Thallium

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poisons containing thallium sulfate, once sold commercially and colourless and nearly tasteless in food or drink, were withdrawn from general sale across most of the world by the 1970s precisely because the same properties that killed rodents made the compound so dangerously easy to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.

Thallium's name captures the fresh green of the spectral line that revealed it, thallos evoking the first green shoot of spring, and it belongs with caesium, rubidium and indium among the elements named for the colours of their light. Its darker reputation, however, is as a poison: thallium salts are colourless, odourless and tasteless, and their effects, including hair loss and nerve damage, can mimic other illnesses, which earned thallium a grim renown as the poisoner's poison and a place in criminal history and detective fiction, notably Agatha Christie's novel The Pale Horse, whose accurate description of thallium poisoning is said to have helped diagnose real cases.

The element has no ancient or alchemical tradition, being a spectroscopic discovery of the 1860s. The same heart-imaging use mentioned above, thallium-201 stress testing, exploits a biological quirk: the body's potassium-handling machinery cannot easily distinguish thallium ions from potassium, so the radioactive isotope is taken up by healthy heart muscle in rough proportion to its blood supply, letting cardiologists read areas of reduced flow directly from the resulting scan, a genuinely useful medical inversion of the same chemical mimicry that makes thallium so toxic in the first place.