Electrodynamic boundary conditions at metal interfaces

The boundary conditions appropriate to the hydrodynamic model of metals are derived. Under appropriate circumstances one of these boundary conditions reduces to the Forstmann and Stenschke boundary condition or the Boardman et al. boundary condition: it need not reduce to either. The number of boundary conditions applicable at metal-vacuum and metal-metal interfaces is shown to be the same in apparent contradiction to several simple analyses. Explanations are given for this apparent discrepancy.