  It is shown that the existence of single critical point of the fluid
described by van der Waals equation of state is not the hypothesis and it is
the consequence of the thermodynamic conditions of liquid-vapor phase
equilibrium. It is also shown that the thermodynamic conditions of liquid-vapor
phase equilibrium of the fluid give equalities to zero of first and second
partial derivatives at constant temperature of pressure with respect to volume
at critical point which are usual conditions of existence of critical point.
