II. On evaporation and dissociation. —Part I

Abstract 1. The phenomena exhibited by gases when exposed to varying temperatures and pressures have been shown by many eminent observers to be explicable by an extension to molecules of the laws of motion of matter which are known to be true in the case of large bodies. Such molecules of gas are supposed to be in a state of very rapid motion, the free path of each molecule bearing a very large ratio to the diameter of the molecule. As a liquid is formed by the condensation of a gas, it is clear that its molecules are in closer proximity to each other, and that the average free path of each molecule in the liquid state cannot be nearly so great as in the gaseous state. It was pointed out by Naumann (Ann. d. Chem. u. Pharm., 1870, 155, 325; see also Ramsay, Proc. Roy. Soc., 1880, April 22 and December 16) that it is conceivable that an explanation of the closer proximity of molecules in a liquid than in a gas may be that two or more gaseous molecules have united to form complex molecular groups, analogous to those complex molecules which are known as chemical compounds, in which two or more elements exist in combination. On the other hand, it is held by some that the difference between gas and liquid is due solely to the greater proximity of the molecules in the liquid state, by reason of which they come within the sphere of mutual attraction, but do not necessarily coalesce to form groups of molecules analogous to the group of atoms in the molecule of a compound.