  The critical properties of an infinitely long Ising strip with finite width L
joined periodically or antiperiodically are investigated by analyzing the
distribution of partition function zeros. For periodic boundary condition, the
the leading finite-size scaling of partition function zeros and its corrections
are given. For antiperiodic boundary condition, the critical point of 2D Ising
transition is one of the loci of the zeros, and the associated non-analyticity
is identified as a first-order phase transition. The exact amount of the latent
heat released by the transition is 4/L.
