The scientist who killed the poisoners

James Marsh clenched the small piece of paper with his fingers while grinding his teeth, avoiding shouting any expletives in front of the jury before him. By his haughty looks and that of the judge, he knew that they had completely rejected the results of his analysis and that they had been carried away by the jocular comments of the defense lawyer. The only different look was that of the accused of murder, John Bodle, who smiled slyly knowing that he had escaped the murder of his grandfather. In these dark conditions, the first reliable detection for arsenic was born.

The yellow that faded

Arsenic is one of the most popular poisons in history. It can be easily obtained by grinding a wide variety of minerals and foods, so it was relatively easy to get a small dose of homemade cleansing arsenic without anyone asking many questions. For that reason, it became a widely used poison from Roman times to the present day, so popular that in the nineteenth-century France it was nicknamed “dust to inherit.”

There are different arsenic compounds, capable of causing different effects on our health, but the best known and used as poison in modern times is arsenic trioxide. Since the nineteenth century, this compound was sold as slaughterhouses precisely for the same properties that made it a powerful weapon of crime: it has neither smell nor taste, and it is a fine white powder very easy to hide in any food and drink.

In 1832, all newspapers followed the trial of John Bodle, the grandson of an important and wealthy family. One morning, the Bodle was drinking coffee quietly when they began to feel very sick, feeling nauseous and paralyzed their limbs. Most of the relatives managed to survive but the family’s elder, Geogie Bodle, could not stand it and died. The police thought they could have been poisoned, and the main suspect was the only one who had not drunk coffee and who “coincidentally” was the beneficiary of multiple inheritance if everyone died: the grandson John Bodle.

To prove this theory, forensics extracted a sample of the victim’s stomach contents and took it to James Marsh, a Scottish chemist who worked for the army and combined the investigative tasks with some forensic evidence.

Marsh applied the usual procedure to detect arsenic: the Gutzeit method. The stomach contents were contacted with hydrogen gas, which would react with the possible arsenic to form a yellow compound that was deposited on a strip of paper. In the nineteenth-century trials it was important to convince the jury with more than numbers, so forensic chemists appeared on a rostrum with the yellow spot on the paper, explaining that such color would only be possible in the presence of arsenic.

When doing the test, Marsh obtained the characteristic yellow spot, so he prepared to testify and put the defendant behind bars. But there was a problem: the trial was late and was postponed one day. That delay could be significant, but it was critical for Marsh. The yellowish compound that demonstrated the presence of arsenic was unstable, and disappeared within twenty-four hours. Without more samples of the victim, the only thing he could teach the jury was a practically white paper. While the chemist swore and perjured the jury that there was a yellow spot, Boyde was released for lack of evidence.

Marsh was deeply affected by this fact, deciding to focus his research on finding a method that would detect arsenic in a more stable way and without a doubt. Over the next four years he tested several hundred chemical reactions with arsenic, looking for one that gave a more stable and colorful compound. Finally it came up with the solution: if arsenic was combined with zinc and sulfuric acid, arsenic vapor formed, a black smoke that easily stuck on any surface. The dark appearance of the smoke and the black-stained samples were obvious and were especially spectacular for the jury. In addition, it had the advantage of being able to indicate the concentration of arsenic in the sample, measuring the amount of vapor released and avoiding false positives.

In this way, Marsh published the method that ended up being baptized with his name, and began to be used in the forensic laboratories of some capitals. But it wasn’t until a year later, that a new and controversial case of poisoning brought him to fame.

Marie Lafarge’s orange blossom sugar

The trial of Marie Lafarge was especially followed by the press day by day, due to much resembling a mystery novel. Marie Lafarge was the bastard daughter of a noble French family. She was taken care of by both families, but the rejection of her nobles always led her to worry about faking an economic status she really didn’t have. Wishing to get rid of her, her father made a marriage of convenience with Charles Lafarge, a justice of the peace with whom he went to live in a renovated monastery in the palace.

The idea of ​​living in a palace attracted her at first to Marie, but seeing that her husband was of peasant origin and was always drowned in debt, she began to draw up a plan. On one of Charles’s work trips in Paris, he sent several love letters and a piece of Christmas cake. As soon as he ate it, Charles suffered nausea and dizziness which he attributed to the contamination of the cake during transport. The symptoms continued for a few more months, and doctors attributed it to cholera, a very common disease in France at that time. When he returned home, Marie dedicated herself to taking care of him and preparing her food, adding a very suspicious “orange blossom sugar” to her plates, the amount of which increased as Charles became more and more sick until she died.

Charles’s family watched Charles’s disease closely and they had been suspecting Marie for some time, to the point of taking samples of the food she gave him. When Charles finally died, they took her to trial accusing her of poisoning.

In the trial they showed that Marie had bought kills just the day before sending the cake and on her husband’s return, so the arsenic was the main suspect as a weapon of crime. She said the arsenic was for mouse traps set in the palace, but when checking the traps there was only a mixture of flour and water.

Seeing that the case was being followed by the newspapers, the judge wanted that there was no possibility of error when judging Marie, so he asked the police chief to test with the new detection technique that is triumphing in Paris , the Marsh method. Without knowing too much about the issue, the police chief agreed, asking two town apothecaries to try the method with the contents of Charles’s stomach. It was negative for arsenic.

With great uncertainty, the apothecaries tried to detect arsenic in the dishes and glasses of the house, without results. They were about to rule out the poisoning, when it was decided to try the food samples that the family had taken to the police for the case. Black smoke appeared and they found that they had enough arsenic to poison ten people.

With the mixture of results they had, they preferred to call an expert in the Marsh de Paris test, who found that the apothecaries were not using the correct concentrations of zinc and sulfuric acid. The last experiment was done in the court itself, with everyone’s view of the glass apparatus that began to release a small amount of arsenic vapor from the sample from the victim’s stomach. The definitive test to imprison Marie with life imprisonment.

The Marsh method played such an important role in the trial that from that moment it was taken as the reference test to detect arsenic. In addition, it caused a positive side effect: the decrease in cases of poisoning. No one wanted to risk killing coffee in the cafe if they could find it easily in the stomach. And if chemistry is used to kill, we can also use it to find the killer.


  • Currently we perform other types of much more sensitive tests such as atomic absorption spectroscopy, a technology that allows us to directly detect arsenic atoms. Thus, the chances of making a mistake like apothecaries are virtually nil.
  • In the current forensic analysis, the tests are performed several times to rule out that someone can be charged because of some anomalous result.



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