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On January 9, 1836, a cold and foggy day, Lacenaire and Avril were led to the guillotine at the Saint-Jacques barrier on the south side of Paris. Though the authorities had tried to keep the time and place of execution secret and had hastily erected the guillotine at night, a crowd of five hundred people appeared — as happened at almost every execution.

Avril went first, and the execution proceeded without incident. Then Lacenaire looked at the executioner and said: “Nothing simpler. I am not afraid.” 14 But when he placed his head on the block, the blade made it only halfway down before becoming stuck in one of the grooved sides. The executioner hauled it up again, and as he did, Lacenaire looked up at the triangular blade. It was the last thing he saw.

Lacenaire had one last contribution to offer. On the eve of his death, he had spoken to a Dr. Lelut of the Bicêtre Prison. One of the scientific questions of the time was whether consciousness continued after the head was severed from the body. Did decapitation instantly end consciousness? This was just the sort of thing that interested Lacenaire, and he had promised that he would give a signal by winking — specifically, closing his left eye and leaving the right one open. Dr. Lelut stood over the basket that caught the severed head of the criminal, but could see no movement of the eyelids at all.

ii

People have used poisons to dispatch their enemies from ancient times. Through experience or experiment, several reliably fatal substances were identified, including mercury, antimony, hemlock, and henbane. But over the centuries the most popular poison has been arsenic. From ancient Greece to Renaissance Italy, arsenic murders were common. In seventeenth- and eighteenth-century France, white arsenic (the powder form) was even called poudre de succession, or “the inheritance powder,” because of its common use.

The advantage of arsenic as a poison was that it was tasteless and colorless, so that it would be undetectable when mixed with food or drink. Also, the symptoms of the poison could be confused with cholera, a very prevalent disease at the time. Thus police and judges had no way to ascertain whether a victim had died of poison or something else. The only way to convict someone was to catch him or her in the act of administering the poison.

That began to change at the end of the eighteenth century. In 1787, the German chemist Johann Daniel Metzger discovered that when substances with arsenic content were heated over charcoal and a copper platter was held above the vapors, the platter became covered with a white material that was arsenous oxide. 15

It was now possible for investigators to test for arsenic, though their methods didn’t work well under certain conditions. It was found, for example, that traces of arsenic occur naturally in the human body. One man in particular was to help solve these problems: Mathieu Joseph Bonaventure Orfila, today remembered as the “father of toxicology.” Born in Minorca in 1787, the young man developed a passion for chemistry and medicine. In 1811, he went to Paris, where he became a doctor of medicine and set up his own laboratory to study poisons. Two years later he published a two-volume work on toxicology that was highly regarded throughout Europe.

In his lab, Orfila experimented on animals to try to understand the chemistry of poisons, particularly arsenic. He showed how the poison passed through the stomach and intestine to other organs such as the liver, spleen, and kidney and finally permeated the nerves themselves, 16 demonstrating that even when no poison remained in the stomach, it would show up in other parts of the body. He further discovered that the arsenic was easier to detect when the animal tissue was charred.

Orfila still faced the problem posed by trace elements such as iron, zinc, and iodine, which occur naturally in the body and may conceal the presence of arsenic. The solution was found by James Marsh of the Royal British Arsenal, who developed the Marsh tube. Marsh’s