gems, Gourjon, mineralogy, history of mineralogy, chemistry, orpiment

Pott’s Exercitationes chymicae is a collection of previously issued chemical dissertations, revised and enlarged with additional notes, experiments, and discussion. It treats the sulphurs of metals; orpiment; the selective dissolution of substances; terra foliata tartari; and preparations involving the acids of vitriol and nitre with spirit of wine. Of particular gemological and mineralogical interest is the dissertation on orpiment (auripigmentum), in which Pott discusses its names, occurrence, physical appearance, composition, preparation, reactions, and distinction from related arsenical sulphides. He describes natural orpiment as a mineral composed of arsenic and sulphur and notes that material could be fraudulently substituted for native cinnabar; he also reviews chemical and alchemical treatments involving orpiment, while critically testing claims made by earlier authors. The work is representative of early eighteenth-century experimental chemistry, combining mineralogical description with laboratory operations such as sublimation, solution, precipitation, distillation, and fixation.