing her bed and walls with the soap, it was a common practice. All her neighbours admitted to using the soap too. "A search of Cotton's house produced no evidence she had any arsenic in her possession," said Mr Herdman. "No traces were found in her kitchen utensils either. To overcome this dilemma, the prosecution had to present hard evidence that Cotton was seen to actually purchase the arsenic and find a witness to strengthen their claim."The prosecution turned to assistant chemist Thomas Detchon, who claimed Cotton had bought three-penny-worth of soft soap and arsenic from him four years before. Cotton had given her name as Mary Ann Booth, he told the court, and the sale of the arsenic was witnessed by a Newcastle woman called Elizabeth Robson. "At the time Detchon supposedly sold the arsenic to Cotton, she was living in Pallion, Sunderland, many miles away from his chemist in Newcastle," said Mr Herdman. "She was also five months' pregnant, yet he made no statement saying the woman was pregnant. It is possible, though, that her clothing concealed it. A more damning aspect is that, after Cotton's committal trial, Elizabeth Robson stated to the authorities in Newcastle that Mary Ann Booth was not Mary Ann Cotton. "But Detchon's statement was never questioned thoroughly, and his word alone was the sole pillar of the prosecution's evidence that Cotton was seen to purchase arsenic." Another prosecution argument involved the quantity of arsenic found in her "victims" doses, they claimed, which were administered with drinks or prescribed medicine.