  The standard treatment of relativistic thermodynamics does not allow for a
systematic treatment of mixtures. It is proposed that a formulation of
thermodynamics as an action principle may be a suitable approach to adopt for a
new investigation. This third paper of the series applies the action principle
to a study of mixtures of ideal gases. The action for a mixture of ideal gases
is the sum of the actions for the components, with an entropy that, in the
absence of gravity, is determined by the Gibbs-Dalton hypothesis. Chemical
reactions such as hydrogen dissociation are studied, with results that include
the Saha equation and that are more complete than traditional treatments,
especially so when gravitational effects are included. A mixture of two ideal
gases is a system with two degrees of freedom and consequently it exhibits two
kinds of sound. In the presence of gravity the Gibbs-Dalton hypothesis is
modified to get results that agree with observation. The possibility of a
parallel treatment of real gases is illustrated by an application to van der
Waals gases. The overall conclusion is that experimental results serve to pin
down the lagrangian in a very efficient manner. This leads to a convenient
theoretical framework in which many dynamical problems can be studied.
