  We report the observation of bottom-charmed mesons B_c in 1.8 TeV p-bar p
collisions using the CDF detector at the Fermilab Tevatron. The B_c mesons were
found through their semileptonic decays, B_c -> J/psi lepton X. A fit to the
J/psi lepton mass distribution yielded 20.4 +6.2 -5.5 events from B_c mesons. A
test of the null hypothesis, i.e. an attempt to fit the data with background
alone, was rejected at the level of 4.8 standard deviations. By studying the
quality of the fit as a function of the assumed B_c mass, we determined M(B_c)
= 6.40 +- 0.39 +- 0.13 GeV/c^2. From the distribution of trilepton intersection
points in the plane transverse to the beam direction we measured the B_c
lifetime to be tau(B_c) = 0.46 +0.18 -0.16 +- 0.03 ps. We also measured the
ratio of production cross section times branching fraction for B_c -> J/psi
lepton neutrino relative to that for B+ -> J/psi K to be: 0.132 +0.041 -0.037
(stat) +- 0.031 (syst) +0.032 -0.020 (lifetime)
