In this paper the concepts "equilibrium" and "reversibility" are examined from the standpoint of the number of independent variables which define a thermodynamic system. The endeavor is made to show that the number of independent variables which are selected is crucial to the basic concepts as well as to the second law of thermodynamics. If this number is less than the minimum required to define the system completely, the equation of state for the internal energy will be incomplete. It is our practice to employ incomplete equations of state, and this results in processes which we consider to be irreversible. The Kelvin-Planck and Clausius principles are shown to be somewhat related to the first law of thermodynamics, and their validity arises from the fact that the equations of state which are used are incomplete. If these equations were complete, that is, if the number of independent variables were not less than the minimum required to describe the systems completely, we should have complete conversion of heat into work, as well as work into heat. The difficulties in realizing complete equations are considered.
The entropy concept is considered in detail, and it is shown that there are serious limitations in its application. 1. INTRODUCTION.