
66 The Principles of Thermodynamics
forbidden about that. Stated differently, while perpetuum mobiles of the second kind
are impossible, their reverses are commonplace!
FIGURE 3.10 The ideal heat engine and
its reverse.
FIGURE 3.11 The ideal refrigerator and
its reverse.
Therefore we see circumstances where certain processes are perfectly legitimate,
but their reverses are absolutely forbidden. This is another way of saying that certain
processes are irreversible. The nature and characterization of such irreversibilities is
the body and soul of the second law. As Planck points out, the situation is dramati-
cally different with perpetual mobiles of t