  The kaon number density in nucleus+nucleus and p+p reactions is investigated
for the first time as a function of the initial energy density $\epsilon$ and
is found to exhibit a discontinuity around $\epsilon$=1.3 GeV/fm$^3$. This
suggests a higher degree of chemical equilibrium for $\epsilon >$
  1.3 GeV/fm$^3$. It can also be interpreted as reflection of the same
discontinuity, appearing in the chemical freeze out temperature (T) as a
function of $\epsilon$. The $N^{\alpha \sim 1}$ dependence of (u,d,s) hadrons,
whith N the number of participating nucleons, also indicates a high degree of
chemical equilibrium and T saturation, reached at $\epsilon >$1.3 GeV/fm$^3$.
Assuming that the intermediate mass region (IMR) dimuon enhancement seen by
NA50 is due to open charm ($D \bar{D}$), the following observation can be made:
a) Charm is not equilibrated. b) $J/\Psi/D \bar{D}$ suppression -unlike
$J/\Psi/DY$- appears also in S+A collisions, above $\epsilon$ $\sim$1
GeV/fm$^3$. c) Both charm and strangeness show a discontinuity near the same
$\epsilon$. d) $J/\Psi$ could be formed mainly through $c \bar{c}$ coalescence.
e) The enhancement factors of hadrons with u,d,s,c quarks may be connected in a
simple way to the mass gain of these particles if they are produced out of a
quark gluon plasma (QGP). We discuss these results as possible evidence for the
QCD phase transition occuring near $\epsilon \sim $1.3 GeV/fm$^3$.
