§ 2. External and Internal Pressure.

Between the centers of every two molecules of the substance, let there act an attractive force (the van der Waals cohesive force), which, while vanishing at distances accessible to observation, decreases so slowly with increasing distance that it can still be regarded as nearly constant even within distances that are large compared to the average distance between two neighboring molecules of the substance. The consequence of this is that the van der Waals cohesive forces exerted by the surrounding molecules on every molecule located in the interior of the vessel act very nearly uniformly in all possible directions in space and therefore cancel each other out, so that the movement of individual molecules occurs like that of ordinary gas molecules and is not significantly modified by the van der Waals cohesive force. Therefore, although the latter falls outside the scope of the forces considered by us in Part 1, the movement of the molecules can still be calculated exactly according to the principles established there.

Only on the molecules that are very close to the boundary of the substance does the van der Waals cohesive force act predominantly inward. These molecules are therefore forced to reverse their direction by two kinds of forces. Firstly, by the counter-pressure of the wall on the gas, and secondly, by the van der Waals cohesive force. Let the intensity with which the former force acts on the molecules adjacent to a unit area be called $p$, and that of the latter $p_i$, so that the molecules adjacent to the unit area of the boundary surface of the substance are forced to reverse by the total force

1. $p_g = p + p_i$

Here, $p$ represents the external pressure measured by instruments, while $p_i$ represents the "internal pressure" caused by molecular attraction.

Now let a part $DE$ of the vessel wall with surface area $Ω$ The Greek letter Omega, commonly used to denote area in 19th-century physics. be flat. The total force

which, in the state of equilibrium, acts on the molecules hitting the surface $DE$ in a unit of time and forces them to reverse, is—according to § 1 of Part I—equal to the total momentum momentum: the product of a particle's mass and velocity estimated in the direction of the normal $N$ The "normal" is a line perpendicular to the surface. to the surface $DE$, which the molecules would carry through this surface in a unit of time if the—