LIII
conductor than air. 278. Sensation of heat and cold, when in
contact with certain bodies. 279. The genuineness of gems
tested by touching them. 280. Wind conducts heat thus pro
ducing cold. 281. Dipping the hand into melting iron. 340.
Red hot cannon-balls can be wheeled to the gun's mouth in
wooden barrows partially filled with sand. Lava has been known
to flow over a layer of ashes underneath which was a bed of ice,
and the ice did not melt. (About propagation of heat by convec
tion, see § 146.)
§ 130. a) Radiation, Reflection and Absorption of Heat.
The warmer body causes by means of the thermal waves, pro
ceeding from it vibrations of the atoms in a colder body. Hot
bodies may send out rays of heat, which produce heat without
heating the air. In standing before a fire or exposing ourselves
to the sun's heat, we feel warm in consequence of the radiation
of heat. 282. This radiation takes place in all directions round
a body, but the rays themselves are emitted in a right line. This
heat is even propagated in a vacuum. Radiant heat is propor
tional to the temperature of its source, but like sound its intensity
is inversely as the square of the distance.
§ 131. Bodies always tend to exchange their heat in order
to assume an uniform temperature or “the Equilibrium of Tem
perature”.
§ 132. Three cases are possible in this respect:
1. If the bodies exchanging their temperature are of the
same substance and mass, the warmer body loses as many de
grees as the cooler one gains, f.e. 1 lb. at 100° C and 1 lb. at
20° make a mixture of 60°.
2. The bodies have the same substance but unequal mass.
If the cooler one has n times more mass, it gains n times less de
grees than the warmer one loses; if the cooler one has n times
