Page:Elementary Text-book of Physics (Anthony, 1897).djvu/220

 CHAPTER II.

EFFECTS OF HEAT.

184. The Kinetic Theory of Heat.—In order to describe more easily certain of the effects of heat, it is advantageous to have an idea of the theory by which they are explained. This theory, the kinetic theory of heat, asserts that the molecules of all bodies are in constant motion, and that the heat of a body is the kinetic energy of its molecules. The idea that heat consists of the motion of the least parts of matter was introduced into science by Newton, of course with a very imperfect knowledge of the facts. The apparently unlimited production of heat by mechanical work led Rumford and Davy, more particularly the latter, to assert the equivalence of heat and motion. This theory was afterwards displaced for many years by the influence of the French school of physicists, who considered bodies, at least in their mathematical discussions, as assemblages of stationary particles, and heat as a separate substance. It was revived by Mohr, who showed its very general applicability in the explanation of ordinary heat phenomena. Since the discovery of the conservation of energy, the reasons in its favor have been very much strengthened and its foundations securely laid by the complete success attained with it in explaining the laws of gases.

We will use this theory in its general form in the description of some of the effects of heat, and will discuss it more fully in § 221 seq.

185. Expansion of Solids.—When heat is applied to a body it increases the kinetic energy of the molecules, and also increases the