ON EVAPORATION AND DISSOCIATION. 
73 
of any molecule is proportional to its mass, and to the square of its velocity, and 
since the mean energy of one of the complex molecules is equal to the mean energy 
of a single gaseous molecule, it follows that the square of the velocity of the com¬ 
plex molecule is --th of that of the gaseous molecule, where n represents the number 
of gaseous molecules condensed to form a complex molecule. It follows, then, that 
after the complex molecule has been resolved into its constituent simpler molecules, 
heat must be imparted to each of the latter to increase its velocity, until the energy, 
^ MV 3 , becomes equal to that of the remaining molecules. (See Naum ann, ‘ Ther- 
mochemie/ p. 93, ed. 1882).'" 
(e.) If molecules, in becoming gaseous, acquire increased velocity, it is obvious 
that owing to their collision, the motion of the atoms in the molecule is also 
increased, a certain proportion of energy being absorbed. 
Supposing the existence in liquids of complex molecular groups, the analogy is 
evident between the evaporation of a liquid and the vaporization of such a compound 
as is stable when solid or liquid, but dissociates partly or completely when gasified. In 
the first case the simple molecules are like each other ; in the second, generally unlike. 
§ 6. (2.) The density of gases is found in some cases to increase as the temperature 
falls to the condensing-point. This may be explained by assuming the existence of 
complex gaseous molecules (Playfair and Wanklyn, Trans. R.S.,Edin., xxii. (3), 441 ; 
‘Annalen,’ 122, 245 ; also Naumann , ‘Annalen,’ 155, 325); or it may be viewed, as 
before stated, as caused by greater molecular proximity, when mutual attraction 
comes into play. Similarly with those substances in which dissociation takes place in 
the gaseous state, the density of the mixture of gases increases with fall of tempe¬ 
rature ; and this is doubtless caused by the increase in number of complex molecules. 
(As regards this statement, see § 3.) 
§ 7. Solids and liquids, when confined in a closed space, give off from their 
surfaces gaseous molecules, until there exists in that space a certain number. 
These gaseous molecules are termed collectively, a saturated vapour. After this 
limit, conditioned by the temperature of the liquid and gas, is reached, evaporation 
is balanced by condensation ; the pressure exerted by the vapour is termed the 
vapour-pressure (vapour-tension) of the substancec Similarly, substances which 
are known to dissociate, when confined in a limited space, give off vapour, or a 
mixture of vapours, the molecules of which are wholly, or in part, simpler than 
those forming the solid or liquid. Such a vapour, or mixture of vapours exerts 
a pressure conditioned by the temperature. This pressure has been termed the 
pressure (or tension) of dissociation. As in the case of bodies that do not 
dissociate into unlike molecules, the pressure is dependent on the temperature, 
* Exception may be taken to the above statement on the ground that it is yet unknown to what the 
energy of a liquid is due ; and that the relation between energy and temperature, with liquids, has not 
been ascertained. 
MDCCCLXXXVI. 
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