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the same temperature, is called the Specific Heat of that body.
(More about this see under 3, § 136.)
§ 133. When a ray of heat falls upon the surface of a
body, part of it is absorbed and raises the temperature of the
body, another part darts off in such a manner, that the angle
made by the incident (falling) ray and the surface of the body
is equal to the angle formed by the reflected ray and that sur
face.
§ 134. The absorbing and reflecting powers of bodies are
very different in various bodies. Some absorb much and reflect
little, some reflect much and absorb little. Bodies, which the
rays of heat pass through we may call diathermal and those,
which reflect them, athermal bodies. (Crookes's Radiometer.)
The reflecting power of some substances may be seen in
the following instances: Plants placed in contact with a wall.
283. Reflecting power diminished by black colour, 285; cooking
vessels should be black and rough, 286; white and polished
vessels cool slowly. 287. Nocturnal radiation prevented by
clouds or smoke, which reflect the rays of heat. 288. 289.
§ 135. A great heat may be produced by concave mirrors,
which catch the parallel rays of the sun and reflect them in such
a way, that they coincide in one point, the focus of this burn
ing mirror
Remark. It is said that Archimedes burnt the Roman vessels in the
harbour of Syracuse by means of such mirrors. 284. (See No. 84 page 210.)
§ 136. 3) Specific Heat. We may compare bodies as to
the quantity of heat they absorb, when their temperature rises
through a certain number of degrees. The unit chosen for this
comparison (the thermal unit) is the quantity of heat nece
ssary for raising 1 lb. of water through one degree of Centigrade.
§ 137. For finding the specific heat of any body the most
reliable method is that of mixture. With a certain quantity of
