In early 19th century Europe, arsenic was the poisoner’s weapon of choice. It was cheap, sold to kill rats, impossible to taste or smell, and its symptoms looked like cholera or food poisoning. Investigators could do little more than feed leftovers to a dog or inspect a stomach lining. Mathieu Orfila changed that. Born in 1787 in Mahon on the island of Menorca, he studied medicine in Valencia, Barcelona, and Paris, published his Treatise on Poisons in 1813, and by 1816 was royal physician to King Louis XVIII. He became a French citizen in 1819.

This episode explains the idea that made him the founder of forensic toxicology: poisons do not vanish after digestion but are absorbed and collect in particular organs, where they can be recovered. Orfila proved it by extracting arsenic from the liver, spleen, heart, and muscle, and he adapted the Marsh test of 1836 so it could handle human tissue. The story also covers the hundreds of dogs his research consumed, which troubled even his contemporaries, and the 1840 murder trial in which he placed visible arsenic before a jury.

• The Marsh test mixes a sample with zinc and sulfuric acid to form arsine gas, which leaves a silvery black mirror on heated glass when arsenic is present.
• Orfila destroyed organic matter first with nitric acid and potassium nitrate and scaled up the apparatus so that organs and blood would not clog the reaction.
• In the Marie Lafarge case, local doctors found no poison in her husband Charles until Orfila reran the analysis and found a lethal amount in the tissues.
• He served as dean of the Faculty of Medicine in Paris for 18 years from 1830 and was removed in the upheaval of 1848.
• Elected president of the Academy of Medicine in 1851, he held the post until his death on March 12, 1853, at 65.