Arsenic, and to a lesser extent, thallium. “Inheritance powder”. Also known as “the poison of kings” and the “king of poisons”.
Historically, arsenic was the “go-to” poison for expediting the passing of a relative who stood in the way of one’s inheritance or claim to a title. Arsenic is odourless and easily incorporated into food and drink, such as elderberry wine. (Don’t spill it on the old lace.) A dose as small as 100 mg to 300 mg can be lethal. (One teaspoon is roughly equivalent to 5,000 mg.) The symptoms of arsenic poisoning are not very distinct, and its presence was hard to detect. The advent of the Marsh test in 1836 had an impact on the popularity of the chemical as a lethal condiment. The test rendered the chemical traceable.
Arsenic did have legal applications. It is toxic to insects and bacteria and therefore has been used as a wood preservative and insecticide. However, its potent toxicity has led to its ban in most countries.
Ironically, “organic” arsenic is used as a feed additive for chickens and pigs. It increases weight gain, improves feed efficiency and prevents certain diseases.
After WW1, the US built a stockpile of 20,000 tons of a type of arsenic, to be used as a chemical weapon. The stockpile was neutralized with bleach and dumped in the Gulf of Mexico in the ‘50s(!).
Thallium is also lethal in small doses. Water-soluble and nearly tasteless, it too has a history of being used as a murder weapon.
For more on this topic, see The Elements of Murder: A History of Poison by John Emsley. For a short article on arsenic, its illicit use and the downfall of its popularity, see “Sex, lies and arsenic: how the ‘king of poisons’ lost its crow’.”
Thank you for reading.