Thallium was discovered in 1861 by the English scientist William Crookes, who spotted a brilliant green line in the spectrum of a residue from sulfuric-acid manufacture; Claude-Auguste Lamy isolated the metal independently soon after. Crookes named it from the Greek thallos, a green shoot or twig, after that green line. Thallium is a soft, heavy, bluish-grey metal, extremely toxic, once used in rat poison and now handled with great caution. Used in rat poisons and depilatories until its dangers were recognised, thallium now serves mainly in specialised electronics, in infrared optics and glass, and in low-temperature thermometers, while a radioactive isotope is used in heart-imaging scans.
Thallium's ease of criminal misuse was starkly illustrated by the British poisoner Graham Young, who murdered and sickened several coworkers with thallium sulfate at a photographic-supplies factory in Hertfordshire in the early 1970s before being caught, a case that helped drive tighter restrictions on the sale of thallium compounds in the United Kingdom and elsewhere. Rat poisons containing thallium sulfate, once sold commercially and colourless and nearly tasteless in food or drink, were withdrawn from general sale across most of the world by the 1970s precisely because the same properties that killed rodents made the compound so dangerously easy to misuse against people.