For as the body declines in vigour we
tend to cold at every time of life, and especially in old age, this
age being cold and dry. We must remember that the nutriment coming
to each part of the body is concocted by the heat appropriate to the
part; if the heat is inadequate the part loses its efficiency, and
destruction or disease results. (We shall speak more in detail of
causes in the treatise on growth and nutrition.) Whenever, then,
the hair in man has naturally little heat and too much moisture enters
it, its own proper heat is unable to concoct the moisture and so it is
decayed by the heat in the environing air. All decay is caused by
heat, not the innate heat but external heat, as has been stated
elsewhere. And as there is a decay of water, of earth, and all such
material bodies, so there is also of the earthy vapour, for instance
what is called mould (for mould is a decay of earthy vapour). Thus
also the liquid nutriment in the hair decays because it is not
concocted, and what is called greyness results. It is white because
mould also, practically alone among decayed things, is white. The
reason of this is that it has much air in it, all earthy vapour
being equivalent to thick air. For mould is, as it were, the
antithesis of hoar-frost; if the ascending vapour be frozen it becomes
hoar-frost, if it be decayed, mould. Hence both are on the surface
of things, for vapour is superficial.
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