Time-scaling in the thermodynamics of irreversible processes

We present a general scheme to deal with situations in the thermodynamics of irreversible processes in which some of the variables vary on a much faster time-scale than others. The theory is based on a transformation of variables of a type first introduced into thermodynamic fluctuation theory by Tisza. It is shown that this transformation simplifies the calculation of time-correlation functions. A systematic approximation procedure is presented for the time evolution of the slow variables.