Arsenic

Arsenic trioxide is nearly tasteless, dissolves in food and drink, and produces symptoms that pass for cholera, which made it the poison of choice for centuries and earned it the nickname poudre de succession, inheritance powder. That ended with James Marsh, who in 1836 devised a test sensitive enough to show a visible metallic mirror from micrograms of arsenic and to display it to a jury — the birth of forensic toxicology. Its worst chapter is unintentional: tube wells drilled across Bangladesh and West Bengal from the 1970s to escape surface-water pathogens tapped aquifers with naturally high arsenic, exposing tens of millions of people in what the WHO has called the largest mass poisoning of a population in history. Industrially it survives in gallium arsenide semiconductors and, until recently, in pressure-treated timber.