Introduction.

repelled, but attracted by D and E and therefore driven back toward its original equilibrium position original: "Ruhelage". If every molecule oscillates around such an equilibrium position, the body has a fixed shape; it is in the solid state of matter original: "Aggregatzustand". The only consequence of thermal motion heat-induced movement will be that the equilibrium positions of the molecules are pushed somewhat apart, and therefore the body is slightly expanded. But if the thermal motions become increasingly vigorous, one finally reaches a point where a molecule is forced between its two neighboring molecules from the equilibrium position A to A'' (Fig. 1). It is then no longer driven back to its old equilibrium position, but leaves it permanently. If this occurs with many molecules, they must crawl past one another like earthworms; the body has melted. Even if one finds this conception perhaps crude and childish, and even if it may be significantly modified later—and specifically the apparent repulsive force perhaps being a mere consequence of motion—in any case, one will admit that when the motion of the molecules has grown beyond a certain limit, individual molecules must be torn away entirely from the surface of the body and fly out freely into space; the body evaporates. If it is located in a closed vessel, the vessel fills with freely flying molecules, and these penetrate back into the body here and there; as soon as the number of re-entering molecules is on average equal to the number of those tearing away, one says that the space of the vessel is saturated with the vapor of the body in question.

    A sufficiently large closed vessel, in which exclusively such freely flying molecules are located, provides the image of a gas. If no external forces act on the molecules, they fly for the vast majority of their movement like fired musket balls original: "Flintenkugeln" in straight-line paths with constant velocity. Only when a molecule happens to come very close to another or to the wall of the vessel is it deflected from its straight path. The pressure of the gas is explained by the impact effect original: "Stosswirkung" of these molecules on the wall of the vessel.