INTRODUCTORY AS TO HEAT 125 



mysterious thing which they called phlogiston, a thing which 

 every inflammable substance was supposed to possess. In 

 the process of burning, the phlogiston was supposed to 

 escape. 



The Kinetic Theory. But the modern explanation of 

 heat is based upon the kinetic theory. You learned in the 

 last chapter that the theory of gravitation explains many 

 things. But you will find that this kinetic theory is even 

 more remarkable as to the number of things that it explains. 

 You have just learned that kinetic energy means energy 

 at work. Now the kinetic theory is a theory of motion. 

 You have learned that all substances, whether gaseous, 

 liquid, or solid, are composed of very small particles. A 

 single drop of water is composed of many millions of these 

 particles. They are called molecules, and the kinetic theory 

 is the theory that molecules are in motion. That is to say, 

 they move as much as conditions permit them to move. 

 In a solid substance this motion is, of course, very limited, 

 while in a gas it is not so limited. This kinetic theory 

 helps us understand why gases diffuse so rapidly. It also 

 helps us understand why liquids evaporate; molecules 

 at the surface are continually flying off into space. This 

 theory is too complex for full explanation in this book, 

 but even a very general idea of it will help you understand 

 many common phenomena. 



If heat is due to the activity of molecules, then the 

 process of heating a substance is simply a process of mak- 

 ing its molecules move more rapidly, and the rise in tem- 

 perature of the heated substance is one of the results of 

 this acceleration (increase in speed) of its molecules. So 

 we may say that heat is one of the results of molecular 



