Marsh test
The Marsh test is a historical technique developed in 1836 for detecting arsenic, a substance often associated with undetectable poisoning in murder cases. Invented by chemist James Marsh, this test significantly contributed to forensic science and the legal system by providing a more reliable means of identifying arsenic in food, body tissues, and other substances. Prior to its invention, existing methods often failed to convince jurors due to limitations in visibility and specificity. The Marsh test involved heating a sample with strong acid and zinc, which converted arsenic into arsine gas, allowing for the collection of a metallic arsenic mirror that could be presented as evidence in court. The test gained notoriety during notable trials, including that of Marie Lafarge, leading to increased convictions for arsenic poisoning and a decline in such crimes. Although now largely replaced by modern techniques like molecular-absorption spectrophotometry, the Marsh test played a crucial role in establishing the importance of scientific evidence in legal proceedings and highlighted the historical challenges of detecting poison in criminal cases. The evolution of arsenic detection methods reflects broader advancements in forensic science and the ongoing quest for justice in the face of deceptive practices.
On this Page
Subject Terms
Marsh test
DEFINITION: Arsenic detection technique invented in 1836.
SIGNIFICANCE: James Marsh’s highly sensitive arsenic test revealed murderers and helped to establish the value of the testimony of scientific experts in criminal trials.
For centuries killers used arsenic, in large part because it was undetectable. Many murderers went unpunished while in some cases innocent persons were instead convicted of poisoning based on and confessions extracted by torture.
By the early nineteenth century, scientists had devised simple tests for the presence of arsenic in other substances. Chemists performed one standard method by bubbling hydrogen sulfide gas through a solution of the test material. The formation of a yellow precipitate—arsenic sulfide—revealed the presence of arsenic. The test did not always persuade jurors, however.
In 1832, an eighty-year-old English farmer named George Bodle drank his morning coffee and soon experienced severe stomach cramps. He suffered symptoms characteristic of poisoning and then died. When a justice of the peace investigated the death, he discovered that John Bodle, George’s grandson, had bought arsenic from a local pharmacist. John was arrested and tried for murder.
Using a standard arsenic test, chemist James Marsh analyzed the coffee that George Bodle had drunk as well as Bodle’s organs. A yellow precipitate indicated the presence of arsenic. By the time Marsh displayed the in court, however, the precipitate had become discolored, and an unconvinced reached a of not guilty.
Disappointed by these results, Marsh invented a new arsenic test, one that revealed arsenic itself rather than a chemical reaction by-product. Marsh performed his technique by heating a sample of food or human tissue with strong acid to destroy organic matter and dissolve any arsenic. Next, he added pieces of metallic zinc, which converted dissolved arsenic to arsine, a gas. When arsine gas passed through a heated glass tube, it decomposed into hydrogen gas and metallic arsenic. Marsh collected metallic arsenic on the cooler part of the tube, where it formed a black, shiny deposit. A tube that contained such an “arsenic mirror” could be sealed and stored.
In 1840, the Marsh test became the first analytical method of introduced during a criminal trial. Marie Lafarge stood accused of killing her husband, Charles. Her weapon, according to the prosecution, had been cake, milk, and eggnog brimming with arsenic. Experts, including the great forensic toxicologist Matthieu-Joseph-Bonaventure Orfila, used the Marsh test to analyze food evidence and samples of tissues from Charles’s body. They found arsenic, and Marie was convicted and sentenced to life imprisonment.
When Marsh’s technique was performed with care, it could detect as little as one-fiftieth of a milligram of arsenic, and it could identify any trace amounts of arsenic in the chemicals used to perform the test. Although it was not perfect, the Marsh test proved adequate to expose a number of murderers in famous poison trials of the Victorian era. Furthermore, the test improved the chances of being caught and convicted and the number of arsenic poisonings decreased significantly.
The Marsh test is no longer the primary means of detecting arsenic. One of the most reliable tests in modern times is the molecular-absorption spectrophotometry in aqueous solution.
Bibliography
Bradbury, Neil. A Taste for Poison: Eleven Deadly Molecules and the Killers Who Used Them. St. Martin’s Press, 2022.
El-Ghiaty, Mahmoud A., and Ayman O. S. El-Kadi. "The Duality of Arsenic Metabolism: Impact on Human Health." Annual Review of Pharmacology and Toxicology, vol. 63, 2023, doi.org/10.1146/annurev-pharmtox-051921-020936. Accessed 16 Aug. 2024.
Emsley, John. The Elements of Murder: A History of Poison. New York: Oxford University Press, 2005.
Gerber, Samuel M., and Richard Saferstein, eds. More Chemistry and Crime: From Marsh Arsenic Test to DNA Profile. Washington, D.C.: American Chemical Society, 1997.
"Marsh Test." ChemEurope.com, www.chemeurope.com/en/encyclopedia/Marsh‗test.html. Accessed 16 Aug. 2024.
Wagner, E. J. The Science of Sherlock Holmes: From Baskerville Hall to the Valley of Fear, the Real Forensics Behind the Great Detective’s Greatest Cases. Hoboken, N.J.: John Wiley & Sons, 2006.