Physical vacuum as an instanton paramagnetic medium

An investigation is made of the properties of the vacuum in QCD in terms of four-dimensional color magnetostatics. The permeability of the instanton gas is calculated as a function of the large external color field. The equation of state is also studied, showing how a first order phase transition occurs. This leads to a bag model, in which quarks are confined inside a dilute phase with unity permeability, in equilibrium with a dense vacuum phase of infinite permeability outside the bag. Finite temperature effects for such a system are discussed. (A.W.)