Two extreme models of a simple fluid are considered. The first is the ideal gas in which the atoms are described simply by plane waves; in the second every atom is considered as harmonically coupled to the «bubble» within which it sits and the «bubble» systems is assumed in turn to form a free gas. Once a suitable soft-core interatomic potential has been chosen and a mixture of the two types of «fluid» has been considered, the single-particle potential is self-consistently determined. Then, by minimizing the system free energy, the distribution of the atomic population between the two «fluids» at thermodynamic equilibrium is found. Finally the various thermodynamic functions of the mixture are determined. The «two fluids» model is found to be actually practicable.