Kinetic Theory of Heat
The atomistic idea has not been limited to the structure of matter by any means. It is associated with several properties or characteristics of matter such as heat, gas-pressure, and light or radiant energy. The early idea of heat was that it was a fluid. Count Rumford (1753-1814) a little more than a century ago was one of the first to relegate this idea to the discard by experiments which suggested that heat was a mode of molecular motion. It is true that Roger Bacon (12141294) suggested heat was a matter of agitated particles such as molecules, but as in other phases of science, experimental data were lacking. Mayer and Joule (1842) discovered that mechanical work was converted into heat and that the amount of heat was always exactly proportional to the amount of work and vice versa.
It had long been recognized as a direct consequence of Newton’s laws of motion, that energy could not be destroyed. This is the well established law of conservation of energy. For example, if a moving body collided with another body, the resultant mechanical energy remained equal to the total before the collision. The total mechanical energy required to produce a certain amount of heat (agitation of the molecules of the body in which the heat was generated) would remain unaltered. The molecules of the body now possessed so much more mechanical energy; in other words, they were sufficiently more agitated so that their excess mechanical energy was just equal to the mechanical energy required to produce the amount of heat that the body received.
Thus we see not only the atomistic idea of matter used to account for a change in temperature (due to the production of a certain quantity of heat), but also Newton’s laws of motion applied to the molecules of the body. It is a beautiful picture of unification. For example, suppose a large iron ball is dropped from a height and it strikes a thick iron plate of great mass. The mechanical energy of the falling ball at the moment before striking the plate is proportional to its mass and the square of its velocity. Actually the ball will bounce a few times, but let us assume it always hits in the same place and comes to rest there. The energy of the ball is now distributed among myriads of molecules of iron both in the plate and in the ball. Owing to the minuteness of the agitated molecules we cannot see their increased movements, but we can feel the result of the increase. The two bodies are of a higher temperature at first only near the points of contact, but the heating effect rapidly spreads and equalizes. This is what we term conduction of heat and is really a communication of the increased agitation to other molecules. It should be noted that the molecules of a body are always in a state of agitation (excepting at absolute zero), the degree of agitation being indicated by the temperature.