done during the Evaporation of a Lnquid. 403 
desirable. We shall see later that the point in question is of more 
than passing importance. Accordingly I endeavoured to obtain 
an expression for w— wq in terms of other quantities. An expression 
was obtained corresponding to matter in the critical state as 
follows. 
It will be convenient to consider first the exact meaning of 
the quantities on the right-hand side of equation (1). When a 
molecule is ejected from the surface of a liquid and passes out of 
the sphere of influence of attraction of the liquid molecules, it may 
undergo a change in the configuration of its atoms during the 
journey. This we would in fact expect since each molecule in the 
liquid state is under the action of the forces of attraction of the 
surrounding molecules whose resultant effect is a more or less 
radial force, whose centre is in the molecule, tending to separate 
the atoms from one another, and from which the molecule is 
relieved when passing into the gaseous state. This change in 
configuration may modify the law of attraction between the 
molecules. When the molecule is out of the sphere of action 
of the molecules of the liquid, or in the perfectly gaseous state, 
it may not yet be in a state of internal equilibrium. And the 
adjustment of equilibrium may give rise to a displacement of 
energy some of which may be algebraically communicated to the 
surrounding molecules. Thus an evolution or absorption of heat 
may occur when the molecule undergoes bombardment by other 
molecules after it has passed out of the influence of the molecules 
of the liquid. This heat is evidently the quantity u—wu,. It will 
be obvious that the same reasoning applies if the molecule passes 
into the gaseous state in stages, due to the existence of a surface 
transition layer. During each stage a change in internal energy 
equal to du will take place. But =du may then not be exactly 
equal to the value that w— wu, would have in the absence of the 
transition layer. 
Next it. will be necessary to consider briefly the equilibrium 
of a substance. When in equilibrium the external pressure and 
force of contraction due to molecular attraction, called the intrinsic 
pressure, is balanced by the pressure due to the motion of trans- 
lation of the molecules. We may suppose, as in the kinetic theory 
of gases, that the motion of translation of the molecules takes place 
parallel to three lines at right angles to one another, one-third of 
the molecules moving parallel to each line. If n denote the 
number of molecules crossing a cm.’ per second of a plane at right 
angles to one of the lines, and p and P, denote respectively the 
external and intrinsic pressure in dynes, I have shown* that 
Mts [Pin = (pena Se NOR THs eangonnseenee (2), 
* Phil. Mag., July 1912, pp. 103—109. 
