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.