The path of a transformation is defined and discussed in the framework of classical thermodynamics. The Ehrenfest's approach to the theory of higher order transition is generalized to heterogeneous systems of r components in the presence and in the absence of electromagnetic fields. By introducing a state label the phase transitions, in proximity of singular points, are analysed in terms of the stability theory. Systems, in the presence and in the absence of a magnetic field are discussed in some detail. It is shown that the Ehrenfest approximation about the transition of 2nd order is physically unstable.