Crime Library: Criminal Minds and Methods

Forensic Toxicology

First Scientific Case

One of the most important discoveries was that of the "arsenic mirror" as a method for detecting the poison.   In 1787, Johann Daniel Metzger was the initial inventor, at least for the method of detecting arsenic in solutions if not yet in the human body.  He discovered that when arsenious oxide was heated with charcoal, it formed a black mirror-like deposit on a cold plate held over the coals.  That substance was arsenic.

In 1806, Valentine Rose took this further by showing how arsenic could be detected in human organs.   He used nitric acid, potassium carbonate, and lime, evaporating that mixture into powder form and treating it with coals to get the mirror substance. 

Around the same time, in 1813, when he was only 26, Mathieu Joseph Bonaventure Orfila, now known as the "Father of Toxicology," published a book, a Treatise of General Toxicology.   In it, he summed up everything known about poisons at the time and offered classifications.  He had tried to demonstrate the various tests for poison detection and had found them to be highly unreliable.  Assuming that toxicology was not yet a real science but could become one, he refined Rose's method to achieve greater testing accuracy.  It was Orfila who showed with tests on animals that after ingestion, arsenic gets distributed throughout the body. His fame won him a prominent position at Paris University, where he started to consult on criminal cases.  He was further assisted by the work of the next prominent scientist.

In the 1830s, James Marsh tested the coffee of a supposed victim of poisoning, but was unable to explain to a jury how he had found the arsenic, so he decided to improve his methods to make them more demonstrative.   In a closed bottle, he would treat suspected poisoned material with sulphuric acid and zinc.  From this bottle emerged a narrow U-shaped glass tube, with one end tapered, through which arsine gas emerged to hit zinc and escape.  The escaping gas could be ignited, and it then formed the expected black mirror substance.  His method, known as the Marsh Test, was sufficiently precise to test very small amounts of arsenic—and to help a jury better comprehend it.

Orfila used the Marsh Test to analyze the soils of cemeteries for the presence of arsenic, so that exhumed bodies that had absorbed it from the soil would not help to falsely convict anyone.

Bodies of Evidence
Bodies of Evidence

Jurgen writes that the case that brought the "science of poisons" into public view and established the use of science in the courtroom was the prosecution in France in 1840 of Marie LaFarge, 24, for the murder of her husband, Charles LaFarge.  Brian Innes provides details in Bodies of Evidence.

Marie Lafarge, notably unhappy with her arranged marriage to the owner of a rat-infested forge, was accused of using arsenic as a murder weapon.   She had bought a relatively large amount during the months preceding the death, allegedly to exterminate the rats, and her husband had become violently ill before he died in the manner consistent with arsenic poisoning.  Servants claimed Marie had stirred white powder into his food.  A local pharmacist tested the food and found arsenic.  The circumstances were clearly against her. 

However, even the prosecution experts could not determine with the Marsh Test that the contents of Lafarge's stomach contained arsenic, so they requested that the body be exhumed to test the organ tissues.   That was done, and still the tests were negative.

Yet food from the Lafarge household did test positive for arsenic.

The experts were stumped.

Mathieu Orfila
Mathieu Orfila

Enter the renowned Mathieu Orfila.   He examined the materials the experts had used for arsenic testing and then went alone to an office and re-performed the same test, proving that it was not the method that was at fault but its practitioners.  They had bungled it.  Orfila was able to detect the presence of arsenic in Lafarge's body, and to prove that it had not originated in the soil surrounding the coffin.  Based on his results, Marie was declared guilty and sentenced to death.  Her sentence was later commuted to life.

Doctors and pathologists came to be recognized as the first forensic scientists, and the first procedures for proving poisoning in court were formalized. As yet, they could not measure the actual amount in the body, so they had to rely on building a convincing circumstantial case as well.

In 1851, writes Innes, Britain passed the Arsenic Act, which restricted the sale of arsenic-laced rat poison to persons over 21 who were also known to the seller and who signed a register.

That did not stop people who were determined to kill.