We consider the main physical notions and phenomena described by the author in his mathematical theory of thermodynamics. The new mathematical model yields the equation of state for a wide class of classical gases consisting of non-polar molecules provided that the spinodal, the critical isochore and the second virial coefficient are given. As an example, the spinodal, the critical isochore and the second virial coefficient are taken from the Van-der-Waals model. For this specific example, the isotherms constructed on the basis of the author's model are compared to the Van-der-Waals isotherms, obtained from completely different considerations.