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II. On Evaporation and Dissociation .— Part I. 
By Professor William Ramsay, Ph.D., and Sydney Young, D.Sc., Lecturer and 
Demonstrator of Chemistry in University College , Bristol. 
Communicated by Professor Stokes, Sec. R.S. 
Received August 4,—Read Nov. 19, 1885. 
[Plates 1, 2.] 
§ 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. cl. 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. 
These views also apply to the molecular constitution of solids; it is generally held 
that solids differ from liquids only in the arrangement and restrained motion of their 
molecules. 
§ 2. In support of the first view the following points have been urged :— 
(a.) The rotation of the plane of polarised light is supposed to point to molecular 
grouping in liquids and solids, which does not exist in gases. 
(b.) The existence of allotropic forms of elements, and of isomorphous forms of 
