Arsenic

Arsenic reached the alchemists as two brilliantly coloured sulfide minerals. Orpiment is a golden yellow arsenic trisulfide, and its Latin name auripigmentum means gold pigment; realgar is a scarlet arsenic sulfide whose name comes into European languages through the Arabic rahj al-ghar, powder of the mine. Both were mined, ground and used as pigments across the ancient and medieval world, in Egyptian painting, in Chinese and Persian manuscripts and in European panel painting, and both are seriously poisonous.

Their importance in alchemy goes beyond colour. Orpiment's golden appearance made it an obvious candidate for a substance containing the principle of gold, and arsenic compounds do in fact whiten copper, producing an alloy that convincingly resembles silver. That effect is documented in early recipe collections, and it is one of the concrete practical results that kept transmutation credible: something was visibly happening to the metal.

Isolation of the element is usually credited to Albertus Magnus, around 1250, by heating orpiment with soap, although earlier preparations in the Arabic tradition are likely. Arsenic has a recognised alchemical symbol and appears throughout the corpus, but it was never one of the seven planetary metals and was never assigned a planet. Like antimony, bismuth and zinc, it is a substance the tradition knew well and could not fit into its own scheme.

Arsenic sublimes rather than melting at ordinary pressure, passing straight to vapour above about 600 degrees C, and the vapour smells of garlic. That odour was a recognised warning sign in mining and metalworking for centuries.

Arsenic trioxide is a white powder, very nearly tasteless, soluble in hot liquid and lethal in doses well under a gram. The symptoms of acute poisoning, violent vomiting and diarrhoea followed by collapse, resemble cholera and ordinary gastric illness closely enough that in an age without toxicology the difference could not be demonstrated. It was cheap and freely sold as rat poison.

This combination made it the standard instrument of domestic murder in early modern Europe, so much so that it acquired the name poudre de succession, inheritance powder. The Affair of the Poisons in the France of Louis XIV, running from 1677, produced dozens of arrests and executions and reached into the court itself.

The remedy was analytical. James Marsh, a chemist at the Royal Arsenal at Woolwich, published in 1836 a test in which the sample is treated with zinc and acid, the arsine gas produced is burned, and the arsenic deposits as a mirror-bright black film on a cold porcelain surface. It is sensitive to a fraction of a milligram, it works on tissue taken from a body, and it can be shown to a jury. Its first celebrated use was the trial of Marie Lafarge in France in 1840. Arsenical murder did not stop, but it stopped being safe.

The element's other Victorian career was entirely legal. Scheele's green and Paris green, copper arsenite pigments, coloured wallpaper, fabric, candles and confectionery, and the resulting chronic exposure was widespread and poorly understood. Arsenic was also taken deliberately as a tonic and complexion treatment.

In 1910 Paul Ehrlich and Sahachiro Hata introduced Salvarsan, an organic arsenic compound, as a treatment for syphilis. It was the first drug designed to attack a specific pathogen rather than to relieve symptoms, and it began modern chemotherapy. Arsenic trioxide is today a standard and highly effective treatment for one form of acute leukaemia.