  We offer the parameter, interpreted as the "chemical potential", which is
sensitive to the first order phase transition: it must decrease with number of
evaporating (produced) particles (hadrons) if the (interacting hadron or/and
QCD plasma) medium is boiling and it increase if no phase transition occur. The
main part of the paper is devoted to the question: how one can measure the
"chemical potential" in the hadron inelastic processes. Our definition of this
parameter is quite general but assumes that the hadron multiplicity is
sufficiently large. The simple transparent phenomenological lattice gas model
is considered for sake of clarity only.
