Section I.

Fundamentals of van der Waals’ Theory.

§ 1. General Views of van der Waals’.

When the distance at which two gas molecules perceptibly act upon one another is negligibly small compared to the average distance from one molecule to its nearest neighbor—or, as one might also say, when the space occupied by the molecules (or rather their spheres of influence) vanishes in comparison to the space filled by the entire gas—then for each molecule, that portion of its path traveled during interaction with other molecules also vanishes in comparison to the path of its center of mass traveled in a straight line or solely under the influence of external forces. In such a case, the Boyle-Charles Law The ideal gas law, combining Boyle's law (pressure-volume) and Charles's law (volume-temperature). holds for the gas in question, regardless of whether its molecules are simple material points, rigid bodies, or arbitrarily complex aggregates term: aggregate (a cluster or combination of smaller units forming a whole molecule). In all these instances, the gas in question is called an ideal gas.

The gases occurring in nature fulfill this condition of the ideal gas state only incompletely, and therefore a theory which also takes into account the finite extent of the spheres of influence of the molecules is highly