  Recent differential measurements of elliptic flow are used to probe several
hydrodynamic scaling predictions. Eccentricity scaling is observed for Cu+Cu
and Au+Au collisions at $\sqrt{s_{NN}}=~200$ GeV, suggesting essentially
complete thermalization of the high energy density matter produced in these
collisions. An estimate of the speed of sound is also obtained from the
eccentricity scaled elliptic flow. The predicted mass scaling is observed for
particles with transverse kinetic energy KE$_T$ up to $\sim 1$ GeV. For KE$_T$
values above $\sim 1$ GeV, valence quark number scaling compatible with
partonic degrees of freedom is observed.
