Page:International Library of Technology, Volume 93.djvu/28



1. Molecular MotionAll bodies are supposed to be made up of a large number of very small parts or particles, called molecules, which are too small to be seen, even with the aid of the most powerful microscopes. These molecules vibrate to and fro, and, since each molecule has some weight, though very little, it possesses a certain amount of energy due to its motion. According to the modern theory, neat is the energy that a body possesses, due to the continual vibration of its molecules; that is, heat is the energy of molecular motion.

2. Path of Molecular Motion.The molecules of a solid body are supposed to move with comparative slowness in fixed paths, and these paths are supposed not to change so long as the body remains in the solid state. This is due to the fact that in a solid the molecules are close together and attract one another very strongly. If the solid is heated until it melts, thus becoming a liquid, the molecules vibrate more rapidly, and no longer have definite paths of motion, but are free to move in and out among one another. When a liquid is heated, the vibratory motion increases until finally