: 
a relation which is independent of the density of the substance, 
where 7’ denotes the temperature and m the molecular weight of 
a molecule relative to that of hydrogen. Its deduction was based 
on the fact that the temperature indicated by a thermometer is 
independent of the nature of its bulb, and consequently of the 
molecular attraction it exerts on the surrounding molecules. 
Now let us consider the behaviour under certain conditions 
of a substance which is contained in a cylinder as 
shown in figure 1, in which slides a piston A. The 
system is kept at constant temperature. Suppose 
that the substance is a liquid and that the piston is 
in contact with its surface. Also suppose that the 
material of the piston consists of solidified liquid, in 
which case there is no change in density of the liquid 
as we pass from it into the material of the piston. 
Now suppose that the piston is imstantaneously 
removed from the liquid surface to some distance 
away. Then initially an molecules will leave the 
liquid surface per second, namely those that are able 
to overcome the attraction of the liquid, where a is 
a fraction. The substance may also be in such a state that a is 
unity, in which case the liquid surface is bodily projected after 
the piston. The properties of the substance corresponding to 
different values of a will now be considered. 
It will first be shown that if a=1 the substance cannot give © 
404 Mr Kleeman, On the Nature of the Internal Work 
Fig. 1. 
rise to another phase of the substance which is in contact with it _ 
in equilibrium. For suppose that A and B in figure 2 are two 
= a 
Fig. 2. 
homogeneous slabs of the substance which are in equilzbrium in 
contact with one another under the same external pressure, and 
that for the slab A the value of ais unity. It follows then that 
at any instant all the molecules in the boundary surface of the — 
slab_A move bodily into the slab 6. And since there is equilibrium — 
an equal number must move from the slab B into A. But this — 
can only be the case if the density of the slab B is the same as 
that of A. It will be easy to see that this also holds if in the 
beginning we suppose that a transition layer exists at the boundary 
of the slabs. For this transition layer may be cut up into an 
infinite number of homogeneous slabs, each pair of which may be 
treated in succession in the same way, beginning from the A side. 
a 
