40 OUTLINES OF PHYSICAL CHEMISTRY
The Kinetic Theory
We must not neglect at this point to give a short survey of what has been termed the Kinetic Theory of the gaseous state.
The apparent volume of a gaseous mass is only filled to a very small extent by material molecules. These molecules are not in a state of rest, but move about in straight lines. They move through space with a very great velocity until they meet the wall of a vessel or another molecule; then the original direction of motion and the velocity are altered, in conformity with the laws of impact of elastic bodies. These collisions are so very frequent that the path of the molecule is zig-zag and made up of very short straight lines. The kinetic
energy -^- is not absolutely the same for each molecule
of the same gaseous mass, because the velocity v is more or less variable. But for the necessities of the theory and for calculations, we assume that all the molecules of the same gaseous mass have the same average kinetic energy.
It is in virtue of this kinetic energy of the molecules that a gas tends to expand and occupy any space, however large ; and the impacts of the molecules against the walls of the enclosing vessel constitute the gaseous pressure. It can be shown that this pressure is proportional to the number of molecules contained in unit volume of the gas, and to the kinetic energy of the molecules :
If, at constant temperature, we change the volume of a gaseous mass, it follows that the number of molecules contained in unit volume is changed, and consequently
1 p denotes the pressure and v the volume of the gas, n expresses the number of molecules, m the mass, and v the average velocity of a molecule.
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