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There is a similar doubt as to whether the warmth of wool, skins,
feathers, and the like, is derived from a slight inherent heat, since
they are animal excretions, or from their being of a certain fat and
oily nature that accords with heat, or merely from the           and
separation of air which we spoke of in the preceding paragraph;[91] for
all air appears to possess a certain degree of warmth when separated
from the external atmosphere.