The entire success of a criminal poisoning depends on the process imitating the effects of a natural disease. So said Edinburgh's professor of forensics Robert Christison in 1836. If he was right, then the odds ran strongly in the murderer's favor when arsenic was involved.
In 19th century Britain, arsenic was the poisoner's substance of choice. From 1750 to 1914, it featured in 237 cases to come before the English courts. The second most common substance -- opium -- trailed a long way behind, with just 52 cases.
And while the fact that anyone ever finished up in the dock, not to mention at the end of a hangman's rope, showed that not all arsenic poisoners got away with their crime, the majority almost certainly did. The poison was particularly popular with impatient heirs, keen to get their hands on their fortunes, hence its name, the inheritor's powder.
The element called arsenic will pass quite safely through the human body provided it remains in that elemental state. What most people mean by arsenic is the compound arsenic trioxide, known in the 1800s as white arsenic. A harmless-looking powder, resembling flour or sugar at a quick glance, white arsenic is virtually undetectable in hot food and drink and fatal in small doses.
"If you feel a deadly sensation within and grow gradually weaker, how do you know you are not poisoned?" asked the London newspaper The Leader in 1855. "If your hands tingle, do you not fancy it is arsenic? ... Your friends and relations all smile kindly upon you; the meal ... looks correct but how can you possibly tell there is not arsenic in the curry?" Of course you couldn't and the idea was terrifying.
And then there was the difficulty in diagnosing cases of arsenic poisoning. As Robert Christison pointed out, the first requirement in catching a murderer is to recognize that a murder has been committed. Doctors at that time had only the patient's symptoms to go on, however, and the clinical signs of arsenic -- vomiting and diarrhea -- were easily mistaken for those of common diseases such as food poisoning, dysentery and cholera.
In 1862, the London toxicologist Alfred Swaine Taylor, giving evidence in a murder trial, said he knew of at least eight cases where death had at first been recorded as due to cholera. Only after suspicions had been raised later and the body exhumed was the true cause found not to be disease at all but an irritant poison such as arsenic.
And if the poison was administered in small doses over a period of time then the chances of being caught were particularly slim.
Some poisons such as cyanide and strychnine work according to a strict timetable and dispatch their victims in a predictable manner. Arsenic, by contrast, is mysterious and shilly-shallying, behaving more like an infectious disease, so that the nature and length of the victim's suffering depends partly on their genetic make-up and general state of health. Death from acute arsenic poisoning can take anything from two hours to four days, although victims have been known to linger for a fortnight. For most, though, the misery lasts at least 24 hours.
To confuse matters further, human beings are capable of building up a certain tolerance to arsenic if they go about it carefully enough. In 1851, a community of peasants living on the Austria-Hungarian border were found to be taking arsenic in what would normally be lethal doses. They believed that the poison was good for their health and took it as a tonic, starting with a tiny sub-toxic dose and gradually increasing it.
Defense lawyers quickly seized on this to try to sow doubt in juries' minds. Was this case really murder? Perhaps the arsenic in the dead person's food or body had been self-administered for health reasons, only this time the victim had gone too far?
And then there was arsenic's ready availability. For a few pence and with no questions asked, it was possible to buy enough to wipe out the entire neighborhood. A cartoon in the humorous magazine Punch in 1849 shows a child so small she is barely able to see over the counter asking a chemist: "Please mister will you be so good as to fill this bottle again with laudanum and let mother have another pound and a half of arsenic for the rats." The chemist, who appears to be in his early teens, replies "Certainly ma'am. Is there any other article?"
Charles Dickens's magazine Household Words imagined a particularly sinister scene: "A thin, respectable-looking man in spectacles, with dark hair and whiskers and wearing a long brown coat, calls at a chemist's shop ... and asks for an ounce of arsenic to kill rats....He has a design to poison his wife, her mother or a man to whom he owes money ... and he has now got a stock in trade for the carrying out of his intentions."
In 1851 -- not before time, some people thought -- the British government introduced The Sale of Arsenic Regulation Act after a crescendo of complaint and concern from the medical profession, the press and the public. The legislation required the seller to record the buyer's name and address and the buyer to sign a register, which became known as the poison book.
Gradually through the 19th century and on into the 20th, arsenic became harder and harder to obtain. This, together with better diagnosis of poisoning cases, finally led to the end of arsenic's deadly career as a murder weapon. It was, however, extremely nasty while it lasted.
Sandra Hempel is the author of the book The Inheritor's Powder: A Tale of Arsenic, Murder, and the New Forensic Science.