Many alchemists throughout history have held the curious belief that for a process to be successful it must be repeated many times. So it was common for substances not only to be distilled, but for the products of distillation to be returned to their residue and distilled again, perhaps several hundred times. This process of alchemical redistillation was called cohobation.
It was so common a technique that a special piece of apparatus was designed to make the whole process automatic, a flask with return tubes from the neck for the vapour to condense and pass back into the base of the vessel. It was called a blind alembic or a pelican. The latter name derives from the medieval heraldic image representing the ancient legend of the long-necked pelican wounding its breast to feed its young on its blood, the curve of the bird’s neck resembling one of the curved return tubes on the flask.
The origins of the word cohobation are mysterious, though the OED surmises that it may derive from an Arabic dialect root for “repeat”.
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