  We report on our ASCA, Keck, and ROSAT observations of MS1137.5+6625, the
second most distant cluster of galaxies in the Einstein Extended Medium
Sensitivity Survey (EMSS), at redshift 0.78. We now have X-ray temperatures,
velocity dispersions, and X-ray images for a complete, high-redshift sample of
EMSS clusters. ASCA observations of MS1137.5+6625 yield a temperature of 5.7
+2.1, -1.1 keV and a metallicity of 0.43 +0.40,-0.37 solar (90% confidence).
Keck-II spectroscopy of 22 cluster members reveals a velocity dispersion of 884
+185, -124 km/s. ROSAT observations show that MS1137.5+6625 is regular and
centrally concentrated. A beta-model fit yields a core radius of only 71 h^{-1}
kpc (q_0=0.1) with beta=0.70 +0.45,-0.15. The gas fraction is 0.06 +/-0.04
h^{-3/2}. This is the highest redshift EMSS cluster with a possible cooling
flow (~20-400 solar masses/yr). The velocity dispersion, temperature, gas
fraction, and iron abundance of MS1137 are statistically the same as those
properties in low-z clusters of similar luminosity. We derive a high-z
temperature function for EMSS clusters with 0.5 < z < 0.9 and compare it with
temperature functions at lower redshifts, showing that evolution of the
temperature function is relatively modest. Supplementing our high-z sample with
other data, we demonstrate that neither the cluster L-T relation, nor cluster
metallicities, nor the cluster gas fraction has detectably evolved with
redshift. The very modest degree of evolution in the L-T relation inferred from
these data is inconsistent with the absence of evolution in the X-ray
luminosity functions derived from ROSAT cluster surveys if a critical-density
structure formation model is assumed.
