In some cultures, it was believed that there are substances, so-called ordeal poisons, which could enable one to distinguish between innocent and guilty persons. One such ordeal poison was the Calabar bean of Africa. The Calabar bean is the seed of a plant known as Physostigma venenosum, which is native to tropical Africa. The active ingredient is known as physostigmine.

calabarplant  Physostigma venenosum

The Calabar bean was first brought to the attention of Westerners by the English physician and botanist William Freeman Daniell.  On an exploring expedition in Africa in 1845, he came to the region known as Old Calabar (in what is now Nigeria).  In a paper delivered the following year in Britain, Daniell described the culture of the inhabitants of this region.  He reported on their system of justice, which was administered by a court consisting of the king and various chiefs.

If a person brought before the court was judged guilty, he or she had to undergo an ordeal called “chopping nut.”  The beans of the Calabar plant, called esere by the natives, were pounded in a mortar and then soaked in water until a milky white fluid was produced.
calabarbeans Calabar beans
The prisoner was forced to drink this extract and then made to walk around until the poison took effect.  If the prisoner died, this was considered proof of his or her guilt.  If the prisoner happened to vomit up the poison before it took effect, and thus survived the ordeal, he or she was then judged to be innocent and set free.  Did this trial really distinguish between the guilty and the innocent?  It would not seem likely in terms of modern science and medicine, but one scientist and historian, Walter Sneader, suggests that a person confident of his or her innocence might have swallowed the potion rapidly, which could have overwhelmed the stomach and caused the person to vomit.  On the other hand, a guilty person, fearing the test, might have held the liquid in the mouth and swallowed it slowly, facilitating absorption and avoiding vomiting.  It has also been suggested that perhaps the priest or chief who administered the poison could have predetermined the outcome by preparing the poison especially strong or weak (e.g., by using dried beans instead of fresh ones to reduce the potency).  This could have been based on a judgment of the person’s actual guilt by the person administering the poison, or perhaps for some other reason (assuming this practice took place at all).
There was great interest in the Calabar bean in Britain after Daniell had reported on it.  Robert Christison of the University of Edinburgh, one of the founders of toxicology and forensic medicine, studied the poison.  Christison experimented with the poison on animals, and, as you read, he also experimented on himself.  He was fortunate to survive his self-experimentation.
christison  Robert Christison
The active ingredient, the alkaloid physostigmine, was isolated in 1864, and it has found some use in medicine.  It was first used to reduce pressure in the eye in some types of glaucoma.  It is still used to some extent today for this and other purposes, including treatment of myasthenia gravis (an autoimmune disorder).