Did You Know? ~ The Marsh Test for Arsenic

In the 19th century, arsenic was nicknamed the “inheritance powder,” due to its prevalent use in murdering inconvenient relatives. When used skillfully, arsenic induced cholera-like symptoms in victims. Deaths were often attributed to natural causes, but even if poisoning was suspected, it wasn’t always possible to identify arsenic. That changed after the murder trial of John Bodle.

James Marsh

In 1832, James Marsh, a skilled scientist, was called as a chemist by the prosecution in Bodle's murder trial. Bodle was accused of poisoning his grandfather with arsenic-laced coffee. Marsh was given a sample to test in his lab. He performed the standard test for arsenic at the time, mixing the suspect sample with hydrogen sulfide and hydrocholoric acid. In the presence of arsenic, the chemicals form yellow arsenic trisulfide, and that’s what happened to the sample he was given in this case. However, in the trip from the lab to court, the yellow arsenic trisulfide deteriorated, and could no longer be used as proof of the presence of arsenic. John Bodle was acquitted due to reasonable doubt.

Marsh knew John Bodle was guilty, and he was frustrated by his failure to prove that to the jury. As a result, he developed a test so effective, it directly contributed to a drop in the frequency of arsenic poisonings.

His test involved reacting a case sample with zinc metal and sulphuric acid. This produced arsine (a very poisonous gas). The gas passed along a heated glass tube, which caused the arsine gas to decompose. The resulting arsenic metal was deposited as a silvery-black film on the sides of the tube. The tube could then be sealed and kept as evidence.

The concentration level of arsenic in a sample was determined by comparing the Marsh test results with photographs of test results from samples of known arsenic concentration. The test was so sensitive it could detect arsenic for as little as one-fiftieth of a milligram. James Marsh first described this test in The Edinburgh Philosophical Journal in 1836.

James Marsh was correct about John Bodle’s guilt. A decade later, Bodle confessed. He'd gotten away with murder.