  A new statistical ensemble is examined using the example of classical
one-component simple fluid. It's logical to call it an open ensemble, because
its peculiarity is the inclusion in the consideration some surrounding area.
Calculations point to the necessity of taking into account the restricting
surface, exactly when the system is not separated by anything from the bath,
and the whole medium is uniform.
  The "surface tension coefficient", included in the partition function
corresponds to the interface of the fluid and hard solid, due to the strict
compliance of probability and potential limitations. The number of surface
particles corresponds exactly to near surface number density distortions
(oscillations) arising in the neighborhood of fluctuation cavities.
  In contrast to grand canonical ensemble, an open statistical ensemble
satisfies the scale invariance requirement: general term of the included
subsystem distribution corresponds to that of the original system.
  It is this ensemble which should be used where consideration of a truly open
system is required, since it properly integrates the surface terms.
Furthermore, this ensemble may be employed in studies of small systems, since
it has no lower limits for the volume of the system. Finally, it is useful in
the investigation of fluctuations. For example, it demonstrates that the
variance (the mean square deviation) of the number of particles is divided into
the bulk and surface terms.
