  The paper considers the geometry of the Modified Solid-Liquid-Vapour equation
of state. This model describes a substance state in three phases.
Thermodynamics states are points on Legendrian or Lagrangian manifolds in the
corresponding contact or symplectic spaces in terms of differential geometry.
The conditions of applicable states and the first order phase transition are
given for the Modified Solid-Liquid-Vapour equation of state. The Lagrangian
manifold, singularity curve and the phase transition curves are plotted for
methane.
