Circulation
Circulation is the repeated, continuous distillation of a liquid within a closed vessel, so that vapour rises and condensate falls back again in an endless cycle, refining the substance with each pass. It was carried out in a special double vessel, the pelican, whose side-arms return the condensate to the body. It belongs to neither Pernety's zodiacal twelve nor Ripley's gates, but it was a standard and important laboratory operation. It aimed to exalt and subtilize the matter through unceasing motion.
See also the concept page: Circulation
Historical Development
Circulation is continuous internal distillation: in a sealed vessel, vapour rises, condenses, and returns to the body to rise again, refining the substance through endless repetition. The pelican, with its returning side-arms, was the apparatus emblematic of the operation, and the closed cycle it enabled was prized for producing especially pure and 'exalted' substances.
Unlike the operations in the two twelve-fold schemes, circulation is not a numbered gate or zodiacal station; it is a general technique that supports many of them. It is included here as a major additional process because of its central role in the preparation of quintessences and spirits.
Textual Tradition
Circulation is described across the medieval and early-modern technical literature on distillation, from the treatises on the quintessence to the printed distillation manuals. Its apparatus is documented by historians of laboratory practice and figures in the work of Bruce Moran on distilling knowledge.
Modern Scholarship
Historians of apparatus and technique treat circulation as evidence of sophisticated closed-system design, an important step toward controlled, reproducible chemical processing and toward the reflux methods of later chemistry.