Until the 19th century it was relatively easy to get away with murder. Arsenic was the perfect weapon because it’s practically tasteless and odorless, symptoms are often mistaken for natural diseases, and best of all there were no tests to detect it. Chemists saw the need to identify this poison in samples and experimented with methods to test for arsenic. A reliable technique was developed by James Marsh in the early 19th century and became known as the Marsh Test. During the dramatic murder trial of Marie Lafarge in 1840, the Marsh Test was thrust into the spotlight and validated the use of chemical tests as evidence for the courtroom.

Arsenic is a chemical element that occurs naturally in the environment in food, water, air and soil. Arsenic trioxide, also known as white arsenic, is a poison that people used to eliminate political rivals, spouses, and to collect inheritances for hundreds of years.6 It became known as the “king of poisons” and the “poison of kings,” as well as “inheritance powder.” Arsenic was famously used by the Medici and Borgia families to kill their enemies, and may have contributed to the death of Napoleon Bonaparte.6

Arsenic was readily available to anyone for hundreds of years in apothecaries because it was a common ingredient in cosmetics, medicinal tonics, and pest control. The poison was virtually odorless and tasteless when mixed with food and beverages so murderers could slowly poison their victim to make it look like a natural death.6 The symptoms of arsenic poisoning include abdominal pain, nausea and vomiting, diarrhea, respiratory problems, and joint pain. So unless someone got caught red-handed sneaking a suspicious white powder into food and drinks, arsenic poisoning was usually misdiagnosed as diseases like cholera, influenza, or pneumonia.3,4

Marsh’s Big Break

The first step toward a valid and reliable test to identify arsenic came at the end of the 18th century. A Swedish chemist named Carl Wilhelm Scheele (1742-1786) developed a test that could detect the presence of arsenic in 1775. Scheele’s method required arsenic powder to be mixed with metallic zinc and nitric acid. When heated, this solution formed arsine gas that smelled like garlic.2 But, a positive result obtained from Scheele’s test was not objective or reliable and the results were difficult to present in a courtroom. In another early technique chemists dissolved a suspected sample of arsenic in hydrochloric acid and “bubbled” hydrogen sulfide gas through the mixture. Any arsenic present would form King’s yellow.2 This test was problematic because the results were not stable enough to present to a jury, something British chemist James Marsh would learn the hard way.

In 1832, James Marsh (1794-1846) was called as an expert witness in the murder trial of John Bodle, who was accused of poisoning his grandfather with arsenic. The prosecution wanted Marsh to see if he could discover arsenic in the stomach contents from the grandfather’s corpse. When Marsh used the hydrogen sulfide test on the sample he got King’s yellow, which is a positive result.2 By the time he presented his results to the jury the substance had degraded and as a result the jury acquitted Bodle. Marsh was so frustrated by the verdict in the Bodle trial that he decided to develop a reliable test that could test for arsenic in tissue samples and stomach contents.2  It took four years but he was successful.

Marsh’s breakthrough came in 1836 when he essentially improved on Scheele’s test. For the Marsh Test, as it came to be known, hydrochloric acid was added to a tissue sample or stomach contents. Shavings of metallic zinc were added to the mixture and then heated.2 When arsine gas formed it was trapped and funneled through a tube, where it was heated again. The arsine gas decomposed into a gray metallic film that became known as the “arsenic mirror.”2

This was a big deal because the Marsh Test could reveal arsenic in samples taken from a dead body, and, unlike smelly arsine gas or King’s yellow, the “arsenic mirror” film was stable enough that it could be used as forensic evidence. Despite these advances, the Marsh Test remained an obscure test that wasn’t used that often. It wasn’t until the sensational murder trial of Marie Lafarge in 1840 that the Marsh Test got the publicity it deserved.

The Black Widow

Marie Lafarge (1816-1852), née Marie-Fortunée Capelle, was born in to a middle class family and was distantly related to French aristocracy. She was orphaned as a teenager and went to live with an aunt and uncle. When she was 23 years old they hired a matchmaker in the hopes of marrying her off. One of her suitors was Charles Lafarge, a foundry owner who claimed to own a beautiful estate in the Beyssac region of France. Marie was reluctant to marry Charles because he was rough and ill-mannered. She went ahead with the marriage anyway in 1839 because she was impressed by his grand estate. But it wasn’t long before she realized she was the victim of a bait and switch. She found out that Charles’s beautiful chateau was actually rat-infested and dilapidated, and his foundry business was bankrupt and Charles needed her dowry to keep it afloat.2 Marie was miserable from the beginning and wanted to end her marriage.

In December of 1839, Charles became sick while he was traveling after he ate some cake that Marie sent him. When he returned home Madame Lafarge made sure to prepare his food and drinks like a good devoted wife. Charles’s condition quickly deteriorated and he died in January of 1840.2

Charles’s family and servants were suspicious of Marie because she asked for arsenic to kill the rats in the chateau and she always prepared his food and drinks. Marie was eventually arrested and put on trial. The French newspapers covered the case exhaustively, and the public was split on their perception of Marie. Half of France thought Madame Lafarge was an innocent young bride who was the victim of circumstance, and the other half thought she was a conniving black widow out for Charles’s money.

The prosecution asked local doctors to perform the Marsh Test on samples taken from Charles’s body. But these tests were inconclusive and lawyers on both sides questioned the validity of the results and the competency of the doctors that performed the tests. So the judge decided to consult the premiere authority of toxicology and advocate of forensic medicine Mathieu Orfila.2

The Father of Toxicology

Mathieu Orfila (1787-1853) published Traité des poisons tirés des règnes minéral, végétal et animal; ou, Toxicologie générale in 1814, which was one of the earliest studies of the chemical nature of poisons and their effects on the body.5 In 1830, he became the dean of the Medical Faculty of Medicine of the University of Paris. Throughout his career Orfila worked to make chemical analysis part of forensic medicine as well as studying decomposition. He was a renowned expert witness and is recognized as the “father of toxicology.”1

Orfila agreed to be an expert witness and to test Charles’s body for arsenic. He had Charles’s corpse exhumed and collected samples from the body and soil from the grave to make sure that any arsenic he found did not come from the environment. When Orfila performed the Marsh Test the telltale metallic arsenic “mirror” formed on the gas container, a positive result for the presence of arsenic. This was the first high-profile use of the Marsh Test in the courtroom. After Orfila presented the results to the jury, Marie was found guilty and sentenced to death. She maintained her innocence and her sentence was later commuted to life.1

The Lafarge Trial marked the beginning of the use of chemical testing in murder trials and the end of undetectable poisons. The media circus around the Lafarge trial became a platform to demonstrate the reliability of the Marsh Test and the effectiveness of toxicology. It was used to test for arsenic well into the 20th century, until technology advanced enough for more sophisticated methods. Today toxicologists use sophisticated methods to discover arsenic in tissue samples such as high performance liquid chromatography and inductively coupled plasma mass spectrometry.




2.Crime and circumstance: Investigating the history of forensic science



5.Introduction to forensic sciences, second edition

6.Arsenic exposure and toxicology: A historical perspective


Dolly Stolze is a researcher and editor at Strange Remains, a blog site that specializes in forensic science, bioarchaeology and bizarre history. She has a master’s degree in forensic anthropology from California State University, Los Angeles.