§ 1. Van der Waals' Fundamental Views.

The experiments conducted according to this method by Gay-Lussac¹) and later by Joule and Lord Kelvin²) did not yield a result that definitively decided the question of the existence of these attractive forces; however, the latter two succeeded in experimentally proving the existence of these attractive forces through expansion experiments with gases using a more indirect method.³) Specifically, they showed that a gas which is forced under pressure through a porous plug (without heat transfer to the outside) undergoes a slight cooling, whereas calculations show that a perfectly ideal gas A theoretical gas that follows the gas laws perfectly, with no volume or inter-molecular forces. would not change its temperature in this process.

The simultaneous existence of an attractive long-range force and an elastic core of the molecules admittedly possesses a certain improbability. In particular, it seems diametrically opposed to the assumption discussed in Section III of Part I, that two molecules repel each other with a force inversely proportional to the 5th power of the distance. Nevertheless, both assumptions could provide a certain approximation of reality if the molecules exerted a weak attraction on each other at greater distances, but at very small distances exerted a repulsion nearly inversely proportional to the 5th power of the distance. The attraction would then have to grow far more slowly with decreasing distance than the repulsion, so that the former would not be taken into account during collisions alongside the repulsion, which predominates enormously at very small distances.

We will, of course, leave a more precise formulation of any possible assumptions to the future and, in the following, unconcerned with the connection to the hypotheses discussed in Part I, adhere strictly to the prerequisites of Van der Waals, which—entirely in the spirit of our theory—we again regard as an image that is accurate only in some respects. After all, being conscious of our lack of knowledge regarding the true nature of molecules, we have never—

¹) Memoirs of the Society of Arcueil original: "Mem. d. l. soc. d'Arcueil" I, p. 180, 1807; cf. Appendix to Mach, Principles of the Theory of Heat original: "Princ. d. Wärmelehre", Barth, 1897.
²) Philosophical Magazine original: "Phil. mag." ser. III, 26, p. 369, 1845. Joule's Collected Treatises, German by Sprengel.
³) Philosophical Transactions original: "Phil. trans." 1854, p. 321. 1862, p. 579.