
244 The Principles of Thermodynamics
n are the same, and equal to -P. Let us concentrate only at the end points l and n.
Therefore, there are three possibilities: i) the points l and n have a common tangent,
ii) the one at n lies above that at l, or iii) the tangent at n lies below that at l. Let us
consider possibility ii) first. Let ( f
,v
n
) be a point on the tangent at l. In other words,
the tangent at l intersects the vertical line v = v
n
at f
. Clearly, f
n
> f
. Since the
slope of the tangent at l is -P it follows that
f
− f
l
v
n
−v
l
= −P →−P(v
n
−v
l
)= f
− f
l
< f
n
− f
l
→ g
l
< g
n
(12.44)
This means that ii) represents the case when the liquid phase is more