A nineteenth-century chemist developed a test that could detect a fiftieth of a milligram of arsenic

The 'Marsh test' revolutionized forensic toxicology after a failed 1832 prosecution where a suspected poisoner was acquitted because the evidence had deteriorated before the jury could see it. James Marsh’s breakthrough involved mixing suspected samples with zinc and sulfuric acid to produce arsine gas; when ignited, the gas left a distinct silvery-black metallic deposit on a cold porcelain plate.