  We show that thermally activated interstitial and vacancy defects can lead to
first order melting of a vortex lattice. We obtain good agreement with
experimentally measured melting curve, latent heat, and magnetization jumps for
YBCO and BSCCO. The shear modulus of the vortex liquid is frequency dependent
and crosses over from zero at low frequencies to a finite value at high
frequencies. We also find a small surface tension between the vortex line
liquid and the vortex lattice.
