Thus a constant motion of currents and
interchange of particles is produced until, as in a vessel of water, the
whole body comes to an equal temperature. Air is heated in the same way.
In case of a hot stove, the air that touches it is heated, becomes
lighter, and rises, giving place to cooler and heavier particles, which,
when heated, also ascend. It is owing to this process that the air of a
room is warmest at the top and coolest at the bottom. It is owing to
this principle, also, that water and air can not be heated by fire from
above. For the particles of these bodies, being non-conductors, do not
impart heat to each other; and when the warmest are at the top, they can
not take the place of cooler and heavier ones below.
Another principle of heat (which it shares with light) is _Radiation_,
by which all things send out heat to surrounding cooler bodies. Some
bodies will absorb radiated heat, others will reflect it, and others
allow it to pass through them without either absorbing or reflecting
Thus, black and rough substances absorb heat, (or light,) colored and
smooth articles reflect it, while air allows it to pass through without
either absorbing or reflecting.
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