Arsenic

Arsenic the metal was obtained by roasting orpiment or realgar, its two common mineral sulfide sources, and reducing the resulting oxide with charcoal, a process that yields a brittle, grey, metallic solid with an unusual, semi-metallic character, notably sublimating directly from solid to vapor at ordinary pressure when heated strongly rather than passing through an obvious liquid phase, a striking behavior that marked it out from the true metals early observers were more familiar with. Far more commonly encountered in practice than the metal itself was white arsenic, the trioxide, a fine white powder obtained as a byproduct of roasting arsenic bearing ores, notably in the course of smelting other metals such as tin and copper from ores that carried arsenic as a natural impurity, meaning white arsenic accumulated in metalworking flues and furnaces as an unavoidable byproduct long before anyone deliberately manufactured it as a poison.

White arsenic's reputation, well earned, was as the single most notorious poison of the pre-modern and early modern world: odorless, tasteless in the small doses that killed slowly enough to resemble natural illness, and, critically, undetectable by any chemical test until the nineteenth century development of specific arsenic assays, it became the poison associated with countless real and rumored murders across European history. Physicians also used it, in carefully controlled small doses, as a genuine if dangerous medicine for certain skin and blood conditions, a therapeutic use that persisted, remarkably, into the early twentieth century.

Arsenic the element is identified without dispute as the metalloid As, and the historically far more significant substance, plain arsenic as a poison, is identified as arsenic trioxide, As2O3, an identification resting on very solid ground given the extensive medical, forensic and toxicological literature on the compound stretching back centuries. The genuinely important distinction for accuracy is that most pre-modern textual references to arsenic, in medical, alchemical and legal contexts alike, mean this white oxide rather than the metallic element, a distinction not every historical source is careful to draw and one modern readers should not collapse.

The development of the Marsh test in 1836, a specific and sensitive chemical assay for arsenic that could detect the poison in a victim's remains long after death, is widely credited with transforming arsenic poisoning from an almost undetectable crime into a forensically provable one, a landmark moment where analytical chemistry directly reshaped both criminal law and the practical history of a substance long used precisely because it could not be caught. Arsenic's toxicity was also long known to be dose dependent in a way that made it genuinely useful, if dangerous, in the small quantities physicians prescribed, a therapeutic window modern medicine still exploits with more tightly controlled arsenic compounds for a small number of specific conditions today.