Albertus Magnus

The Dominican friar who isolated arsenic, taught Aquinas, and refused to claim he had ever made gold.

Albert of Cologne is conventionally credited as the first European to isolate elemental arsenic, around 1250. His De Mineralibus is a careful natural history of stones and metals based on what he could observe and replicate. He wrote alchemical texts and believed transmutation was theoretically possible — but in his own surviving work he never claims to have done it, and he distinguishes natural philosophy from the mystical strands then circulating. The chemistry and the teaching are real (Aquinas was his student). The transmutation theory was a hedge, not a result.