Fans of golden-age detective fiction may well be familiar with the Marsh test - the method used to detect the presence of arsenic. Its inventor, the chemist James Marsh, developed it while working at the Royal Arsenal, Woolwich.
There were already tests for arsenic available, and Marsh was familiar with these. Indeed, he appeared in court as a prosecution witness on poisons - and it was that experience which led him to develop his own, more sensitive test. In 1833 he had used the test developed by Samuel Hahnemann, which produced a yellow precipitate when arsenic was present, to establish that there was arsenic in the coffee one John Bodle of Plumstead had given his grandfather. However, by the time the case came to trial, the yellow precipitate had degraded; the jury was unconvinced; and Bodle escaped conviction. Although it now seems to be accepted that lack of forensic evidence prompted the acquittal, a contemporary report in the Spectator suggests that the real issue was whether John or his father, the victim's son, was responsible for the death. Whichever version is correct, it was around that time that Marsh began his work of refining the test for arsenic.
Marsh was highly successful, developing a new test in 1836 (less than three years after the Bodle trial) which could not only measure the quantity of arsenic present, but also detect tiny amounts. It made him famous, but by no means represented his only achievement. As Ordnance Chemist at Woolwich Arsenal and assistant to Michael Faraday, he invented electromagnetic equipment as well as developing a screw time fuze for mortar shells. Nonetheless, it is the Marsh test which has kept his name alive over the intervening centuries.
Arguably, that fame is well-deserved. The test soon came to public attention - not in a British trial, but in the French case of Marie Lafarge who was convicted of poisoning her husband. Awareness that the poison (then readily available for purchase) was now detectable may have turned other would-be poisoners from temptation, and saved lives.
As for the Bodle murder, John Bodle would later confess that he, not his father, was guilty. Having already been acquitted of the crime, he could not be tried again.