CLASSICAL FLUIDS OF NEGATIVE HEAT-CAPACITY

It is shown that new parameters X can be defined such that the heat ca pacity C(X)=T(partial derivative S/partial derivative T)X is negative, even when the canonical ensemble [i.e., at fixed T=(partial derivativ e U/partial derivative S)Y and Y not-equal X] is stable. This implies an extension of the classical theory of polytropes from ideal gases to general fluids, As examples of negative heat capacity systems we trea t blackbody radiation and general gas systems with nonsingular kappa(T ). For the case of a simple ideal gas we even exhibit an apparatus whi ch enforces a constraint X(p, V) = const that makes C(X) < 0. We then show that it is possible to infer the statistical mechanics of canonic ally unstable systems-for which even the traditional heat capacities a re negative by imposing constraints that stabilize the associated nonc anonical ensembles. Two explicit models are discussed.