Spirit (Latin: Spiritual) is a concept of alchemy whose meaning is sometimes unclear. Generally, alchemists report fumes and gases, which means opposite to body, indicating heavier substances. A third term that joins this is soul, the most abstract of the three. Possibly it refers to a (re) vitalizing principle of matter. Concrete specifications are missing in such terms in alchemical texts.
In the alchemical sense, spirit refers to the same understanding as in common speech usage: there is "something", we just do not know how to perceive or measure it. In the pre-modern times, gases were not easy to describe. For example, during a distillation of beer, it was clear that there was something out of the beer, but it was not visible. The condensate, reasonably drinkable as a kind of gin, precipitated in the cooler seemed to arise from nothing. In this period, a pigeon (as a sign of mind) is shown in pictures of distillation.
With the evolution of the concept of gas, the necessity of the alchemist spirit broke out.
In some older names, the term spirit still exists in this sense:
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