Page:International Library of Technology, Volume 93.djvu/29

 the attractive force that holds the body together is overcome, and the molecules pass off into space. The substance is then known as a vapor or gas. The molecules, therefore, of a gas vibrate much more rapidly than the molecules of a solid or a liquid body. Furthermore, the molecules of a gaseous body have no fixed paths of motion. Consequently, when one molecule strikes another, it rebounds and keeps moving in the direction of the rebound until it strikes other molecules, thus causing the gas to expand indefinitely unless confined in a vessel under some pressure. Hence, gases are said to be perfectly expansible and perfectly elastic.

3. Heat may be derived from three sources, known as physical, chemical, and mechanical sources. The earth and the sun are physical sources of heat; the burning of wood, oil, or coal is an example of a chemical source of heat, since burning is a chemical action; and the rubbing together of two pieces of wood until they become warm by friction and the hammering of a piece of iron until it becomes hot, are examples of mechanical sources of heat.

4. Definition.—The term temperature is used to indicate how hot or how cold a body is; that is, to indicate the velocity of the vibration of the molecules of a body. A body having a high temperature is said to be hot; a body having a low temperature is said to be cold. When a body, as, for example, an iron bar, receives heat from any source, the vibrations of its molecules become more rapid and its temperature rises; on the other hand, when a body loses heat, the vibrations of its molecules become less rapid and its temperature falls.