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In alchemy, digestion is a process in which gentle heat is applied to a substance over a period of several weeks.
This was traditionally performed by sealing a sample of the substance in a flask, and keeping the flask in fresh horse dung or sometimes in direct sunlight. Today, practitioners of alchemy use thermostat-controlled incubators.
- John Reid's Course on Practical Alchemy - II Chapter 2 (November 1, 2010)