  In the first part of the paper, we introduce the concept of observable
quantities associated with a macroinstrument measuring the density and
temperature and with a microinstrument determining the radius of a molecule and
its free path length, and also the relationship between these observable
quantities. The concept of the number of degrees of freedom, which relates the
observable quantities listed above, is generalized to the case of low
temperatures. An analogy between the creation and annihilation operators for
pairs (dimers) and the creation and annihilation operators for particles
(molecules) is carried out. A generalization of the concept of a Bose
condensate is introduced for classical molecules as an analog of an ideal
liquid (without attraction). The negative pressure in the liquid is treated as
holes (of exciton type) in the density of the Bose condensate. The phase
transition gas-liquid is calculated for an ideal gas (without attraction). A
comparison with experimental data is carried out.
  In the other part of the paper, we introduce the concept of new observable
quantity, namely, of a pair (a dimer), as a result of attraction between the
nearest neighbors. We treat in a new way the concepts of Boyle temperature
$T_B$ (as the temperature above which the dimers disappear) and of the critical
temperature $T_c$ (below which the trimers and clusters are formed). The
equation for the Zeno line is interpreted as the relation describing the
dependence of the temperature on the density at which the dimers disappear. We
calculate the maximal density of the liquid and also the maximal density of the
holes. The law of corresponding states is derived as a result of an observation
by a macrodevice which cannot distinguish between molecules of distinct gases,
theoretical and experimental data are compared, observations in three scales,
macro, micro, and nano, are studied.
