19-07E

The theory of the first-order phase transition based on the quartic density measure is proposed. It is shown that the jump of density during the isothermal process of the vapor-liquid phase transition of the first order starts and occurs in a newly arisen two-phase system between the parent phase of vapor and the droplet of liquid. The condition of a new phase nucleation in the form of a liquid droplet is the appearance under the high external pressure of equal probabilities 1/2 for the most probable vapor states and the most probable liquid states. The theory is developed from the first principles: the potentials of interaction between particles are set. The van der Waals isotherm is a prototype for consideration. According to small parameters inherent in active forces, respective phase spaces are chosen. A staged calculation of the grand partition function in mutually connected phase spaces is performed.