The Strange Story of Napoleon's Wallpaper - Page 2
Since the Middle Ages, arsenic had been a popular way of poisoning people. Arsenic itself is a grey metal, which is not very poisonous. It generally occurs in nature as a mineral, arsenic sulphide (arsenikon is the latin word for yellow pigment), but another form is white arsenic, or arsenic oxide. This is extremely poisonous, although the symptoms of arsenic poisoning could be confused with those of many other illnesses, and it was also very difficult to detect arsenic after the death. All in all, a handy way to get rid of someone.
Politicians, popes and parents were all victims at different times. Indeed white arsenic became known as 'inheritance powder'. Arsenic poisoning happens because arsenic binds very strongly to sulphur groups. Many proteins contain one or more sulphur atoms, and enzymes (also proteins) are responsible for regulating the body's internal chemistry. An enzyme with an arsenic atom bound to the sulphur will not be able to function. Hair is interesting because the protein keratin contains sulphur atoms. If you ingest arsenic, then some of it will bind to the sulphur atoms in your hair, and a sample of hair will last a long time. Since hair is constantly growing, it can even show how the level of arsenic in someone's body can change over time. There was a poisoning case in the 1950s when a Sergeant Marymont murdered his wife. Modern analytical techniques found bands of arsenic in her hair, which corresponded to the times that he had been home on leave. He had increased the dose each time he was home, until finally he killed her. But these advances in forensic chemistry are relatively recent.
However in 1832, 11 years after Napoleon's death, there was an interesting and significant development for the followers of arsenic poisoning. A certain John Bodle was brought to trial in the UK, accused of poisoning his grandfather by putting arsenic in his coffee. James Marsh was the chemist called by the prosecution. He carried out the standard test of the time, which was to pass hydrogen sulphide gas (the one with the 'bad egg' smell) through the suspect solution. If arsenic was present, it would be shown by the appearance of a yellow precipitate of arsenic sulphide. Unfortunately, although James Marsh did find arsenic, the yellow precipitate doesn't keep very well, and by the time it was shown to the jury it had deteriorated. The jury were not convinced, and John Bodle was acquitted.
James Marsh was somewhat angry, especially when John Bodle later confessed that he had poisoned his grandfather after all. And so James Marsh set out to devise a better test. The result was the 'Marsh Test', which worked by adding zinc metal and sulphuric acid to the sample suspected of containing arsenic. If arsenic is present, the resulting chemical reaction converts the arsenic into arsine (each arsenic atom has three hydrogen atoms attached) which is a (very poisonous) gas. This gas is then passed along a heated glass tube, which causes the arsine gas to decompose, and the arsenic metal is deposited as a silvery-black film on the sides of the tube. The tube can then be sealed, and kept as evidence. Not only could minute amounts of arsenic be detected, but the test was very specific, since virtually no other element behaves in this way. With a chemical test this good, the age of the arsenic poisoners was drawing to a close! The only problem with the chemical test is that your arsenic-containing sample is used up in the course of the test - in that sense it is a destructive test. And the Marsh Test is not sensitive enough to be able to detect arsenic in hair samples. In the 20th Century, techniques such as X-ray fluorescence spectroscopy replaced the Marsh Test. These non-destructive tests are incredibly sensitive, and can detect really tiny quantities of elements. James Marsh would have been amazed!
Of course Napoleon had died some years before the Marsh Test was devised, and in any case no-one had suspected arsenic poisoning at the time.