James Marsh is an ordinary chemist, just keeping his head down and doing his job. But when he sees a murderer go free, he has to do something crazy. Something like . . . chemistry.
It's possible that John Bodle committed the world's most useful murder. He put arsenic in his grandfather's coffee. He wasn't alone. In 1832, when the murder was committed, arsenic had acquired the nickname "inheritance powder." Arsenic poisoning was not a great way to go. If victims were lucky, they got a headache, got drowsy, and lapsed into a coma. Less lucky victims got vomiting and diarrhea followed by convulsions.
Bodle's case hinged on the testimony of James Marsh, a chemist, who had tested the coffee for arsenic. Unfortunately, the existing test was not particularly sensitive. It involved exposing the suspicious substance (usually a liquid) to hydrogen sulfide. Within a few minutes, a yellow powder would precipitate out of the liquid. This only worked for a few hours. Eventually, the precipitate would disappear, and the liquid would turn clear. A clear bottle did not tend to convince juries. It certainly didn't convince Bodle's jury.
Marsh was angry when Bodle went free. He was even angrier when Bodle outright confessed to the crime once he was out of danger. He was so angry that he spent the next few years developing a test for arsenic that was both sensitive, and would leave him something to show juries.
Marsh's test was just a little more complicated than the existing one, but got an entirely different result. He added zinc and sulfuric acid to the mix. He knew that the zinc would strip away any companions the arsenic might have found, and leave with some extra electrons on its hands. He also knew that, when exposed to acid, this electron-rich arsenic would turn to gas. When gas leaked from the solution, he heated it. Arsenic is a metalloid. Heating the gas and letting that heated gas flow over a glass surface leaves a shiny metal gleam on the glass that doesn't go away.
In 1840, Marsh took his new test to a new trial. Marie Lafarge was accused of murdering her husband with arsenic. The Marsh test showed the jury tangible evidence, and Lafarge was convicted. Unlike Bodle, she never confessed, which gives us a rather bitter ending to the story. Although the Marsh test was so reliable that it was used into the 1970s, many people still believed that Lafarge was innocent, and Marsh was a villain.